Electronic Load Project - TLV171 & IRFP250 with KiCad Files

[Kleinstein]

For the first tests the DUT should be a reasonable clean voltage source. The SMPS type charger is not suitable. Its more like transformer + rectifier and filter cap. The relatively slow 120 Hz ripple would not be so bad. So a voltage regulator is not absolutely needed, but makes it easier.

The waveform of interest should be the stationary signal, at least at first. Ideally this should be rather close to background noise.
A next step than would be the signal when connecting the DUT  (use single capture mode on the scope).

For connecting the probe it should be good enough to use the ground clip, no need to use the spring contact that is need for high frequency.

[not1xor1]

For the first tests the DUT should be a reasonable clean voltage source. The SMPS type charger is not suitable. Its more like transformer + rectifier and filter cap. The relatively slow 120 Hz ripple would not be so bad. So a voltage regulator is not absolutely needed, but makes it easier.

The waveform of interest should be the stationary signal, at least at first. Ideally this should be rather close to background noise.
A next step than would be the signal when connecting the DUT  (use single capture mode on the scope).

For connecting the probe it should be good enough to use the ground clip, no need to use the spring contact that is need for high frequency.

I suggest to do it Jim William's way (page 4) 

[t1d]

I played with the scope, a bit more, to try to learn it. I forgot to print each and every screen; a rookie mistake. But, I did write down the info, for some that I forgot to print.

Setup:
x10/GND Wire made to Meter Output GND/20MHz BW Limiter
@ 1A Load
Still using the original Wall Wart.

Positive V Reg
#33 Input pin – Stop print = 59.52Hz Sawtooth/432.0mV
#35 Output pin - Stop print = 50KHz Ring/76.00mV

Negative V Reg
#37 Input pin – Stop print = 60Hz Sawtooth/276.0mV
#38 Output pin - Run print   Missing Info = 49.75KHz Ring/72.00mV

Anyway, even just playing around, I thought the data might be interesting and tell us more about the original Wall Wart.

Is the amplitude of the mains 60Hz hum acceptable? 432.0/276.0mV? The 50KHz ring is evident on both the positive and negative rail...

Thanks.

[Kleinstein]

The 60 Hz hum level is acceptable for an unregulated source. However it is really odd to see 60 Hz and not 120 Hz.

For the curves, it's not clear what is actually measured. The supply should normally be cleaner than shown, especially with no load.

[t1d]

The 60 Hz hum level is acceptable for an unregulated source. However it is really odd to see 60 Hz and not 120 Hz.

For the curves, it's not clear what is actually measured. The supply should normally be cleaner than shown, especially with no load.

Well, as I said, I was just fooling around. I did mention the load; it was "@ 1Amp."

I'm not sure what you meant by "the  curves." What I was measuring was the input and output of the circuit's positive and negative rail voltage regulators.

Anyway, I am making up a new circuit voltage supply and a new transformer based DC PSU, with rectification and regulation, to use as the Device Under Test. Health issues have me slowed down, but I will get it done.

[t1d]

I am just bumping this thread, until I feel better and can get back to it. If someone wants to do the troubleshooting, and you're in the USA/48, I will send you a free test unit. Just send me a private message...
t1d

[t1d]

I have not worked on this project in a long while. Hopefully, this will be a re-start.

Where I think that we left off…
We were testing the single MOSFET test board. It has a few limitations; 1) the test rig heat sink and fan are not adequate for real burn-in testing, 2) the power supply needs to be improved and 3) the Test PCB layout is not optimal.

Additionally, we were looking at various waves forms…
The wave form on the power supply had a questionable 60Hz frequency. This made it impossible to look at the wave forms on the MFET input pin and across the sink resistor.

And, I did not have a power supply that would push the circuit to its design limit of 60 watts per MFET.

The two MFET board has been completely designed, with a better layout, and is ready to be produced, when testing is completed.

I think that is the essence of things. If I missed anything, please let me know.

Playing, today...
I have two new pieces of test equipment; A Siglent SMD3065X DMM and a SPD3303X-E PSU. Having these new toys inspired me to do some casual testing with the test rig.

I used a transformer-type AC wall wart, to power the DUT. (I think it may be suspect as well, but it would do for playing around.) I set the PSU (DUT) to 30v/2.1a. I was able to run the load up to a full 30v/2a without the DUT PSU going into constant current mode and the MFET did not go into saturation. I did this quickly, for fear of thermal damage to the MFET, due to the heat sink/fan limitations. The case temperature was 90*C and climbing, when I stopped the test. These results are promising, indeed.

I am not sure of how to relate the Operating Junction Temperature (175*C max) to the case reading. The Junction to Case perimeter is 0.7 max. An education, please?

A small scare…
I did have two oscilloscope probes in place; one on the MFET input pin and one one the Sink Resistor input pin. While adjusting the probe placement of the Sink Resistor probe, I encountered a spark. I do not know how it occurred.

I did check both probes and both oscilloscope inputs with the scope’s calibration output. Everything looks as it should be. I also cycled the scope’s power and it came back up just fine. It did not burn the probe point at all. So, I am hoping that I did not do any damage.

Speaking of probes, I have ordered all the parts necessary to make up some high-quality Kelvin probes. So, if they would be helpful for taking any particular readings, please let me know.

Where to go from here...
Build the transformer based power supply and fully test it. Once completed, place it in the project case. Also bodge the test rig into the project case to attach the MFET to the large heat sink. Do some static load testing of the MFET. Do some variable/wave form load testing of the MFET by inputting waveforms through the AUX input.

As I have offered previously, I will provide a complete test unit to anyone in the USA/48 that can do serious/professional testing. I am not trying to make a commercial unit and, as I have done, I will release all of the project information.

So, your thoughts and suggestions as how to proceed would be much appreciated.

[Kleinstein]

The junction temperature is determined by he case temperature plus the temperature difference inside the case. So the temperature should be power divided by junction to case thermal resistance plus the case temperature. However the case temperature should be measured at the metal towards the heat sink, not the plastic body. The plastic surface can be closer to the junction temperature.

Testing to maximum power is usually on of the less critical points. Testing is more about checking that there is no oscillation, even with a slightly difficult power source (with some inductance - like a slightly longer cable to the supply or an added ferrite ring (to one cable).

With a new meter one could also check if the set current is constant. E.g. set the supply to some 5 V (to avoid excessive heat at the load) and check it a 1 A current is really constant over a time of a few minutes. A possible problem here is the shunt getting worm over time and thus changes its value.

[t1d]

Thank you so much, Kleinstein, for your continued support.

I will be sure to do your tests, after I get the power supply straightened out. I am working on that, now.

I have a transformer with appropriate voltage and I am doing some testing to look at its output.

[t1d]

Setups
Oscilloscope: x10—20MHz Filter=On – 5mV/Div – 20mS/Div (60Hz = 16.7mS)
DUT: Simulated by DC PSU = set to switch 0vdc/0a and 30vdc/2a, in and out.
DMM = Set to monitor the E-Load’s current load, but the loads were not recorded. Formal E-Load load regulation studies to are to follow.
New E-Load Power Supply Transformer – A used donor from stock on-hand = 120VAC/14.5VAC @ No Load – Single secondary.

General Arrangement
A fly-wire was soldered to the Positive Rail at the input of C4. Another fly-wire was soldered to the Negative Rail at the input of C1. Circuit neutral/ground was accessed through a previously added fly-wire on the output of C1. The readings were taken from these points.

General Procedure
The E-Load was allowed to run continuously. The load was switched from out to in. Readings were taken in both power states and from both rails. The observations were from the rails to circuit neutral/ground.

AC fluctuations were of the primary interest, but DC fluctuations were checked by simply toggling the coupling switch. No DC readings were observed, at all, for the given scope settings.

Conclusions
Positive Rail – AC No Load – Observed fluctuations were <9mVpp.


Positive Rail – AC Full Load – Observed fluctuations were <15mVpp.


Negative Rail – AC No Load – Observed fluctuations were <9mVpp.


Negative Rail – AC Full Load – Observed fluctuations were <15mVpp.


For both rails, maximum fluctuation occurred in instantaneous spikes and were encountered under full load.  Adding the extremes would indicate that the power supply circuit has a maximum fluctuation across the two rails of =<30mVpp. Given the un-optomized nature of the PCB layout, I would think these results to be good. Additional improvements might be had through PCB layout improvements and additional capacitor considerations.

Note: These results only consider a non-oscillating load. An oscillating load might cause quite different performance.

New Test Equipment
It is wonderfully huge fun to set up all the parts and pieces... proper test equipment, proper connector wires... to just press a button to take professional readings from your own design... to press a button to record the results...  Woot!

Did we achieve proper testing technique? What other tests need to be done?
If we wanted to curb the hair-spikes, how might we tweak the capacitance?
Other thoughts/suggestions?

Thanks, to all, for your help.

[Kleinstein]

The supply for the electronic load looks reasonable clean - a big step forward.
Now comes the really interesting part, is the current also not oscillating and constant.  Possible test points would be across the shunt and at the regulating OPs output.
To test a difficult DUT, one could add some inductance in series (e.g. some chokes or just a ling wire).

[t1d]

The supply for the electronic load looks reasonable clean - a big step forward.

Yes, I was well pleased, myself.

Now comes the really interesting part, is the current also not oscillating and constant.  Possible test points would be across the shunt and at the regulating OPs output.
To test a difficult DUT, one could add some inductance in series (e.g. some chokes or just a ling wire).

I think using an inductor would make for a better test procedure and I do have a supply. What value do you suggest?

I want to make sure that I understand how to set up the test... I am to use my PSU in the same manor as the prior testing, but I am to add an inductor in series with the PSU, between the PSU and the E-Load, on the positive lead... Correct?

Then, I am to take readings with the oscilloscope across the shunt resistor and at the regulating OPs output (which would be the MOSFET's input pin, correct?)

I think I have a better understanding of testing, now... This should not be too difficult to accomplish.

I appreciate your very kind help. I look forward to hearing your reply and continuing the tests.

[Kleinstein]

I would do the test first without an extra inductor. That is the simpler test and more normal use. For this test the DUT supply can be set to lower voltage (e.g. 5 or 10 V) to limit the heat at the MOSFET. Besides with the scope it may be worth checking the shunt voltage with a DMM over some time (warming of the circuit).

For the test with the inductor, it would be in series with the supply, probably slightly better in the positive lead  (the common mode capacitance to ground can make a small difference). Something like 10-100 µH should be OK if it can stand the current. There is slight chance the current sink can start oscillation, though it is designed not too. The case with series inductance is about the worst case, the current sink needs to be stable with.

[t1d]

Thank you, Kleinstein...
I will follow your directions and post my results, soon. But, I have not slept, so I will not attempt it, presently. No sleep and electricity do not mix well.<g> Maybe later in the night... I hope so; I'm excited!

[t1d]

Something like 10-100 µH should be OK if it can stand the current.

I did set up the E-Load test rig... But, I stopped, because your warning about the current has great wisdom and it kept speaking to me... sternly. I do think the current is a significant issue.

The small inductors that I have are the type that look similar to 1/2 watt resistors. Surely, a single one of them will not handle the current, even for the moment needed to take a reading. So, I will have to either combine a great many of them, or wrap my own inductor with large gauge wire.

Making one with large gauge wire would be safest. It appears that 55 turns of solid .8mm wire in 12 inches on a 4" air coil will make 100uH. I have a 4" cardboard tube and stranded lamp-cord wire. I will have to learn how strands will effect the calculations. I don't know anything about using inductors, so there is another learning curve. Building it is a delay for the E-Load project, but it should be a fun exercise.

Any suggestions would be very much appreciated, at this point.

[t1d]

I'm just keeping the thread current... I have built the DIY high-amp inductor. To prove it, I found a small circuit for testing inductors and I breadboarded it. It gave every odd results.

So, I checked with my brother and he gave me instructions for testing inductors with a frequency generator and oscilloscope. Everything is set up on the bench, but I hit a bump when setting the frequency to tune the voltage amplitude of the two test points.

I need to change the resistor to a different value. However, I ran out of steam. I will get back to it, asap... maybe even tonight...

[t1d]

I am fighting a respiratory bug, so all progress has been delayed.

@ Mr. Kleinstein
It occurred to me that I need to order the second set of boards that have the improved layout, at some point. But, the question is when/where to do that in the testing process.

It would be a waste to chase problems on the first board, if they were already corrected by the improved layout of the second board. So, when, in the testing process, should I order the second boards?

I would think that maybe that time is now. The constant current circuit is successfully regulating throughout the designed voltage and current limits. Aren't we just tweaking performance issues?

As a reminder, the second board is a dual-MOSFET board, having two mirror regulating circuits. I think it was last posted on page 2, at post #44. To be sure that the current project is posted, I will make an upload in a separate post. But, not tonight, as I have run out of steam.

I would also like to consider adding a feature that would turn off the circuit, once the voltage of the DUT has dropped to a certain voltage. This would be highly useful for testing battery performance, without discharging the battery beyond its low voltage limit. It occurred to me, because I need to cycle some unused batteries, to freshen them.

I think this will only require adding a few components... Is this as simple as adding a pot? I do not know, as this is beyond my knowledge. But, if it is a simple matter, I would appreciate your instruction of how to do it. Here is where I got the idea:
https://www.hackster.io/gleisonstorto/constant-current-load-with-cut-off-voltage-afe3e9

However, if this is a large endeavor that will significantly delay completion of the present design, I would not want to do it.

Thanks to everyone, for your continued help and support.

[Kleinstein]

If there are real errors, these should be fixed with the 1st board. So the 2 nd version could be ordered with a known working circuit. 

Adding a turn off when the voltage drops to far for a battery or similar could be a nice idea. It would need a little more than a pot to set the limit, but not that much. I would consider a OP as a comparator and NE555 as a kind of latching circuit.
Possibly one could add a counter to measure the time it took. A simple version could be just a binary counter with LEDs.

There are a few features one could add with a 2 nd board:

+ turn down the current when the current sink runs out of regulation. If the voltage drops to far the MOSFET would turn on all the way and could cause a spike if the voltage suddenly goes up again. This could happen with a bad contact to the DUT, or just turning on the DUT later.
 A logical solution would be a cross over to constant resistance mode, with a lower limit so that the regulator does not go out of regulation.

+ possibly an amplifier to read the voltage at the shunt(s) with good precision. This may help especially if one does not have a sensitive  meter that can resolve some 10 µV or 1 µV.

[t1d]

Thanks, Kleinstein. I have been playing with a Low Voltage Cutoff Circuit, this morning. The first attempt failed, but I see my error and it should be easily corrected. Maybe I will post my results on another thread, once completed.

However, I did realize that it would add a good amount to the project, so, I will just finish the original design. Simple enough, but more project creep.

The inductor is finished, but I still have not been able to test it and document its characteristics. I will, as soon as I feel better.

Merry Christmas to everyone.

[t1d]

I would do the test first without an extra inductor. That is the simpler test and more normal use. For this test the DUT supply can be set to lower voltage (e.g. 5 or 10 V) to limit the heat at the MOSFET. Besides with the scope it may be worth checking the shunt voltage with a DMM over some time (warming of the circuit).

@Kleinstein - You will recall that, while testing the the new power supply for the circuit, I was able to momentarily subject the circuit to its designs limits of 30v at 2a and it worked without issues. See Post #159. So, I am ready to do this test.

Please give me an overview of what we want to accomplish and the specifics of what needs to be done... Volts, amps, soak time, test points, etc. Are we just looking at the circuits performance over time and warming?

For the test with the inductor, it would be in series with the supply, probably slightly better in the positive lead  (the common mode capacitance to ground can make a small difference). Something like 10-100 µH should be OK if it can stand the current. There is slight chance the current sink can start oscillation, though it is designed not too. The case with series inductance is about the worst case, the current sink needs to be stable with.

I have finished building the DIY High-Amp Inductor. It should handle the full wattage of the design limits without problem... even for the dual MOSFET version, when it is completed.

The inductor has two coils. The first, at the center tap, measures 38.38uH. The two, in series, measure 151.637uH. I find the none-linear nature of the inductance odd, as the coil was made from a single wire pair, but that will have to be a discussion for a separate thread.

[Kleinstein]

The test with a DMM at the shunt is to look for stability of the set voltage. So for drift in the OPs etc., e.g. with warming. If in addition the current is measured one could also check if the shunt changes with temperature.

With inductors it is normal that inductance does not increase linear with the turn. Ideally is should be closer to a square law - not perfect as the coupling between the parts of the coils is not perfect. The test with the inductor is kind of the worst case for stability.

[t1d]

The test with a DMM at the shunt is to look for stability of the set voltage. So for drift in the OPs etc., e.g. with warming. If in addition the current is measured one could also check if the shunt changes with temperature.

May I please have the setup and testing method details. This is really all above my skill level, so the details really help. Thank you for this extra effort.

With inductors it is normal that inductance does not increase linear with the turn. Ideally is should be closer to a square law - not perfect as the coupling between the parts of the coils is not perfect. The test with the inductor is kind of the worst case for stability.

I am glad to know that I likely did not make a mistake when I built it.

[t1d]

I did a soak test, pending Kleinstein’s exact instructions.

Test Initiation
The intent of the test was to monitor the E-Load’s performance at 10v/1a=10w, over time.

Because I am unfamiliar with my oscilloscope, it took me about thirty minutes to find the waveforms and adjust the scope. The E-Load was working during this time. Meaning the E-Load and all the test equipment was fully warmed, when I took the first readings.

Initial Readings
Ambient temp was about 23.8*C.
Time: 2pm
PSU as DUT: set at 9.99Vdc @ 1.4A available. The extra 0.4A on the PSU was allowed to see if the E-Load would drift.
E-Load was set at 0.099945A x 10 (Shunt Factor).  The E-Load does not have provision to limit the voltage, so the voltage was the full voltage available.
MOSFET Face Temp: 36.7*C
Shunt/Sink Resistor Touch Test: Tepid, not hot, or cold.
See PNG2 for waveforms.
Yellow: Probe1 was attached to a flywire soldered to the output of R6, which is tied directly to the input of the MOSFET.
Purple: Probe2 was attached to the input pin of the Shunt/Sink Resistor; R9
Both probe ground leads were tied to circuit ground/neutral.


One Hour Soak Readings
Ambient temp was about 23.8*C.
Time: 3pm
PSU as DUT: set at 9.99Vdc @ 1.4A available. The extra 0.4A on the PSU was allowed to see if the E-Load would drift.
E-Load drifted slightly to 0.099928A x 10 (Shunt Factor).  The E-Load does not have provision to limit the voltage, so the voltage was the full voltage available.
MOSFET Face Temp: 37.0*C
Shunt/Sink Resistor Touch Test: Tepid, not hot, or cold; no discernible change.
See PNG3 for waveforms.
Yellow: Probe1 was attached to a flywire soldered to the output of R6, which is tied directly to the input of the MOSFET.
Purple: Probe2 was attached to the input pin of the Shunt/Sink Resistor; R9
Both probe ground leads were tied to circuit ground/neutral.


Conclusion
At 10v/1a = 10w, the E-Load exhibited minimal drift in the load sunk. 0.99945A – 0.99928A = 0.00017A. 0.00017A/0.99945A = 0.000017% Current Drift.
The MOSFET Face Temperature drifted from 36.7*C to 37.0*C, being a drift of only 37.0 – 36.7 = 0.3. 0.3/36.7 = 11.01%
The Shunt/Sink Resistor performed without labor/heat.
No errant waveforms were encountered.

The minimal current change is considered excellent for a DIY device. The temperature change is attributed to ambient influences and is considered not significant.

I performed the test under my best-guess as to what Kleinstein might instruct. If I did the test mostly correctly, then I am impressed with the results. Pushing the E-Load to its designs limits will be interesting.

[Kleinstein]

There is still some AC part visible in the scope traces. Not very much, but still. This could be something like EMI effects.
The drift measured as the voltage drop over the shunt is only part of the actual drift - it does not include the shunt itself. However it is not that easy to see how much the shunt itself changes with temperature, as one would need a second, better shunt of meter.

The temperature drift is not important. The relevant result from the test is more the temperature rise above ambient of some 13K. This would suggest the heat sink would reach 50 C at about 20 W. So 20 W should be pretty safe even for longer times. And from 40 W on one could expect excessive temperatures.

[t1d]

There is still some AC part visible in the scope traces. Not very much, but still. This could be something like EMI effects.

1) I do very much believe this is just ambient noise. I could turn off the unit and see what is still left on the scope.
2) I did switch the coupling to DC. There was no DC, on either probe.
Do we need to increase the value of the existing capacitors, or add additional ones? What needs to be done?

The temperature drift is not important. The relevant result from the test is more the temperature rise above ambient of some 13K. This would suggest the heat sink would reach 50 C at about 20 W. So 20 W should be pretty safe even for longer times. And from 40 W on one could expect excessive temperatures.

Please remember that the heat sink/cooling for the test rig is wholly inadequate for permanent operations. It is just a ~80mm x 80mm x 2mm aluminum plate with a 60mm 12vdc fan. Even so, in prior testing, I was able to momentarily load the unit to 60w. The face of the MFET went to 90*C and was climbing, so I shut it down, quickly.

The heat sink in the final case is massive and has a 120mm 120vdc computer case fan. I would be glad to install the board in the case, whenever you think best. I had also considered that, if I had the Dual-MOSFET board, now, I could go ahead and install it in the final case. This would make for more accurate testing of the design.

I look forward to your instructions for any hardware changes and the steps for the next test.

I am so grateful for your help. We are making good progress and you are teaching so very much; thank you.