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Saturday, March 20, 2021

Building a Curve Tracer - Version 3

This post will describe the hopefully final version of the Curve Tracer.

Based on the experiences we collected with Version 2, we made some significant changes to make it more reliable, easier to build with commonly available parts, and with greatly increased usability and safety.

Initially, this post will be an unsorted mix of topics that will eventually turn in to a coherent description of the Version 3 Curve Tracer.

 

 Fixing the transformer range switching

The "automatic" range switching that was implemented in my first prototype worked for me, but we've found enough evidence that we could not continue with this method. The original idea was to switch voltage ranges at the DC level, to avoid the in-rush currents. Unfortunately, switching at DC levels, especially using a relays, causes sparks to fly and introduces lethal glitches.. 

We needed another approach to also accommodate a much higher voltage coming from a second transformer.

The solution that I decided to use for our Version 3 CT was to create fixed voltage and current ranges, and not have the unit automatically switch transformer windings while operating the Collector voltage adjustment. This could also inadvertently create higher voltages then intended. With 160V as the goal the adjustment would be too course, and the chances of applying a voltage that is too high is too great. 

The main transformer we decided to use was a center tapped 56VAC at 1A. I wanted at least 2A, so I modified my transformer by de-soldering the middle tap and splitting the two windings. This allowed me to put the two secondary windings in parallel to double the current to 2A with 28V, and when putting the windings back in serial, get the 56V at 1A again. I decided to use a second transformer with 120V at 100mA to bring the output to the 160V level I wanted as the goal.

The best method for the serial and parallel switching and adding the 120V was to switch everything at the AC side, and use one large reservoir capacitor to filter the voltage changes. 

We found small nice AC switches that replaced the relays and because these devices have built-in zero crossing detection, they switch when there is no current drawn. We also found an interesting method to add the 120V transformer into the mix by using another AC switch. 

 


D3, D4, R7, R8, C11 and C12 comprises the circuit to trick the AC-switches into having the required holding current. C5 is the main 2200uF/250V reservoir capacitor.

Mark designed a PCB for this functionality, and we started to test this circuit.


First of all, I found that the toggle circuit to switch the parallel to serial AC-switches didn't work as intended, so I needed to change the switching mechanism.

Unfortunately, both Mark and I experienced failures of the AC switches. This is most likely due to the fact that we switch the large transformer secondary windings from parallel to serial and we probably have to sequence the switching. Due to the change in the way I decided to switch the voltage ranges with a special DPDT on-on-on toggle switch, I had to de-solder one of the AC switches to lift a pin from the PCB. One of the pins broke off during that process, and it turned out that these pins are very flimsy and break off far to easy when they are bent. Although we were initially very happy with this "modern" way of switching solution, it was also a lot more expensive. These 4 AC switches cost more than 20 Euro's. Both relays cost 12 Euro's. The extra additional components to trick the switches into having a holding current also cost some extra components. And then we need to design and add the sequencing circuit. After these failures however, I decided to go the old fashioned but proven and reliable way and turn to relays again.

 

The special on-on-on toggle switch not only switches the relays that switch the transformer windings, but the switch is also used to create 0-40, 0-80 and 0-160V ranges for the Collector Voltage adjustment potentiometer so it always has a full 0-100% scale. The P-MOSFET is needed because the 0-80V range switches both a relays and the voltage range resistor, so they need to be isolated from each other.


Adding Voltage and Current ranges

While we were going through the redesign of the Collector supply controller, I was experimenting with a 120V transformer to see if we could reliably go up the the 160V that I set as the goal for Version 3. Adding that transformer into the mix would cause more switching issues, and I was concerned about the safety aspects with these high voltages. Richard who was continuing to profile Version 2 of the CT was also experiencing problems with the voltage and current settings of his version. He and I continued to blow small signal transistors that we used as the DUT, mostly due to thermal issues when the voltage or current became too high for the poor DUT. This is very easy to do when you try different step settings and the current limit is set too high. With the current configuration, the current limit can be set from zero to 2A and the voltage from zero to 80V both using normal potentiometers. Extending the voltage to 180 would make it even more difficult to adjust. These two adjustments make it too easy to select a higher voltage or higher current the DUT can handle when you are searching for the best step generator setting.

The solution was to create a set of three voltage ranges that can be selected before you start a measurement. This is analog to the way you use the Tek CT's, and that is what people know how to do.

On the front panel, a user can now select the following three ranges: 0-40V at 0-2A, 0-80V at 0-1A and 0-180V at 0-100mA. 

This will be a better solution but the granularity for the current settings especially in the 40 and 80V ranges with 2 or 1 Amp is still too course. To add better control for the maximum current setting,  I also decided to add an attenuation for the current ranges that would give the user the ability to select x1, x.5, x.2, x.1, x.05 and x.02 for each of the tree current ranges, while still be able to further adjust any of these ranges from 0-100% with the Current Limiter adjustment potentiometer.

 

Fixing the Collector Supply

While we were having issues with the Collector Supply stability, I decided to call in the help from my friend Bud, an ex chip designer. He and I worked together remotely as mouse-pal's on a few other projects, most notably on the UPS power supplies for Raspberry Pies and the differential probe, both described in different Blogs. In simulation, it turned out that due to the different Opamps that Mark selected, in combination with the series transistor types, we badly needed different compensation configurations. Over the course of a few weeks, investigating, discussing and trying things out, we eventually arrived at a different design that uses MOSFET's as the series regulator. At this moment, there is almost no stock for high voltage transistors, due to the current part shortages in the car industry where these transistors are used in ignition systems. However, there is an abundance of suitable N-MOSFET types available at reasonably low prices so from a DIY re-build perspective this is much better.

We put together the schematic below, that has several options incorporated we wanted to try and test. In the mean-time we continued development, and so there are a number of things that will change, but at least we can start to test.


 

Mark designed a PCB for this test circuit, and we're currently testing it out at the moment. The leads sticking out from the back are NTC's that will allow me to measure the temperature of the MOSFET's. On the left is the potmeter for the voltage adjustment, and on the right the current limiter.

 


 The board works, but we're experiencing some oscillations that we need to track down and fix.

However, we were anxious to see the results of the new range switching and see if we have fixed the lethal glitches in the changeover switching. Below is the switch from the 0-40V range to the 0-80V range. I drove the 0-40V range just into clipping so you can see the effects better. The 0-40V range has oscillations as you can see, but we have fixed them in the meantime. You can see that there is a very nice hand-over from the parallel windings to the serial windings. Gone are the lethal glitches!


 

 

Protecting the Step Generator

In the post about the Version 2 experiences, I already described the massacre that happened when there was a major catastrophe with the Collector 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

As I already mentioned in that post, 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 180V.

If you realize that there is only a single N or P-junction of a few microns separating the Collector voltage to the Base on a DUT, 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 blown 2N3904 or 2N3906 transistors that I blew up, they all suffered from a blown C-B for the NPN or a blow E-B junction for the PNP. In all cases, that resulted in a short, putting the full Collector voltage back into the Step Gen output.

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 uses MOSFET switches to disconnect the Step Gen output section from harms way when the voltages go beyond the supply rails.


Extending the Step Gen attenuation ranges

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


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 transistors 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 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.


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, 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.


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 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. 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 step selection from 1..7 steps.

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.



Small offset on the Y-axis

Richard has found that on his 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 Opamp a little with a high value resistor to a rail. We're going to see if we can address that problem by using an Opamp that has a built-in offset adjustment. 

 

Stay tuned for more information and updates




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