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Saturday, March 20, 2021

Building a Curve Tracer - Version 3

 The VBA Curve Tracer Project

 

Quick update on the very latest developments:

May 24th
I've found the issue with the Step Gen, unfortunately, we may have a design issue with the over-voltage protection for one of the Opamps that was overlooked. Now that I had a chance to really work with the instrument, I found that I don't like the new offset circuit for the Step Gen very much because it is not transparent for the user. We'll try to fix it, and otherwise go back to the previous circuit. This will eliminate the issue with the over-voltage protection because that Opamp is in the offset circuit we no longer need.  Also the Fault circuit is not fully working yet. We're working on those these issues. With the over-voltage protection disabled, the Step Gen works fine, so I could continue with the verification & calibration, and while doing that, I also updated the document as I went. The good news is that everything is working, and I could even do a stress test with the maximum currents. We passed that with flying colors, but this is still without the top cover on. I need to get my Sandwich wired without the extensions so I can put the cover on. I won't do that yet because I'm still testing and need full access to the boards.  

Below is the IR picture of the 35V @ 2A stress test. Nothing gets above 70 degrees C. The hot spots are the current sense resistors and the bridge. This was to be expected.


The net-net is that the Main board layout is good, the Face plate layout is good, but we need to go through another board turn for the Front board to fix the issues. We'll wait until we have sorted out all the pending problems and challenges. We are already in the process of updating the schematics with more or better information and corrections, and also improve the silkscreen of all three boards. When the schematics and Gerbers are published, they will have the latest modifications.

May 22nd
When I checked my e-mails this morning, I read that Mark found out that he made a layout error to the polarity switch, so that needed a fix too. This information confirmed my own observation and saved me some time and the fix was relatively easy to do. Now the X & Y amps are working. After that I tested the Fault circuit and it seemed to work but no LED. It turns out that these special LED's we use do not have a reliable cathode indication, and also this LED was flipped, making it 4 out of 4. I ended up using my DMM in the Diode mode to verify the correct orientation.

Everything seems to be working now but I still have a strange situation with the Step Gen output measured at the DUT connector in the PNP mode. NPN looks better, but is also not quite right. 

May 21st
We fixed the calibration of the 200V maximum issue by eventually deciding to make a calibration resistor value depending on the Volt potmeter tolerance. The circuit was designed for a 100K value, but we found several specimens with only 90K, the minimum of the 10% tolerance. The 10% deviation of the potmeter value causes a 20% deviation of the maximum DUT voltage. We will now specify three resistor values for a -10%, 0% and +10% deviation, so users can easily calibrate the output to 200V. The other two voltage ranges are heavily influenced by the 200V setting, so this was an important issue to fix.

Having verified the DUT supply side, I switched my attention to the Step Generator circuits and ran into a problem. It turned out that we used a capacitor that should not be there. Removing that fixed the problem I was having. The Step Gen seems to work and a quick check showed that has all the functionality. Now on to the X-Y amplifier section. Both amps where not working. The X-amp was fixed by correctly mounting a protection diode to the input, because it was reversed. The Y-amp took a bit longer to find. It turned out that Mark made a schematic error that also transpired to the layout. The new offset circuit that we designed was connected to the wrong input pin of the Opamp. Fixing that still did not produce the expected results, but I ran out of time. I suspected the polarity switching and send an e-mail to Mark.

May 18th
I was away from home for a few days and otherwise busy with other chores. I also spend a lot of time writing the instructions that will help users to build the instrument and a list of instructions for Mark to update the schematics and the silkscreens to make it all more logical and self explanatory. I did however complete the wiring between the Main board and the Sandwich. Since then I have been testing the functionality step-by-step. As to be expected, I ran into a few snags on the way, like incorrectly installed parts. It turns out that the special LED's we use on the back of the Face plate have a very poor indication of what the cathode side is. As luck would have it, three out of four were flipped. I can't really blame Mark for getting it wrong because the cathode indication is very difficult to determine. Another issue that I have is that I can't yet get the full maximum voltage of 200V. I though we fixed that in the 10x10 version, but it's still there. We know we have a tolerance issue with the Volt potmeter, so we need to account for those +/-10% deviations.

May 10th
Apart from a few missing parts that will arrive Thursday, I was able to construct the Sandwich of the Front board and the Face plate and while doing that, also updated the instructions on how to do it. This combination has not seen a power-up yet, that will happen after I installed the missing parts. 

Here is a quick-and-dirty picture of where we are now:



May 8th
I managed to build-up the Main board and the Back panel and they are in the enclosure together. All the vital signs I can check on the Main board are there! Very encouraging and a well done job by Mark. I've also written a how-to-build section that includes checking the vital signs while completing the building process. I now need a functioning Front board and Face plate sandwich to further verify and test the Main board operation and functionality.

May 5th

The shipment from Mark with the new boards in addition to most other parts have arrived so I can start to complete the instrument. It will be slow going because I also want to complete the how-to-build & calibration instructions and need to verify BOM details as well.

April 24th
The latest revision boards have arrived a few days ago and Mark is busy populating them with the SMD parts. Unfortunately, we are also hampered a bit by the current chip shortages so we had to make some changes and order parts. When Mark has received the last part we're waiting for, he will send the board set and the remaining components to me so I can complete the build and start testing again.

April 8th
The complete set of boards was sent to production. This included a revised Face plate and Front board, the new Main board and the new Back plate. The latest schematics and layouts are added at the end of the Blog. We also started to work on the final BOM's.

March 1st
I got delivery of the two PCB's that Mark designed and populated. Below you can see how the Front panel and the Front board will fit together. This also shows how the interface will look like. This is just a first test on how everything fits mechanically. Spoiler alert; it does, with some minor adjustments. We also identified and know how to fix a number of layout errors and silkscreen changes for the next board turn. We're now verifying the operation and also made some changes we want to implement in the next revision.


Below is the Front board facing the Face plate with all the parts mounted. The two floating potmeters are for the Step Delay and the Offset and are mounted directly on the Face plate (with shorter wires than shown here).

Below is the fully mounted sandwich. The distance between these two boards is adjusted to be 18mm and secured by the toggle and rotary switches and the Current and Volt potmeters that also connect the two ground planes together.

 

 The Main board below is still printed on paper, shown here to give you an idea of how it will all fit inside the enclosure. The green terminal blocks will change to another type in the next revision.

Below is the previous setup with the fully working version of the curve tracer, built with the circuits on 10x10 boards to allow for swapping out new revisions, making measurements and making changes.

Here is the setup I'm working with now, all the bugs are out and again we're fully functional and ready to test the functionality.
 

Version 3 of the Curve Tracer Project

Based on the experiences we collected with Version 2, we made some significant changes to make it a lot more reliable, hopefully easier to build with commonly available parts, and with greatly increased usability and safety. In addition, we would like to see if we can make more measurements.

More information about this overall project can be found here:

The building of the first version, actually a working prototype with a detailed Theory of Operation.
https://www.paulvdiyblogs.net/2017/

There is also a description of the second generation based on the first prototype. This is a fully functional CT but has some problems and shortcomings that we're addressing in V3.
https://www.paulvdiyblogs.net/2021/03/building-curve-tracer-v2.html

During the development and testing phase for Version 3, we use 10x10cm boards that contain the required functionalities so we can easily swap them out with newer revisions. I will describe them below.

The pictures above show that we're working hard on the completion of this version.We now have a fully functional V3 Curve Tracer and we can make measurements. I have started a dedicated Blog with the measurements that we are currently able to make:
https://www.paulvdiyblogs.net/2021/11/making-meassurements-with-v3-curve.html

Note:

This is not a project to fully replace or replicate a Tektronix 577 or 576 Curve Tracer. We do however hope to replace these units as they are currently still used in workshops, laboratories or high-schools/university classes but are dying of age or due to unavailable replacement parts. They are also too expensive and too bulky to allow more students or hobbyists to access them.
We can do many measurements these very versatile instruments can do, but not all. Most likely never will. Not yet anyway. 😏

 

Completion date

The completion goal is slipping but we hope to have a finished and published instrument by the middle of 2022.

On January 6th, 2022, we have reached a design freeze for our 10x10 board setup. Hooray! The next step is to combine all the circuits and design the final layout of the 4 PCB's incorporating the latest changes. The 4 PCB's are the front panel or what we call the face plate, the front board, the main board and the back panel. While we are going through this process, there have already been a few items that we identified, changed and improved. At the same time, we're also working on the final BOM's for each PCB.


Specifications 

Here is a list of the specifications, but note that they are not final yet!

Voltages/Currents (Collector or Drain)

There are three main ranges that can be selected.

  • 0-35V @ 0-2A
  • 0-75V @ 0-1A
  • 0-200V @ 0-100mA

The voltages and the currents of any of these ranges can be adjusted from 0-100% of the maximum values with full load. The voltage output can be selected between a triangle-based Sweep, which is the default, and also DC for leakage measurements.  The maximum current in the DC mode is half of that for the triangle based sweep mode. Selecting the DC mode will automatically change the maximum current to 50% in the X1 current range.

The frequency of the triangle-based waveform is set at about 160Hz but can be tuned by changing a trimmer inside the unit. It can range from 140Hz to 650Hz and can be used to tune the rate of flicker on the display of the oscilloscope.

An indicator will warn the user when voltages higher than approx. 50V are present on the DUT outputs.

There are 5 current ranges to limit the maximum currents in the three voltage modes:

  • x 1  (changed to 0.5 in the DC mode)
  • x 0.5
  • x 0.2
  • x 0.1
  • x 0.05
  • x 0.02

This means that in the 35V @ 2A range, the maximum current can bet set to 2A, 1A, 400mA, 200mA, 100mA and 40mA. A Current Limiter can be used to further adjust the maximum current from 0-100% in any of the selected Current Ranges.

An LED will signal the current limiting (CL) mode, ie when the current to the DUT is exceeding the set current by the Current Range and the Current setting.

Step Generator

The Step Generator feeds the Base or Gate of the DUT and can be set to output a current for BJT devices or a voltage for FET devices.

The Step Generator output can be set to generate 0-7 steps.

A Step Delay can be selected to delay the time between complete step cycles to reduce the thermals developing in the DUT and prevent it from over-heating. When activated, the delay is approx. 40mSec between step cycles and can be increased to more than 250mS. The step cycle can be set from 1 to 7 steps.

The output of the Step Generator can be set in a 1-2-5 sequence.

The selections are: 

  • 5V*) or 5mA
  • 2V*) or 2mA
  • 1V*) or 1mA
  • 500mV or 500uA
  • 200mV or 200uA
  • 100mV or 100uA
  • 50mV or 50uA
  • 20mV or 20uA
  • 10mV or 10uA
  • 5mV or 5uA
  • 2mV or 1uA
  • 1mV or 1uA
  • 500uV or 500nA
  • 200uV or 200nA
  • 100uV or 100nA
  • 50uV or 50nA

*) Note that with the 5V, 2V and 1V ranges, you must reduce the number of steps to avoid clipping against the supply rail of the step amplifier.

An offset can be selected with either a positive (aid) or negative (oppose) offset to the Step output. The offset range has a maximum range of 10V in either direction. Depending on the selected DUT polarity and the selected offset direction, the maximum number of steps in the V-mode will have to be reduced to avoid clipping against the supply rails. For N-channel devices, the positive offset will be limited due to clipping while the negative offset can go the maximum voltage. For a P-channel device the offset will be full in the positive direction, but limited by clipping in the negative direction. 

Both the current and the voltage outputs are corrected for DUT junction voltage drops and other current/voltage drops so composite devices (Darlington) can be tested without influencing the Base or Gate step settings. 

The Step Gen output is protected against damages by high voltages in case of a shorted DUT socket, or a shorted/damaged C-B or D-G junction. A Fault signal will warn the user of this condition as long as it is present and the voltage at the Collector/Drain output will be removed for the duration of the fault condition.

A switch determines the polarity of the DUT for PNP/P-channel or NPN/N-channel type devices in such a way that the origin of the I/V curves always start in the lower left-hand corner of the oscilloscope.

Device Under Test (DUT)

A large variation of DUT's can be connected to the Curve Tracer by means of test sockets or 2mm Banana sockets. Two DUT's can be compared by means of a DUT selector switch that can be off or power the left or the right DUT socket to make comparisons and matching of devices possible.

Many different 2 or 3-pin DUT's can be measured or characterized with the instrument.

Display Device

An analog (CRT) or digital oscilloscope (DSO) is used in the X-Y display mode to show the I/V curves of the DUT. There are two BNC sockets available on the back panel of the instrument to connect the X and Y outputs to the input channels of the scope. An optional Z or blanking signal can be made available through a BNC connector on the back panel for analog oscilloscopes.

A multiplier for the measured DUT current can be used to select between x1 and x10 to amplify small DUT currents on the scope and raise the signal above noise levels.

Enclosure

A mains switch is available on the back of the unit. An indicator on the Face Plate will show that the instrument is powered on.

The overall current consumption of the instrument still needs to be determined.

The instrument can be used with 115V 60Hz or 240V 50Hz mains voltages by means of a selector switch inside the unit. The mains connection is through an 8-type connector with EMI filter, has a fuse and a mains switch on the back panel. The instrument circuits will be earth grounded through the BNC connections to the oscilloscope.

The plastic enclosure measures 25cm x 18cm x 8cm. The overall weight still has to be determined.



The Major Building Blocks

Below is a description of the basic building blocks of this CT. Several of the diagrams and schematics posted are no longer the very latest version or revision. Eventually, that will be the case, and we will publish everything on a Github site, but for now, this shows our work in progress.

After this section with the description of our progress and struggles, there is now also a section where we show some of the measurements we can currently make with the CT, and some of the issues.

 

1. The auxiliary supply

 

 

This is the supply for all the voltages for the Op-amps and other parts. There are actually three segments, all fed by one transformer that has dual primary windings for 230V and 115V based main voltages, and equal dual secondary windings. We need dual secondary windings that are isolated from each other because we need a fully isolated supply for the Step Generator. 

The black tape is there to protect me from touching the mains related voltages, because I did not mount the switch that selects 110 or 240V. The dead bug resistor and LED below it is the power indicator, that will be on the front panel. We don't need the large heat sink areas for the regulators and will change the packages of the LM317 and LM337 on the left to smaller ones on the final layout.

One separate winding from the transformer is used for the Triangle Generator, the DUT power supply and the XY amplifier.  This supply generates voltages of +10V and -5V.  The +10V is used as a reference for a few critical circuits, so this supply is adjustable with a trimmer. 

 

For the final version, we're going to replace the LM337 with a 79L05 regulator because we no longer need a precise -5V due to the way we changed the Triangle Generation. 


The Step Gen supply section is the +15V (called plusStep in the schematics) and -15V (minStep) for the Step Generator circuits. This supply is floating (isolated) from all the other supplies and uses a separate winding from the transformer to accomplish this.

 


The third section is for the X-Y amplifier. It needs -5V (minusXY, the same as minTri)  and +24V (plusXY). Because the raw voltage after the voltage multiplier is too high for a 78L24 regulator, we use a transistor and a reference. The 24V is needed because we need a minimum deflection of 20V on the DSO to show the 200V Collector/Drain supply, and also enough steps that are measured with the DUT current shunt.

 



2. Triangle Generator



The left side of this board above is the Digital to Analog (D2A) section, the right hand side the Triangle Waveform Generator. The knobs you see do not belong to these circuits.
 
The Triangle Generator is the hart of both the Collector/Drain supply and also the Step Generator. It creates the triangle wave form that is the basis for the Collector/Drain supply, and it triggers the Step Generator so the steps are aligned with the triangle signal. Mark and Bud created a different circuit based on the one we had been using so far.



This circuit is quite simple, it creates a free running oscillator that is also an integrator created by the feed-back capacitor of the output Opamp. The value of this capacitor and the trimmer in series determines the triangle waveform shape and the frequency. The frequency can be adjusted between about 150Hz to 650Hz. This will allow you to select the optimum refresh rate based on the used DSO or CRT. The 555 acts as a comparator. The R/C delay set by C65 and the 100K trimmer is there to compensate for all the propagation delays in the Step Gen circuits, mainly the opto-coupler, and is needed to synchronize the triangle transitions with the Step Gen step transitions at the DUT as they are on the display of the DSO.
 
The output of about 7.5Vp-p generated by the Opamp goes to the Collector Supply. The lower two gates are used to create a trigger signal from the square wave for the Step Generator. With these two Gates we can create a single trigger pulse, either a positive going or a negative going, or both, selected by two jumpers. This allows us to synchronize the beginning of the Step sequence by either the top of the triangle, the bottom of the triangle or both. The consequence has been explained in the other two Blogs and will be explained later in the Step Generator circuit. At this moment, we are only using the jumper P1-En, and we could eliminate the other selection all together, but decided to leave it in just in case there could be a use somewhere down the line for a user. The output of the last gate goes through a pulse shape circuit to create a better flank for the opto-coupler output.

On the top of the schematic, there is a provision to create a blanking signal (Z-axis) for CRT based scopes. This output is available through a BNC on the back panel.


3. The AC Power Supply

 


 
The AC supply section is the first stage for the regulated DUT power supply. 
It contains the two transformers for the voltages, the switching of the voltage ranges and the transformer winding switches.
We are using two transformers to create the 35V, 75V and 200V ranges. The ranges are selected by an on-on-on DPDT switch on the front panel that is wired to create three positions. The 35V switches the two secondary windings of the main transformer in parallel, to double the current. The 75V position switches the windings in series, and the 200V position adds the high voltage transformer's 120VAC "on top" of the main transformer to create an unregulated voltage of about 230VDC.
 
Bleeder resistors are used to make sure that high voltages are quickly brought to safer lower levels. We use a beefy relays to switch the secondary windings of the main transformer from serial to parallel, and use another relays to switch-in the 120VAC transformer for the 200V supply and the bleeder resistors.



Adding Voltage and Current ranges

After many deliberations and tests, we decided to offer the following main voltage and current ranges: 

  • 0-35V @ 0-2A
  • 0-75V @ 0-1A
  • 0-200V @ 0-100mA.

The voltages are available with the maximum current load. These ranges are created by switching the AC side of the main transformers, and also by limiting the triangle waveform at the input to the Sweep supply. We use MOSFET's that are activated by the Voltage Selection switch to limit the voltage by limiting the input voltage by resistors and trimmers to ground so we can calibrate each of the three voltage ranges.

We also added a current range selection that will allow you to set the maximum current in any of the three main voltage/current ranges, so you can more easily protect the DUT by selecting a maximum current.

The current range attenuation selections are:

  • x1
  • x.5
  • x.2
  • x.1
  • x.05
  • x.02

This means as an example, that when you select the 75V @ 1A range, you can set the current selector to 0-1A, 0-500mA, 0-200mA, 0-100mA, 0-50mA and 0-20mA, and still use the current setting adjustment to go from 0-100% within any of these ranges.

The current ranges are created by a rotary switch located on the front panel that changes the reference voltage for the current limiter circuit. They work in tandem with the already mentioned Voltage range shunts and this is why they have a multiplier and not an exact number.

Part of the Voltage and Current switching is on this AC board, in particular the transformer switching parts. The rest of the circuits is on the Collector/Drain Supply Board.

 

Current Source

There is also a Current Source circuit on this board. It provides a stable 12.5mA load on the DUT power supply irregardless of the voltage, for regulation stability.

 


 

With these changes we are currently on Revision 8 for this board. 


4. The DUT Power Supply

 


The DUT power supply is one of the two main sections of the Curve Tracer, the other one is the Step Generator discussed below.

The DUT power supply is made up of two sections. One is the AC supply section supplying the raw DC voltages, and the other one the regulated triangle or DC based buffered output section that we call the DUT power supply.

Initially, we used Opamps to control the Voltage and the Current regulators, and power transistors or Darlington types for the regulation but we were having all sorts of problems to provide a clean triangle waveform at voltages ranging from 0-200V and with currents ranging from open circuit to 2A.

While we were having these issues with the stability, I decided to call in the help from my friend Bud, an ex chip designer from LT. He and I worked together remotely as mouse-pal's on a few other projects, most notably on the UPS power supplies for the Raspberry Pi Model 3 and 4 and a differential scope probe, both described in different Blogs on this site.

Bud didn't have our hardware at his disposal, but used LTspice extensively. In simulation, it turned out that due to the Opamps we selected, in combination with the series transistor types, we badly needed different compensation configurations. Over the course of many weeks, investigating, discussing and trying things out, we eventually arrived at a different design that uses MOSFET's as the series regulator. However, we were still having difficulty taming this tiger and were spending a lot of time trying different methods and circuit variations with an attempt to get it right in all conditions.

 
Bud's Wild Hair idea.

Bud could not leave the challenges this DUT supply circuit posed out of his head, and started working on a novel and different solution that would accomplish a better regulation transition from voltage regulation into current limiting. He called it a "Wild Hair" idea.

 
This plus a number of other developments and additions resulted in the busy diagram below. 
 
 
Note:
Some of you may see that this is a diagram coming from KiCad. Mark is using Altium Designer to generate the schematics and layouts, and I had been using DipTrace, that I used for several years, to import the Altium files. This was a very cumbersome process that needed a lot of tweaks and changes. With the recent version of KiCad 6, there is now an import functionality that works really well, so I switched to KiCad now. I can use these files to manipulate and show on this Blog, but Altium will stay the lead EDA.
 
This circuit looks pretty complicated but I will break it up into a number of sections and explain it in more detail.
 
This new circuit uses discrete trans-conductance amplifiers for the voltage and current controllers. This type of amplifier converts a voltage at the differential inputs into a current at the output. This is what we were trying to do, kind of, earlier but in a much more complicated way. The two regulators are now fully discrete using matched transistors (Bud was a chip designer after all), and the LTspice results looked very promising. Gone are the troublesome transitions from voltage regulation to current regulation and back and no more desperate hugging of the Opamps to the rails.  
 

The DUT supply Voltage Regulator

 
The major difference between our Curve Tracer and many others, like the bench Tektronix units, is that we do not use transformers to create a half-sine wave sweep supply, but use a triangle based wave form. This was also done with the Tek 5/7CT1N scope plug-in units that I was very familiar with, because I built one myself, described in another post on this Blog.  
 
In the bench designs where transformers are used, they use a large number of taps on custom made transformers to create different voltages. It is relatively easy to limit the current from these taps by switching a resistor in series. We use a different method to create the voltage because we do not have access to these multi-tapped transformers and we do not want to be limited to twice the mains frequency of 100 or 120Hz for the Sweep frequency.
The other limitation is that you need very complicated and sturdy custom made rotary switches and many relays for switching the transformer tab selections and also for the many resistor selections. These complicated switches are one of the major reasons that many of the Tek CT's are no longer serviceable.

We use an electronic method to generate different voltages and also to limit the current. This is analog to a Lab power supply with the major difference that a Lab supply is only needing this for a DC voltage, and we need to do the same for DC but also for a triangle waveform. This means that the current limiter needs to limit the highest voltages (peaks) of the triangle waveform, and that with the frequency of the triangle, because it is repetitive. The transition from voltage regulation into current limit and regulation is a touch challenge if you also consider that the voltage can range from 0 to 200V, and the current from 0 to 2A.

This is why, after going through many different variations, we decided to use discrete trans-conductance amplifiers for both the voltage and the current regulator circuits.
 

The Voltage Regulator

 
Below is a reduced schematic just dealing with the voltage regulation circuit.
 
 

 
The triangle signal from the triangle generator comes in on the left. The input voltage is about 7.5Vp-p. The switch selects the sweep or DC mode for the supply. In the DC mode, a voltage is supplied that is the same as the p-p voltage level of the triangle waveform, 7.5V. 
 
The trimmer after the switch is used to calibrate the maximum output at 200V in that range. Next are the two MOSFET switches that will limit the input voltage going to the amplifier. There are two switches, one to setup the 35V range and one for the 75V range, each with their own trimmer to calibrate these two voltages. The two MOSFET's switch the triangle signal or DC level with a resistor to GND to reduce and set the voltage for the ranges. One is used for the 75V range and the 35V range switches a resistor in parallel to reduce switching glitches. The signals that drive the MOSFET switches also drive the relays that select the three voltage ranges. This part is located on the AC Supply board.

The trimmers to set the maximum voltages of 200V, 75V and 35V are needed to compensate for part tolerances. In particular the value of the 100K Voltage selection potmeter. This part has a tolerance of +/-10% and that will also create a +/-10% on the output voltage of the Collector supply. We measured a number of these potmeters and they are indeed ranging from 90K to 110K. In some cases, the trimmers will be insufficient to adjust the voltages, and in those cases the resistors in series with the trimmers will have to be changed.
 
The 1K5 resistor in series with the 100K voltage potmeter is there to set the minimum output level, low enough to have good regulation.

The triangle input signal is entering a trans-conductance amplifier created with matched transistor pairs. The input is protected with a set of limiting diodes. The output of the amplifier is protected with another set of diodes and is used to drive the Gate of the main MOSFET, Q1. The Drain is connected to the positive DC supply. The Source connects through a little balance resistor via a current sense resistor to the main GND.  

Driving the parallel MOSFET's
 

Quite novel is the circuit Bud designed to drive the parallel MOSFET's. In most circuits that I know, there is a separate Opamp to drive the second (or more) MOSFET and it must make sure that the load is in effect really shared. That kind of a circuit is a little more difficult to realize in our setup. Good load sharing is not so simple to do in reality, because with the MOSFET's in the linear mode, a minute change in the Gate drive will cause a major change in the conductivity and hence the temperature. Bud came up with a "current duplicator" circuit where he uses an Opamp that measures the current through the main MOSFET, and drives the "slave" MOSFET to conduct the same current. This works really well.

We have seen situations by which the Opamp has been blown during a fault situation, so we added diodes to the rails to protect it.

Voltage Regulation
 
The feed-back loop to regulate the voltage is going through a 100K Sweep Voltage potmeter located on the front-panel and that connects to a summing point to the regulator Opamp through a 1K5 resistor. This resistor sets the minimum output voltage to about 2Vp-p to aid in the regulation and also protects the potmeter when it is in the lowest setting with the highest output voltage of 200V.

More details about the regulation principle can be found in Blog 1 that deals with the Theory of Operation and it explains how the regulator is tied to GND, and regulates "downwards". This is analog to the Harrison Labs (HP/Agilent) method of designing Lab Power Supplies. (Tech Letter)

The Current Regulator/Limiter

 
The current controller seems to be a little bit more complex, but is actually not much more complicated than the voltage controller. The complexity comes from the fact that we want current ranges specific for the voltage ranges, and on top of that further current attenuation to limit the maximum current going to the DUT.
 

 
In principle, the current controller measures the current through the main MOSFET and compares that with a set reference. This reference is further attenuated by the resistor selections of the current ranges, and by the current setting potmeter on the front panel. If the measured current is larger than the set current, the Gate of the main MOSFET is pulled down to reduce the voltage and hence the current. The second MOSFET will dutifully follow along. An LED on the Front Panel will alert the user that current limiting (CL) is in effect.

The output current is measured by using shunts of different values to make them equal in the three voltage/current ranges. There is a 10R shunt for the 200V range, this will produce up to 1Volt across this resistor at 100mA, the maximum for this range. We switch other shunt resistor values in parallel to the 10R to create the 1A and the 2A ranges to reduce glitches when switching. The shunts are switched in circuit by MOSFET's that are activated with the same switch that controls the other voltage range settings. 
 
The reference voltage for the current controller Opamp is set at 1V by a voltage divider located on the left side of the schematic. The different current shunts for all three ranges will produce a 1V level at the maximum (100%) for the ranges, 100mA for the 200V, 1A for the 75V and 2A for the 35V range.

We also create current attenuation for each of the three voltage ranges by changing the current reference voltage through resistors to GND. The attenuation values are x1, x.2, x.5, x.01, x.02 and x.05. This selection is made by a rotary switch that is located on the Front Panel. The reference voltage is buffered by a unity-gain Opamp and fed into the trans-conductance amplifier.
 
The trans-conductance amplifier is made-up of two pairs of matched transistors.  The output of the amplifier drives a 2N3904 transistor that is pulling the Gate of the main MOSFET down to regulate it. The output is also going to a pair of transistors that switch the Current Limit LED so there is a visual indication on the Front Panel when Current Limiting is in effect i.e. when the current to the DUT is larger than set, and therefore limited.
 
Below is the current limiting in effect. Note that in this particular screen shot, the triangle wave-form is going negative, with the zero Volt level at the top of the display.


 
The current limiting keeps the triangle voltage from reaching higher levels (here going down) by simply "stopping" it, creating the horizontal line. As soon as this limit is reached, an LED on the Front Panel will inform the user of the Current Limiting effect.
 
Note the clean edges top and bottom and the utter lack of wiggles and glitches during the transition from voltage regulation (the triangle slopes) and the current limiting taking over (the flat portion). This is a far cry from our earlier versions so we made the decision to go forward with this design. 

The Current Source & Compensation

To increase the stability of the regulation, we use a Current Source that pulls about 12.5mA from the supply, regardless of the voltage range. This circuit is currently located on the AC Supply board. In the 35V range with 2A and the 75V range with the 1A maximum, 12.5mA will not be noticed. In the 200V range with a maximum of only 100mA however, this will create an error so to speak of 12.5%, because it is part of the 100mA budget, which is too much.  In order to compensate for the approximately 12.5mA, we need to keep the 1V current reference at a maximum of 100mA. We do that by adding a parallel resistance circuit that can be used to calibrate and subtract the current of the Current Source from the output so the maximum net current is still 100mA. This is the circuit around R47, R52 and Pot4 in the middle of the diagram.

Output and thermal stress test

In order to verify that the unit can provide the maximum voltages with the maximum currents while all thermals are OK, I ran a few stress tests with the maximum load for every voltage range with the maximum current, by using (ventilator cooled) load resistors across the output. I ran the tests for about 10 minutes each so see if we had any issues.
 
I used an IR-camera ( a new toy for me) to have an overview and make detailed measurements.
 

Obviously, the two MOSFET's are the most involved. The picture above shows that the thermal balance between the two is excellent, and I also measured that the temperature of the hot-spot on the device package itself was not above 50C. The three tests passed with flying colors, although this was with everything in free air. We need to do the tests again when everything is inside the enclosure, but it looks like we have the thermals under control with a normally operating CT and don't need a fan. More extreme/fault tests are described below.
 

DUT protection circuit

 
During my extensive testing of DUT's with the previous version CT, I had a major disaster happening that blew out many parts on the DUT power supply and also the Step Generator. We needed some sort of protection from this happening again.  More of this is described in the Step Generator section, but the solution we adopted was to disable the DUT supply output that could find it's way into the Step Generator circuits. In my earlier case, it turned out to be caused by an almost shorted C-B junction, but can also happen by a pilot error creating a short on the DUT socket.
 
We now have an input coming from the detection circuit on the Step Generator and that feeds-in to this supply to kill the output by disabling the triangle or DC input the same way as the Voltage ranges do. We use a discrete SCR to control the triangle voltage and use an R/C filter to keep the voltage off for a few milliseconds before it releases the triangle again, making it fully automatic and dynamic.

At the same time, an LED is lit on the Front Panel to indicate the Fault situation to the user.

 

 
The DC mode switch circuitry is not shown here.

High Voltage Warning Indicator

We wanted to warn the user when dangerous voltages are present on the DUT output connectors. We determined that this is at about 50V and higher. Mark came up with a simple method that just uses a single transistor to drive the LED on the front panel when the 50V threshold is exceeded. After adding the DC mode, we needed to make sure it tripped with both voltages at the same level. The negative DC voltage enters through R29 and R30, and is blocked by C1 to reach the lower value resistors. The triangle voltage enters though C1, R1 and R2. We use two resistors in series to take care of the up to 200V, and it also allows a trimming of the tripping point. The diode clamps the voltage level for the transistor. This circuit is currently on the AC Supply board.



 

5. The Step Generator

The Step Generator PCB consists of two parts, the Digital to Analog (D2A) circuit with the step generation and second the buffered output amplifier that drives the DUT.

The D2A section

 

The left side of this board above is the D2A section, the right hand side the Triangle Waveform Generator we already discussed. The knobs you see do not belong to these circuits.
 
Below is the Digital to Analog part.
 
 

The optical isolator in the top left of the diagram is where a pulse comes in that is generated by the triangle wave form generator on the right hand side of the board. This circuit, on the left hand side of the board has floating power supplies that are separate from the rest. That's why we need an opto-isolator to continue to create the electrical separation. 
 
The edges of the pulse are aligned with the triangle transitions. The signal goes through two gates and enters a CMOS 4040 counter. The counter outputs a count for every transition of the trigger pulse. The resistor ladder network at the output sums the counter outputs and thereby creates the stepped staircase waveform, which is the basis for the Step Generator. The output of the resistor ladder network is buffered by an Opamp to preserve the equality of the steps, while the gain can be set by a trimmer to calibrate the individual step size. Another Opamp creates an inverted signal and together they are used to activate N- or P type DUT's.


The output of the 4040 counter is also going to a CMOS BCD-decimal counter that is used to set the number of steps per cycle. The output resets the 4040 counter, starting a new cycle. The selection of 0 through 7 steps is done through a rotary switch located on the front panel. The 0 step setting is useful for leakage measurements.

To control the thermal heat that can develop in a DUT during a measurement, a delay circuit around a 555 timer is used and driven by a potentiometer with a switch on the front panel and is used to turn the delay feature on and make the delay variable. This function sets a delay between complete step cycles, which can be from 1 to 7 steps. See a more detailed description below.

The Buffered Step Gen Output Section

 



 
 
 
The input for this section comes from the previous circuit, the D2A section. The D2A section provides positive for NPN or negative for PNP stepped wave forms of zero to 7 steps and go to a buffered output section. The switch that is located on the front panel selects the polarity. This input signal is summed by an Opamp together with the Volt output feed-back signal, coming from an Opamp buffer.
The output goes to another Opamp that is used to create a positive or negative offset to the stepped wave form. The offset controls are on the front panel. The output goes to a complimentary transistor buffer. The output from this amplifier goes through a resistor, selected by a 16 position rotary switch that sets the output current or output voltage going to the DUT Base or Gate.
 
In the case of a current, there is a feed-back loop to the Opamp that also deals with the offset. The feed-back is needed to keep the output to the DUT linear and compensates for the junction voltage drop or drops in case of Darlington configurations and the likes. A similar feed-back loop is needed in the voltage mode to create steps of the selected voltage size. The Voltage mode is selected by placing a 1K resistor from the output to ground. This will also create uniform range settings between the current and voltage mode settings making the front panel selection very easy and straightforward.
 

Protecting the Step Generator

The above diagram shows the latest version of all the protection circuits we added, which is needed when the DUT power supply voltage accidentally connects to the DUT Base or Gate connection, potentially injecting up to 200V into this circuit, and destroying it.
 
In this version, we used two optical isolation circuits that fire when voltages beyond the +/- 15 Volt supply rails are detected. The optical isolation is needed to keep the Step Gen circuit floating from the rest of the CT. This fault signal goes to the DUT power supply and is used to "strangle" the triangle or DC signal to the input of the supply, removing the output and therefore protecting the Step Gen and DUT from harm.

In the post about the Version 2 CT experiences, I already described the massacre that happened when there was a major catastrophe with the DUT power supply. This event showed that the Step Gen was not protected from the high voltages that could make their way into the Base/Gate circuit and cause havoc in the Step Gen circuits

Opamps have a really hard time dealing with voltages on the inputs that are greater than the supply voltages. In our case, they are +/- 15V, while the Collector supply can be as high as 200V.

If you realize that there is only a single N or P-junction of maybe a few microns separating the Collector from the Base on a DUT die, its easy to see that this can go horribly wrong. If you blow the Collector-Base junction, you have a serious problem. When I examined the 2N3904 or 2N3906 transistors that I blew up, there were several that suffered from a damaged C-B junction for the NPN or a damaged E-B junction for the PNP. In those cases, that resulted in a low junction resistance, putting the full Collector voltage through the Base back into the Step Gen output and blowing-up parts.

The protection we already added as a modification by using a 100K series resistor and clamping diodes to the rails will help to protect the voltage feed-back Opamp, but that still leaves the rest open for destruction.

I looked for days and studied other CT designs and looked for possible protection circuits for high voltage protection for Opamp inputs and did not find any protection methods for voltages over say 40V that could be used in our application.

Not knowing how to go further, I called in the help from Bud again, and after some brain-storming and long days, he eventually came-up with a clever circuit that first used MOSFET switches to disconnect the Step Gen output section from harms way when the voltages go beyond the supply rails.

While we were working with that version, Bud found out by using LTspice that there is still an issue when the output of the Collector supply gets shorted. This can easily happen at the DUT connector, when the Collector gets shorted to the Emitter somehow. This can be caused by a pilot error, or a blown DUT.
 
He came-up with some more measures that will add even better protection to the supply. At the same time, we found that we needed to improve the protection for the Step Generator Buffer circuit due to the changes that were introduced by a change in the offset functionality. There is an additional Opamp that we now need to protect.
 
After a lot of trying things out with LTspice, and discussing and trying different methods, Bud came-up with -in hindsight- a quite simple method to add much better protection, at the same time eliminating a number of parts. Instead of protecting the output of the Step Gen to voltages up to 200V coming from the DUT power supply, we now simply clamp the input to that supply and remove the output when a problem is detected. 
 
The Step Gen circuitry still sees the 200V in case of a short, but only for a very short time, until the protection circuit gets activated and is eliminating the output. The circuits must still be able to withstand the 200V, but aren't required to withstand high power dissipation. The protection is active, meaning that as long as the fault is there, it is active. When the fault is gone, everything goes back to normal. No manual reset required, and no fuses to replace. A fault indicator LED will be added to the front panel to warn the user.

Because both supplies are on different and floating rail supplies, the detection signals that need to drive the DUT power supply need to be isolated with optical-isolators.
 
Below is a rudimentary schematic showing the output protection method for the Step Generator circuitry.
 

 

The output of the Step Gen Buffer circuit, also going to the Base/Gate of the DUT, comes in on the left hand side of the diagram. Diodes D1, D2 and the two opto-coupler diodes work together to create a fault signal. When the Base/Gate voltage is going beyond one of the +/-15V supply rails, actually at +/-18V,  one of the opto-couplers will fire and turn on the dual transistors configured as an SCR. The SCR flips and will turn on both MOSFET's. Q3 will strangle the input to the DUT power supply and completely remove the output voltage. Q4 is used to turn on a fault indicator on the front panel so the user is alerted.

The R/C set by C1/R9 will release the SCR after about 16mS. When the fault is no longer there normal operation continues otherwise the output remains clamped.

After I got a working board, I collected enough courage to test the safety feature. I used my lab supply to inject a DC voltage with a low current to the output of the Step Gen, and slowly increased the voltage. At +/- 18V the safety circuit cut in, and eliminated the triangle signal going to the MOSFET output section, reducing it to zero and lit the warning LED. Success!
 
Now I need to find enough courage to really put this circuit to the test by applying the full 200V triangle waveform the the Base connector of the DUT. I also need to do some short tests on the 35, 75 and 200V volt ranges. Initially, I avoided doing these tests in fear of blowing something up that I didn't have the replacement parts for in case something went wrong.

Step Gen protection Test

 
I finally found the courage to do the real Step Gen protection test. To make the test, I added a small jumper wire between the C-B connections on the DUT test socket. I monitored the X-output with my DSO and used the CH2 connected the the Y-channel, or monitored the Fault signal with a 10x probe.
 
I first selected the 35V range, and selected a few volts for the output so I could see it on the scope. I also selected the X1 current range, but set the CL to minimum.
I first tried the Voltage mode because this has a different feed-back loop. I slowly increased the CL setting so I had a signal and increased it further to just before the Fault kicked in. 
 
 

The triangle is at 17.6V, just below the tripping point. The Y-amp shows the Base voltage.
 
I then increased the sweep voltage until the fault signal came on. This was at 18.4Vp-p. Further increasing the voltage only changed the shape of the output, but not the voltage.
 

 
Switching to the 75V range or as the acid test, the 200V range and turning up the voltage to the maximum 200V did not change the situation. The output and feed-back loops of the Step Gen are now fully protected.
 
I also used a 10x probe to look at the Fault signal in relation to the sweep voltage and this is the result:
 

 
As soon as the triangle waveform goes beyond 18V, the protection kicks-in and that turns off the triangle feed to the DUT supply voltage output. It keeps the signal off for a little while, and then frees it again.

In the Current mode it looks a bit different, but works the same.


There is no Base signal displayed in the current mode. The sweep voltage is again clamped at 18V.
This protection circuit works really well. Kudos to Bud!


Extending the Step Gen attenuation ranges

It had always been difficult if not impossible to measure high gain devices like Darlington transistors with the previous versions of the CT. The lowest setting of 1uA/step was still far too high, so we added 4 more settings by switching from a 12 to a 16 position rotary switch. The added settings will be for 500, 200, 100 and 50nA/Step.

 

New offset circuit

Mark was not very impressed with the way I implemented the offset feature for FET's. I simply used the same circuit for both BJT's and FET's, but that's not ideal. The trouble is that with a normal step voltage for FET's, almost any offset will drive the output into the supply rail. You can circumvent the problem a bit by lowering the internal step voltage, reduce the number of steps or increase the supply voltages, but that's still a work around and not a very good solution.

Mark figured out a way to create a dedicated offset circuit for FET's. The BJT version will stay the way it is. The user will not even know about this, because the switching from one offset circuit to the next will be accomplished by the same BJT to FET selection switch already on the front panel. We just changed it from a DPST to a DPDT version to activate either circuit.

 

Dealing with thermal issues

When you're testing devices with higher currents, there are two effects you have to keep in mind. One is that the self-heating of the device while you are testing, can distort the I/V display because of an effect called looping. This is caused by the way the DUT is activated. With the single step level at the Base, the Collector is getting a raising voltage due to the triangle based supply. The higher the voltage becomes, the higher the thermal heat will be. This typically results in a gain change, so the curve will bend up a little. If the triangle voltage now goes down during the same step, the heat dissipation gets less and the gain changes again so the curve will bend down a little. This causes the typical elliptical looping of the traces. Below is an example of a very minor case on a Tektronix CT.


The thermal heat of the DUT die can increase very rapidly and can get very hot, so much so, that you have to stop the test to let it cool off again. If you don't notice it in time, your DUT may have been damaged or even died of a heat stroke already. With high currents needed for power devices, you can't even run the test for more than a few seconds and the looping of the trace can become so big that it will be impossible to interpret.

There is a way to cheat however. When you select only one half of the waveform per step, as you can do with a triangle waveform, you can eliminate the looping effect. This is what we do now standard with this version 3. When you use a half sine-wave, as the Tek 576/7 do, that trick is not possible. Look at the first Blog of building the CT project to see a more elaborate description.

Professional Curve Tracers allow you to use a pulsed step mode, by which there is a pause after each complete step cycle. This gives the DUT some time to cool down before the next step cycle arrives.

I implemented a similar functionality for Version 3 by adding a few components to the Step Gen circuit. With a potentiometer adjustment, it adds several Milli seconds of delay between step cycles. The delay is synchronized with the end of the step cycle, and can be applied for every number of steps from 1..7.

Below are two screen shots that show this feature in operation on a prototype. The first picture shows the normal operation but shown in the time-base mode of the DSO.
The second picture shows the delayed step function. It starts with about 40mS delay between the step cycles. This can be extended to about 150-200 mS by using a potentiometer on the front panel. This is the practical maximum because the trace starts to "flicker" due to the pause between the X-axis acquisitions. The display can be adjusted somewhat by changing the time/div. setting of the DSO to make it as smooth as possible.


 

6. The X-Y Output Amplifier and DUT circuit



 
 
This circuit contains the X-Y amplifier section together with the select-able IC current shunt and also the DUT test section. The resistors with their heat sinks are the two current shunts. In the final version, there will be single resistors. 
 
The ZIF socket is used to select a number of different DUT pin-outs. It will will not be on the front panel of the final instrument. The diode you see on the left is there due to an omission on the schematic. It is a polarity protection for the DUT supply output voltage in case a user connects something that has a residual voltage.
 
 


This circuit has the X and Y amplifiers to feed the Collector/Drain voltage and Collector current to the DSO/CRT. The Ic current is measured across a shunt resistor that can have two values to provide a 10x gain. 
 
The horizontal amplifier has a gain adjustment such that the Collector/Drain voltage can be calibrated against the DSO/CRT V/Div settings, such that you have a 1:1 ratio.
 
The rest of this circuit deals with the rather complex switching of the DUT polarity and also between the two DUT sockets that makes a comparison between two devices easily possible. There will be two test sockets for DUT's on the front panel that will allow quick insertions for different pin configurations. There will also be two sets of 2 mm banana jacks that can be used with flying leads and grabbers, or they can connect to a separate PCB with SMD pads, or for other type sockets, like a ZIF socket as we used during the testing phase.

Dealing with the XY display noise level

Because I'm using a relatively inexpensive DSO, a Rigol DS2072A, most of the X-axis displays for small signal transistor currents are very noisy on my DSO because I have to use V/Div. settings that are in the mV area and they show a lot of noise. It's not so much the DSO itself that is noisy, but the combination of the DSO input circuitry and the pick-up of noise makes the traces very fuzzy.

Both Mark and Richard use professional scopes and they don't have this issue.  There are two solutions. We can add another Opamp with higher gain, but that will also amplify the noise from the source. The other solution is to use a higher value shunt. Both solutions will allow you to avoid the lower level Volts/Division settings of the DSO. 

I decided to use two different Ic shunt resistors, because using a single 1 Ohm resistor does not make sense. (pun intended)

By adding a toggle switch to the front panel, the user can now select a 1 Ohm and 10 Ohm shunt resistor values, in effect multiplying the IC current by x1 and x10. This will allow you to use V/Div. settings that are a factor ten higher and therefore more free of noise and we will have a better sensitivity at lower currents as well.

Here is a measurement taken with the 1 Ohm shunt, and a 2mV/Div. setting on my Rigol.

Here is the same measurement, but now with the 10 Ohm shunt, allowing you to go to a 20mV/Div. setting.


Note that this was made with the "dual triangle sweep" per step. Also, the small "opening" in the trace for the second step is caused by the period the DSO needs to process the acquisition of the collected data. This "hole" only shows when you make a screenshot. Look at the first post of building a CT for more information.
 
UPDATE
When testing the breakdown voltage (VbrCE0) of a MJL3281A power transistor, I forgot to select the X1 position, and left the 10R as the current shunt. The VbrCE0 caused such a violent current surge that it blew the resistor in two parts and unsoldered one half of it from the board.  That's something we wanted to avoid from happening, so we decided to no longer switch the current sense resistors, but to change the gain of the X-amp. The downside is that this will also amplify the noise we're trying to get rid of, but it is a lot safer.


Small offset on the Y-axis

Richard has found that on his Version 1 CT there is a slight offset of the X-Y picture. This is probably caused by part tolerances. He fixed it by shifting the output of the X-Opamp a little with a high value resistor to a rail. We've added a trimmer circuit that solves the problem by adding a small pos or neg offset calibration to the Opamp. 

 In a later revision, we have modified the offset circuit for the Y-amp, removed the diodes and reduced the voltage swing.

 

We have a fully functioning Version 3 Curve Tracer 

 

 

This is a picture with the latest revision 10 x 10 boards with the last modifications, resulting in a design freeze. 

Here is what you see on the picture:

The red board top left with the transformer on it is the Auxiliary Supply.

To the right of it is the AC supply with the green color and with the very large main capacitor.

Above that is the main AC supply transformer mounted on the back-panel of the enclosure with the main socket and filter, a mains fuse and mains switch. To the right of the AC supply is the 120VAC transformer used to create the 200V. The plastic back panel will be replaced by a PCB.

On the top right is the red board for the DUT power supply. It has the large heat-sinks on it where the MOSFET's are mounted on. The two potmeters are the Voltage selection and the Current setting. The current ranges are selected by a jumper. The switch you see on the lower corner is a prototype of the DC mode selection.

In the middle you see a blue board that is the Triangle Generator and D2A section with the Step Selection. The number of steps is selected by a wire jumper.

To the lower left in black is the Y-Y amplifier board with the DUT section. There are two small coax cables going to my DSO.

The white board in the middle is the Step Gen Buffer output board. The potmeter to the left of it is for the Offset adjustment. Mark had an idea to use a divider jumper to reduce the Step outputs so we could use less expensive rotary switches.  With this 10x10, we can use a simple jumper on a row of pins to select the output settings.  Unfortunately, Mark's idea did not work in reality, so in order to add the 8 lowest settings, I used a small test board with THT resistors and a 16 position rotary switch instead of the jumper selection to create the full range. It is not pretty, but functionally does the job although, not surprisingly,  there is a lot of hum and noise in the lowest uV/nA settings. We will not make another revision for this board but will go straight to the Front Board design.

 

First measurements to test the functionality

Below is a measurement of a 2N3904 as an example (20uA/Step) using the single slope triangle method.

First is the traditional Time-Base picture of the Collector voltage and the Base current and below it the I/V plot in the X-Y mode. The Step is not flat because the Collector voltage will change the gain, and hence the current. You can see that with higher steps (current) the flatness changes quite dramatically. This shows up in the X-Y plot with upwards going slopes at higher Base currents (the Early effect). This is quite typical for a 2N3904. There are other transistor types where the lines stay horizontally flat for every step, meaning that the gain or beta performance is more uniform across the Base current or Collector spectrum. 

You need a Curve Tracer to see this effect.



Also note how much cleaner this X-Y plot is with the single slope method compared to the Version 1b and also the Version 2.

We identified a few issues that we were able to resolve or fix already and are working on a few things we need to try, possibly involving yet another board turn, but the CT is now fully functional and a lot of testing and profiling still lies ahead of us.


The BJT dV/dt problem

While profiling the CT, we stumbled on a rather strange phenomena that we're trying to understand and see if we can explain it, or better yet, design it into oblivion.

This phenomena shows up at very low Base currents for BJT devices, and also with higher Collector voltages and also with higher triangle frequencies. For reference, look at the screen shot above made with a 20uA/Step and a 12V Collector voltage. When you reduce the Base current to lower levels, we see a change in the Step function. There is a sudden drop when the Collector voltage (the triangle) changes direction. This drop in the current creates a double line display for every step. Below is the Time-Base picture to show the situation at 1uA/Step. Below that is the X-Y plot with the resulting double lines due to this drop.



 
BTW, the "funny" transition of the first step in the lower left corner is due to the turn-on point of the B-E junction (at around 600mV) and that is always there on the first step, seen with low step currents. Look again at the Time-Base picture. Note that the very first edge of the first step in the cycle, is a little bit to the right of the start of the triangle. When the triangle voltage goes beyond the junction voltage, the device turns on, but that is with a bit of a delay due to the triangle shape, and that's what you see in the X-Y plot. That's not the issue at stake though.
 
With the help from Bud we now understand that we seem to have introduced an extra stray or "rig capacitance" that is introduced in the C-B junction, which changes the dV/dt constant and creates a charging/discharging phenomenon at the turning point of the Collector voltage, causing the transition. 
 
The size of the the drop is related somewhat to the triangle frequency (dV/dt : the "t" makes it timing related) but even more to the C-B capacitance. Every BJT device will have some capacity, called the Miller capacitance, but our circuits may add about 10pF in addition, which Bud's simulations in LTspice proved. We're now trying to find the source of this "rig capacitance" and may have to go to yet another board turn to see if we can eliminate it.
 
When I started to test more DUT variations, I ran into related issues that are most likely caused by the long wiring and long interconnects with the current setup, and the modifications we had to make to get all steps functioning.

I'm pretty certain this is all caused by our current setup.

To continue to test and profile while trying to minimize this effect, I lowered the base frequency to about 155Hz. My DSO display is still very nice at this repetition rate so no harm is done, although it may be different on an analog scope. The new 555-based triangle generator has a calibration for the frequency that can now be easily set from about 140 to 650 Hz.

 

UPDATE

I did the same test as above with the new Front board and Face plate, and the problem is now completely gone with a frequency around about 200Hz. However it still comes back with higher frequencies (at 1uA/35V) so we're not fully out of the woods yet. More will need to be tested when we have the final instrument in the enclosure.




Moving forward again

 
Mark and I made the decision to not go to more separate 10x10 board turns again, but to go straight to the layout for the face plate and front-panel PCB's we will need eventually. The front board will combine the Step Gen buffer section, the DUT section and the X-Y amplifier section. When we do that we can try to eliminate the long connections and avoid adding parasitic/stray capacitance as much as possible.

 

The Face Plate (front panel)

I had made a face-plate design earlier that we will now start to follow. It is hopefully the final design. (it is not) 

We call this instrument the VBA Curve Tracer. VBA stands for the initials of our surnames. The face plate will be mounted just in front of the front-panel PCB, and it will be a PCB by itself using black as the solder mask color and with white silkscreen for the text.

The screenshot below was taken from the KiCad 3-D viewer. KiCad was used to create the face plate, starting with Fusion 360 to create the outline and Inkscape with a special add-in to create the dials for the silkscreen. I used the process as described in this Youtube post : Create a Face Plate 

Since then we switched to Altium for the Face plate, like we do for all schematics and layouts. The plated mounting holes serve as a connection to the also plated back side and will reinforce the board and also act as a shield for the Front Board that will be sandwiched at the back where many of the components are mounted on. All the plated holes have the metal switches in them and they will be connected to earth ground for shielding and safety. The potentiometer for the voltage adjustment is specially selected because although it has a metal fastening part, the rest is plastic. It can carry voltages up to 200V so we want to be careful.

The four reversed mounted SMD LED's and their driving circuitry will be on the back of the Face plate. 

In the meantime, we decided to rename the "Current Limit" adjustment simply "Current" because of the ambiguity it can cause with the percentage settings. The LED is still a CL warning.



The lower six holes with the E/S, B/G and C/D names will be for 2mm banana jacks that can be used with flying leads, or with a selection of PCB boards with different sockets for all sorts of DUT's, including one for a ZIF socket that we already use now. We'll leave it to others to design these boards for different DUT's.
 
The rectangle holes will be filled with two test adapters that are mounted on the Front board. They are available through AliExpress and sell as a set of two for $16.
 
 
The reason I selected 5-pin sockets is that you can wire them for a variation of DUT pin-outs as indicated on the Face Plate without bending the leads.



DUT Breakdown/punch-through effect issue

I have found that during a VbrCEO breakdown test or an ICEO leakage test, the breakdown or even punch-through of the DUT causes a nasty glitch on the Step Gen supply rails. The reason is that in the NPN-mode, the Emitter of the DUT is connected to the isolated GND of the Step Gen, and this is connected to the negative Collector Supply. The very fast glitches that are caused by the breakdown event are so severe that they can be found in many places. 
 
These glitches also trip the sensitive Step Gen protection circuit and turn on the Fault condition. The Fault detection switches off the Collector supply for a few Milli-seconds and then turns it back on.
 

The top trace shows the glitches on the isolated GND of the Step Gen, and are also visible on the +/-15V rails. Note that the CH1 scale is incorrect, it should be 1V/div. (wrong multiplier) The glitches turn on the Fault detection system, shown here in the bottom trace.
 
Initially, we wanted to see if we could avoid the Fault circuit tripping. We tried a few things but that would only compromise the protection functionality. We decided to leave it as is since it still shows the breakdown voltage, but limits the possibility of damage to the DUT.


This shows the VbrCE0 of a 2N3904 at about 55V. The breakdown also fires the protection circuit which forces the voltage back to zero for 16mS, then releases it again. The Collector current is about 80mA before it is limited.

With that revision, we will also put the complete protection circuit on the Front board and remove it from the Collector supply to keep connections as short as possible.

 

 

Putting it all together

After working with the 10x10 boards and incorporating the Face plate and the Front board, we were able to fix the last changes and revisions.

It's time to pull it all together and show you the final schematics and layouts. A few component values may still change, but we hope to have all the final changes incorporated.

We will have 4 boards that will comprise the CT, and I will only describe the differences with the circuits already explained in detail above.

 

1. The Face Plate or Front Panel.

The Face plate or Front panel is the user interface for the CT, and it houses all the control elements for the instrument.

 


We've made a number of changes to the position of the silkscreen, moved some controls a bit and removed one alignment hole for the Offset potmeter that was no longer needed. We also added rounded corners so the board will fit better in the slots of the enclosure.

 

Below are the circuits on the back of the Face plate. Basically the circuits to drive the LED's that are reverse mounted. There were some value changes but apart from that, they are basically the same circuits as described earlier.



We changed the interconnect wire terminal types from the previous revision, they were too hard to use with small wires that were not solid core.


2. The Front Board.

This is the board where all the circuits for the controls are located on, and it interfaces directly with the Face Plate and the Main board. 

 

The Current & Voltage range setting circuits

 

 

 

The Digital to Analog circuit


The Step Gen Buffer circuit


The Fault Protection circuit

The only change from the previous revision is that we took the Fault circuit components from the Main board and moved them to the Front board. It is the better place here and reduces the number of inter-connecting wires.


The XY amplifier circuits 

We've made two related changes from the previous revision. One was to eliminate the IC current sense 10R resistor (X10 setting) and made a gain change for the amplifier to have the same effect. The reason was that there is a chance to blow-up the 10R sense resistor if you forget to select the X1 setting when you are dealing with high currents. Forgetting that with the gain change is no longer destructive.
We also went from a single 1R current sense resistor to 4 in parallel to reduce the thermal stress at high currents.

 

The PCB layout

 
Here are two 3D impressions of the Front Board with all the component locations.
First the side that faces the Face plate, and below it the side that faces the Main board.




 We changed all but one of the interconnect wire terminal blocks and improved the circuit ground connection between the Front board and the Face plate.



3. The Main Board.

This is where the remainder of the circuits are living.

 AC Power Supply

Triangle Power Supply




Isolated Power Supply





XY Amp Power Supply





DUT Power Supply

Current Source




The PCB Layout

The large cut-out is where the main transformer will be located and fastened to the bottom of the enclosure.
 
Here are two impression from a 3D viewer.
 







 

4. The Back Panel.

The back panel mounts the power switch, the C8 power entry with filter, and the three XYZ BNC outputs to connect the instrument to the CRT or DSO.
 
This is just a Gerber viewer rendering.



Other files will come soon...



Stay tuned for more information and updates...