Electronic Load Project - TLV171 & IRFP250 with KiCad Files

[Kleinstein]

It is possible the capacitor for compensation at the OPs are a little too small. temporarily adding an extra capacitor (even if rather large, like 10 nF) would be a first try.  Just in case one should check of the set signal send to the last OP is stable. This is the usually the simpler part of the circuit, but it is also easy to check.

[t1d]

Hi, Kleinstein!
Thanks! I will give your bodge a try and check it with my new scope, a Siglent SDS1104X-E, which is on its way... Woot!

[t1d]

It is possible the capacitor for compensation at the OPs are a little too small. temporarily adding an extra capacitor (even if rather large, like 10 nF) would be a first try.  Just in case one should check of the set signal send to the last OP is stable. This is the usually the simpler part of the circuit, but it is also easy to check.

I thought about this too hard and confused my noob self... lol... Are we talking about the op amp decoupling caps on the power rails?

[Kleinstein]

The decoupling caps are already large enough, it is about C2 that might need to be a little larger.

[t1d]

The decoupling caps are already large enough, it is about C2 that might need to be a little larger.

Thanks. Glad I asked. I will give it a try.

[t1d]

I changed C2 to 10nF, as Kleinstein suggested, and re-tested the board, under duplicate conditions. All results remained the same, with the exception that the amplitude of the asymmetrical wave form was reduced by half = 0.15V. The maximum the rig is sinking continues to be around 2.32A/Uncalibrated. What to do next?

Thank you, to everyone, for your continued help and support.

[Kleinstein]

If the frequency of the oscillation does not change much with a changes compensation, this would indicate a source outside the loop. So is the supply and the set voltage (e.g. at the pot) clean ?

With some sources there is also the possibility that the voltage source is the culprit:
Some voltage regulators do not like a load that looks like a low ESR capacitor in combination with a current sink.
The simple current sink circuit has an impedance that is a little like a capacitor, possibly low loss, though usually too small to cause trouble.
Normally the RC element at the output already takes care of the problem by adding loss in the critical frequency range.

For a test one would try with an additional (e.g. external) RC parallel to the load. Something like 2-5 Ohms and 100 µF.

[mk_]

What to do next?

Remove D2 too, there is no reason for this diode in testing-mode.

Waiting for your oscilloscop which allows screenshot and then teaching how to use it...
Waiting for a schematic and(!) picture of your setup...

IIRC you use an active charger as powersource. I assume that this source isn`t used to handle a dynamic load - and therefore the whole system can oscillate. So I would use a normal powersupply for first impressions.


 

[t1d]

Thanks, Kleinstein and mk, for your posts. Today is not a great health day; I will reply to your suggestions asap.

It came to me in my sleep that the DUT/Load battery charger is switch mode. Maybe that explains the ac elements. I will have to think on its effects and put the scope on it... (I would want the circuit to be able to handle such a DUT/Load type.) Do I need to lift the DUT/charger's earth ground, for safety?

[Kleinstein]

The ground (PE) connection is there for safety reasons. If the scope and part of the DUT (e.g. the supply) is grounded there is a chance so get some extra signal pickup between the ground points. One also needs to be careful where to put the scopes ground. The connection should be only where there is ground already, other wise there can be ground current flowing, with a powerful supply possible even damaging.

With 2 grounded parts (e.g. scope and supply) both should be powered from the same outlet. It can also help a little to have a ferrite (e.g. clip on) around the power cables.

Lifting a ground connection can be dangerous. Depending one the circuit one might need to use a isolation transformer for the DUT instead. Even with the transformer there are residual dangers from high voltage. The Transformer helps against some dangers, but also adds new, as it can reduce the protection by an GFI.

Unless one really knows what one is doing and with a special isolated scope (and probes) one should never use the isolation transformer for the scope.

The test-point at the MOSFETs gate can show some effect of Ripple of the source, even if perfectly working. So it is not a very good test point. The voltage at the shunt would be a better test point - this should be constant. One should also check the source voltage.  An switched mode charger can have pretty high ripple - so some AC can be normal.

[t1d]

The new scope arrived. I used it to clarify the recurring wave form that I had noted earlier.
- Probe @ x10
- Pot set to 0 (zero)
- DUT is still SMPS Battery Charger
- Measured at MOSFET Input Pin... I will look at other locations later. This test was to replicate the earlier test settings.

The readings are posted within the screenshot.

Looks like 60Hz Mains hum, to me. I guess the questions is "Is it acceptable and, if not, what to do to correct it?" We already increased the cap to 10nF.

I thought the group might find this (a little) interesting.

[mk_]

The new scope arrived. I used it to clarify the recurring wave form that I had noted earlier.
- Probe @ x10

Well, I don`t know Siglents, but if I read the screenshot correct it assumes that you have a 1:1 probe connected, which is somehow different to your 1:10 saying. You also should set the coursors in the averaged Signal as I assume that the hf-noise is not a real signal at the Gate. dY ist therefore 4mV, which is - with a 1:10 probe 40mV...

Anyway, I don`t see any problem as long as the current is stable. So - please show also voltage over the shunt. Take care with GND for the probes. Best will be you use the Source as GND for the FET _and_ the shunt, so you have to invert the shunt-signal on the oscilloscope.

good luck

[Kleinstein]

The Signal may just reflect the output of the charger. So this should be tested first.

Near zero current setting the load circuit is kind of in saturation and might get out due to hum from time to time. This causes a large signal at the gate, but with not much consequence to the load.

The cheap scopes usually have no automatic probe detection. So one would normally enter if a x 1 or x 10 probe is used. The scope will than include the factor in the scaling.

[t1d]

I finally found the x10 probe setting. I will probe the suggested additional points, as soon as I feel better. Thanks for your continued help and your patience.

[mk_]

I finally found the x10 probe setting.

I knew that we have to teach you using it. 

Anyway - most important is measuring the voltage "over" R9 so that you can see if the current is stable or hums also.
And it would be also nice to show the voltage J2 so that you can see how the inputvoltage looks like.

good luck

[t1d]

x10 @ MOSFET Input Pin

[t1d]

x10 @/across Wall Wart Input Plug

[t1d]

x10 @ DUT Input @ 0.5A

[t1d]

x10 @ Current Shunt Resistor to GND @ 0.0A

[t1d]

x10 @ Current Shunt Resistor to GND @ 0.5A

[t1d]

x10 @ Current Shunt Resistor to GND @ 2.325A

[t1d]

x10 Across Current Shunt Resistor @ 0.5A.
This one looked like Post #117. I shifted it left, to justify it.
Lots of random flashing spikes with greater than, or equal to, amplitude to the maximum ring amplitude. Not shown.

[t1d]

Notice the 50KHz spikes, on the shunt. Those don't look good. Shouldn't I look upstream to find the original source? Op amp outputs and inputs?

Of course, being new to the scope, I am not very confident of these readings. Some look so similar that I may have duplicated them, by mistake. And, I may have shifted some of the readings, to make the X/Y Calculation Box legible. Whatever looks worthy of further investigation, I will simply retest.

Thanks, guys.

[mk_]

Notice the 50KHz spikes, on the shunt. Those don't look good. Shouldn't I look upstream to find the original source? Op amp outputs and inputs?

Of course, being new to the scope, I am not very confident of these readings. Some look so similar that I may have duplicated them, by mistake. And, I may have shifted some of the readings, to make the X/Y Calculation Box legible. Whatever looks worthy of further investigation, I will simply retest.

Thanks, guys.

SDS00008.png shows a periodic signal based on a PWM nearby - maybe a switching powersupply nearby which let me assume that this is not a real signal.

Maybe you could send pictures how you measured. I assume that you didn`t measure with the spring at the probe. Instead you measured with the long GND-Clip, which resumes in a lot of hf-pickup, specialy at low voltages. Search net for "how to probe low voltage signals".

btw: would be nice if you could switch off the unused channels.

And please set the active channel to one of the horizontal gridlines so that it is easier to calculate the DC-part of the signal. Best would be that you don`t move vertikal if possible, just change Volts/div or time/div so that it is easier to compare different screenshots.

[Kleinstein]

One should definitely look at the voltage from the source, e.g. with some load (e.g. resistor with some power).

The signal from the shunt looks like there is some periodic signal at about 50 kHz (e.g. from the source) and some ringing following that. With just one signal it is very hard to tell where the source of that frequency is. Likely it's the voltage source as an oscillating load circuit would be high amplitude and normally not with dampened ringing in between.