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.
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:
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.
3. The AC Power Supply
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'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.
The DUT supply Voltage Regulator
The Voltage Regulator
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.The Current Regulator/Limiter
The Current Source & Compensation
Output and thermal stress test
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
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 Buffered Step Gen Output Section
Protecting the Step Generator
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.
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!Step Gen protection Test
The triangle is at 17.6V, just below the tripping point. The Y-amp shows the Base voltage.
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
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.
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.
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.
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
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.
DUT Breakdown/punch-through effect issue
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.
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
The PCB layout
3. The Main Board.
The PCB Layout
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.Stay tuned for more information and updates...