Electronic Load Project - TLV171 & IRFP250 with KiCad Files

[Kleinstein]

The comments cites do not apply to the single MOSFET version. The problem was with the first 2 MOSFET version.

For the board shown the fuse is rather small  - I am not a fan of those tiny fused.

[t1d]

The comments cites do not apply to the single MOSFET version. The problem was with the first 2 MOSFET version.

I thought so.

I am really excited, to be moving on to the prototype stage; I like building, the most. I will do some hardware testing and post the results. If it goes well, maybe I will build a two, or three, MOSFET model. I would need your help, to make the design corrections, that you mentioned.

For the board shown the fuse is rather small  - I am not a fan of those tiny fused.

I agree, but this is just a model... I don't intend to even put it in a case. So, I cheaped-out, on a fuse holder...

Kleinstein, thank you, so much, for all your support, throughout development. You have taught me tons and I am grateful.

I will likely have several extra test boards, if anyone would like one. Just let me know and we will work it out.

[t1d]

Just an update, to say that I have ordered the boards... Woot!

[t1d]

The test boards are to arrive Wednesday, afternoon. That's just 12 days, to have it produced, by Seeed, and delivered, by DHL.

I have also been working on a Two-MOSFET design. What are everyone's thoughts on it? Particularly, the board layout...

[t1d]

I already noticed that I had not included Kleinstein's larger fuse... So, here's that correction... I am still fiddling with it...

[t1d]

The panels did come, yesterday. They took just 12 days, to arrive at my door. The panels look fantastic. I cut up several, with my PCB Tile Saw.

Today, I rechecked all the passive components' values, again. Then, I assembled a single-MOSFET, e-load board, completely; SMD baking and Through-Hole hand-soldering. It looks great! Everything fit, perfectly. There were not any challenges, in the assembly process. I'm sorry, but I do not have a camera, with which to make pictures, to share.

I'm too tired to test it, tonight, but I will, soon. Woot!

[t1d]

I have played with the e-load, a bit. The setting on the DUT PSU was 1v/1a. I did not receive instant joy, at power up.

All of the power supplies (+9vdc, -9vdc, 2.5vref) look correct, all the way to the op amps. A sine wave, of 4mV amplitude was found, on the oscilloscope, at x10, 20mV/D, 0.25mS/D. I forgot to determine its duration; doh...

The output/Pin#1 of the driver op amp appears to be operating (correctly?) When the pot is advanced, the output fluctuates in the direction of change and, then, steadies itself and returns to the baseline (without the need of resetting the pot.)

However, the output/Pin#1 of the load op amp appears to be operating incorrectly. On the oscilloscope, the slightest advancement of the pot causes the output to leap off the screen and I can not find it, wherever it goes... My Tek2215/60MHz scope just may not go fast enough to find it.

Absolutely nothing heats up.

I was not trying to make a serious test. I will do more testing, in a more formal manner, when I feel better.

EDIT: I discovered that the pinout on Pot1 is reversed and is operating in a CCW manner. So, where I said I was advancing the pot, I was actually decrementing it.

[t1d]

EDITED: 11/4/2018

"Load Me Up!" E-Load Testing

Settings
DUT PSU 1v/1a
Tektronix 2215/Analog/60MHz
Oscilloscope Probe x10

All of the supply voltages, +9v, -9v and 2.5vref, test as good.

Initial Issue Found – The input pin of the MOSFET did not have connectivity. I don’t know, if I lifted the pad, or if the PCB was defective. I bodged this, with a bit of wire.

Operations improved. Current now sinks over the range of 0 to 0.0112, in volts, on my DMM. This equates to ~0.10 current draw, as confirmed by the PSU current meter.

DMM Readings – The Driver Op Amp output voltage sweeps from 0v to (Neg)-2.47vdc and seems to have no problems. The Load Op Amp output voltage only sweeps from -107.mV to 0.006 volts and then jumps to 8.87vdc. For the Load Op Amp, you only have to move the pot a very small amount, to reach 0.006 volts.


Oscilloscope Readings – Errant wave forms begin to appear, when the Probe Return/GND is attached and before the Probe pin is even applied. After the Probe Pin is applied, the errors become more clearly defined. See Load Op Amp, below. The big cap (C14/2200uF/25v,) for the cooling fan, effects the probe, when it is in close proximity, even though a fan is not connected.

With the Scope Probe Pin applied, the Driver Op Amp output exhibits a clean voltage line, throughout a voltage pot sweep.

The Load Op Amp exhibits various issues:
- After 0.006 volts, the Load Op Amp signal jumps and disappears. The output voltage jumps to 8.87vdc.
- There is a sign wave of 0.2/uS/D x 2.66 divisions = 5.32uS, having an amplitude range of -20mV to +20mV.
- There is a recurring ring of 0.5mS/D x 3 divisions = 1.5mS, having an amplitude range of -20mV t0 +20mV. This ring repeats approximately ever 4mS/pp 4 divisions, peak to peak. Today, only one ring reproduced. It had approximately one-half, of the amplitude, of the first ring.

As my oscilloscope only has a 60MHz bandwidth, I may not be able to see other problems.

Conclusions – The e-load can only sink up to one-tenth amp. The Driver Op Amp appears to be operating properly. The Load Op Amp appears to crash, when an output voltage of 0.006 volts is attained. It is possible that the capacitive field, of the fan cap, could be effecting the Load Op Amp. But, the Driver Op Amp is also in close proximity and it does not seem to be effected. None of the PCB components heat up.

So, I sure could use suggestions for repairs and additional testing. I look forward to hearing your ideas.

[Kleinstein]

Were is the circuit are the driver and load output OPs.  The OP driving the MOSFET should be at some 1-5 V (actual range is smaller and depends on the MOSFET) at the output. The OP before that should be only influences by the setting of the pot and could be best tested without anything at the "output".

When not attached, the scope probe can pick up some capacitive coupled background. If it is a lot, this can indicate either the circuit oscillating or external disturbance, e.g. from a SMPS.

A 60 MHz Scope should be well fast enough. The main frequencies to worry about are 60/120 Hz and maybe some 100kHz-1 MHz.

[t1d]

Hi, Kleinstein, I sure am glad to hear from you!

Were is the circuit are the driver and load output OPs.

I am not sure what you are asking, here. I think you are asking me what were the voltage outputs of the driver and load op amps. If so, I have retested and made some small corrections and additions, to my post, from yesterday. If what you need to know is not there, now, please ask, again.

The OP driving the MOSFET should be at some 1-5 V (actual range is smaller and depends on the MOSFET) at the output. The OP before that should be only influences by the setting of the pot and could be best tested without anything at the "output".

Here are my notes, about that:
DMM Readings – The Driver Op Amp output voltage sweeps from 0v to (Neg)-2.47vdc and seems to have no problems. The Load Op Amp output voltage only sweeps from -107.mV to 0.006 volts and then jumps to 8.87vdc. For the Load Op Amp, you only have to move the pot a very small amount, to reach 0.006 volts. Note that the Driver Op Amp sweeps negative.

When not attached, the scope probe can pick up some capacitive coupled background. If it is a lot, this can indicate either the circuit oscillating or external disturbance, e.g. from a SMPS.

The power source is a heavy, block wall wart. I am rather confident that it is a transformer type. It is mounted at the power strip... meaning that the transformer is not in the proximity of the PBC.

A 60 MHz Scope should be well fast enough. The main frequencies to worry about are 60/120 Hz and maybe some 100kHz-1 MHz.

See the prior post, for the disturbance information.

So, my guess would be either the Driver Op Amp should not output a negative voltage (that does not seem likely, because the Load Op Amp is going positive, as it should,) or the signal disturbances are sufficient to cause problems. What could I try, to eliminate possibilities?

Thank you, so much, for your continued help and support!

[GigaJoe]

I'm wonder,  if opamp that manage mosfer has enough power to discharge irfp cap in dynamic mode ,  I did same thingi + tr. emmiter follower  on opamp output, to unload opamp  and speed up process , when square input, resistor 10ohm instead 100

[t1d]

I'm wonder,  if opamp that manage mosfer has enough power to discharge irfp cap in dynamic mode ,  I did same thingi + tr. emmiter follower  on opamp output, to unload opamp  and speed up process , when square input, resistor 10ohm instead 100

I can understand a little of this, but it is really above my knowledge.

I have not used a dynamic signal for testing. I just used my bench power supply, set at 1v/1a, run through a 10 ohm resistor, to the DUT input.

Which resistor reference number(s) should I try reducing? R6, I think? The single MOSFET schematic is at Post #19.

Do you think this will correct the existing operational problem? Or, is it just an observation.

Thank you, for your help.

[Kleinstein]

With only a 1 V source and a 10 Ohms series resistor, the maximum current is limited to some 100 mA. With a 0.1 Ohms shunt, this means a maximum of 10 mV at the shunt to get to saturation.  So the range is very limited.  Getting the MOSFET to saturation will cause the voltage to shoot up to something like the 8.x V.  This is about normal. Limiting the voltage to only 1 V is a little overly cautious and leave very little room for the circuit to work.


The "driver" OP - this seems to be the one that provides the negative set point is OK to provide a negative voltage, because the output stage is build o work inverting.

The "load" OP seems to be the one that does the actual current regulation. It might be oscillating or pic up some disturbance and this way goes to it's limits (positive or negative) rather fast.

Because of the small voltage levels one might need to keep hum out, e.g. by having a groundes metal case or at least something like metal plate below the circuit.

For a first test one could try to start with a slower version, by adding an additional capacitance (e.g. 10 nF)  in parallel to the 220 pF feedback capacitance. This should make it less sensitive to oscillation, though slower.

[t1d]

With only a 1 V source and a 10 Ohms series resistor, the maximum current is limited to some 100 mA. With a 0.1 Ohms shunt, this means a maximum of 10 mV at the shunt to get to saturation.  So the range is very limited.  Getting the MOSFET to saturation will cause the voltage to shoot up to something like the 8.x V.  This is about normal. Limiting the voltage to only 1 V is a little overly cautious and leave very little room for the circuit to work.

Okay, I will increase the voltage and try again. But, IIRC, I had the exact same performance at 1v/1a, without the resistor.

The "driver" OP - this seems to be the one that provides the negative set point...
The "load" OP seems to be the one that does the actual current regulation.

Yes, this is proper understanding of the names I am using. Are there better terms, for these?
 

Because of the small voltage levels one might need to keep hum out, e.g. by having a groundes metal case or at least something like metal plate below the circuit.

I still have concern that the load op amp may be effected by the big fan cap. I think I will remove it, for testing. What do you think?

I also read about adding inductance, in series with the MOSFET gate. What do you think about that?
“Ferrite beads are very effective at eliminating parasitic oscillation while minimizing switching losses, because they act like a frequency-dependent gate resistor.”  https://www.microsemi.com/document-portal/doc_view/14693-eliminating-parasitic-oscillation-between-parallel-mosfets

For a first test one could try to start with a slower version, by adding an additional capacitance (e.g. 10 nF)  in parallel to the 220 pF feedback capacitance. This should make it less sensitive to oscillation, though slower.

I will add this, to the testing.

Thank you, for your great help.

[t1d]

Success!

Kleinstein pointed out that I was starving the load and that I needed to add more voltage. I did so and the unit began to perform as expected. Woot! I made no other changes.

I only took the load up to 5v/1a (without the 10 Ohm resistor.) The FET warmed, maybe to around 110*F, i.e., hot to the touch. So, higher load testing will have to wait, until I build out the cooling fan assembly.

Conclusions:
There was nothing wrong, with the circuit design, from the start. After the FET gate trace was repaired, the unit operated correctly. All confusion, thereafter, was due to (my) operator error. I just needed to let the big dog eat. ;-) Thanks, Kleinstein, for straightening me out! My guess is that it will handle 30v/2a, but I make no guarantees.

It is clear that a ten-turn pot is a necessity. I will change it out.

This will be an opportunity to swap the wires, so that the pot operates clockwise. I have added instructions, as how to make this correction, in the KiCad schematic, at the previous post containing the files, so that the PCB board will be manufactured correctly.

Okay, I could not be more pleased than if I had made an LED blink, for the first time. Woot! Fun! Exciting!

P.S. This is a test board, for the design. A two-MOSFET model will be going to the board manufacturer, very soon.

If you would like one of these Single-MOSFET Test PCB boards, send me a PM... Actual shipping cost and, maybe, a few bucks, to help with the manufacturing costs? But, I will not charge any profit. Releasing me from all liability is required.

If you need help with the cost, please, just say so, and why. I want to encourage anyone and everyone, as appreciation, for all the support I have received, myself. I would think we can find a way...

Remember, this board uses SMDs, so factor in your skills. Actually, SMDs are really easy to handle, by several means. I would not let that hold you back. I can talk you through it... Hand-soldering, heat gun, clothes iron... There are lots of tricks that work well.

I do not have a camera, but I hope to add pictures, soon.

[t1d]

I forgot to mention that it seems to take a few seconds, for the op amp to rise up to a new current setting. What should I be expecting? What test procedure should I use to test this performance?

I can not find my old web cam. It is very likely that I threw it away. And, the one on my cell phone does not work. So, I may not be able to post pictures. That's a shame, because it looks pretty neat...

Thanks.

[t1d]

A friend was kind enough to make pictures...
Board on test stand... Fan yet to be attached...
82mm x 69mm = 3-1/4" x 2-3/4"





Can you find the two tiny op amps?

[Kleinstein]

The 2 FETs version still has a mistake in the last plan. So I would not rush to ordering a 2 nd board.

If just a little more power is needed, one could still add a second of the 1 FET versions, using only a part of the circuit (e.g. mainly the load section).

The first measurements showed that to low voltage for the source could be a problem. So for the next version it might be a good idea to have at least a warning LED to see if there is a problem with "starvation" / saturation of the FET. One might also want an adjustable limit for the gate voltage, so that in case of saturation the voltage does not go up way to far. This is because it can cause some possible problem. If one first sets the current sink to a given (even low) value and only than connects the source, this current will initially be very high and only after some 10-100 µs (depending on the circuit) will reach the set value.  The same type of trouble could also happen if the source is disconnected for some time (e.g. a poor contact).  This is a relatively difficult problem to solve and even commercial load circuits may not have good fix here, but it still would be a good idea to have at least some counter measures to limit the current peak.

[t1d]

The 2 FETs version still has a mistake in the last plan. So I would not rush to ordering a 2 nd board.

I am rather careful to copy, study and employ your notes. I think I have probably already made the corrections and just did not post the updates, because I was working on the Single-MOSFET model. What is the posting number and I will double-check.

If just a little more power is needed, one could still add a second of the 1 FET versions, using only a part of the circuit (e.g. mainly the load section).

You had suggested this approach, before, and I am planning on using multiple boards... One Single-Mosfet and one Double-Mosfet. This will give many combination options.

The first measurements showed that to low voltage for the source could be a problem. So for the next version it might be a good idea to have at least a warning LED to see if there is a problem with "starvation" / saturation of the FET. One might also want an adjustable limit for the gate voltage, so that in case of saturation the voltage does not go up way to far. This is because it can cause some possible problem. If one first sets the current sink to a given (even low) value and only than connects the source, this current will initially be very high and only after some 10-100 µs (depending on the circuit) will reach the set value.  The same type of trouble could also happen if the source is disconnected for some time (e.g. a poor contact).  This is a relatively difficult problem to solve and even commercial load circuits may not have good fix here, but it still would be a good idea to have at least some counter measures to limit the current peak.

Good ideas!

[t1d]

The 2 FETs version still has a mistake in the last plan. So I would not rush to ordering a 2 nd board.

Ah... Yes, the need for two resistors, on the Driver Op Amp output. That has been corrected, but I had not posted the new schematic.

I don't remember the second item. Was it how to deal with just using one shunt resistor? If so, I just reverted to using Jay's multiple shunt resistors design.

Here's the update. Please give it a look.

[Kleinstein]

The new circuit looks OK, at least the 2 obvious errors are gone.

The heat sink on the 1 FET version is still relatively small. So 30 V and 2 A and thus 60 W would need a good fan.

One should also to a few more tests, to see if the load is also stable with a not so well behaved source. Regulation gets more tricky with some inductance in series.  If you have a suitable generator, one could also check the step response, to see if there is ringing when the source voltage changes fast.

[t1d]

The new circuit looks OK, at least the 2 obvious errors are gone.

Great! Phew... I feel better that I didn't miss any of your wonderful instructions.

The heat sink on the 1 FET version is still relatively small. So 30 V and 2 A and thus 60 W would need a good fan.

Agreed and I do have one that would be perfect, for this test bench arrangement. However, I would still worry that it is too small. So, I think I will put it into my lab-type case. Actually, doing this should be easier than adding the fan to the test block.

This is the case that I built, for an earlier design, that I did not end up using. (I think you helped me, on that thread, too.) It is a repurposed rack-mount, 100 watt stereo amplifier case. I gutted it and only kept the huge heat sinks. One HS is still in the box. I attached a 120v computer case fan. I think the case HS and fan will handle one Single-MOSFET and one Double-MOSFET board and give me a lot of combination options.

One should also to a few more tests, to see if the load is also stable with a not so well behaved source. Regulation gets more tricky with some inductance in series.  If you have a suitable generator, one could also check the step response, to see if there is ringing when the source voltage changes fast.

Good advice and I wholly agree. I plan to test the auxiliary Function Generator voltage reference input. I will also test the DUT Input, with a frequency. But, I may need to figure out how to amplify the signal, beyond what the FG can provide. I do not have an amplifier, of any sort, presently.

I just want to say "thank you," once again. Without you, this project would have never moved forward. By sharing your knowledge, time and effort, you have enriched my life. I am truly grateful.

[t1d]

I am holding off, on further testing of the Single-MOSFET test design, because of a lack of sleep caused by health issues. In the meantime, and just for fun, here is the current proposed layout, for the Double-MOSFET version. It is greatly improved, over the Single-MOSFET test design, thanks to suggestions made by JS. It is only very slightly larger than the Single-MOSFET test design board.

The blue ground plane indicates that the ground plane has been moved to the bottom.

As is, the 12vdc supply, for the fan, is actually a fully rectified 12vac, which is approximately 12v x 1.4 = 16.8vdc. This voltage will have to be regulated down to 12vdc. What options are there to making the board bigger and adding a 7812 regulator circuit? A zener clamp won't provide enough current, I don't think...

And, I am thinking on using a single power supply circuit, to supply multiple e-load boards. Does anybody know of 9vdc and -9vdc regulators that will provide a minimum of 2 amps? 3 or 4 amps would be better..

I still need to confirm that the locations of the current sense and shunt resistors are adequate. Any comments, in that regard, or other matters, would be greatly appreciated.

[Kleinstein]

The supply for the circuit does not need much current. So I don't see a real need for a high current regulator.

The 2 MOSFET version uses relative low power Shunts, so this would mean a rather low drop at the shunt and thus not very precise regulation.
The Shunts are also rather close to the MOSFETs, so there would be extra heat in this area. Those SMD resistors only get there nominal heat resistance with a relatively large copper area - so 2 SMD resistors close together only provide a slightly higher power rating, not twice the value.  So the size for the SMD shunts is misleading - it's not just the small chip, but should include 1-2 cm² or copper on the board.
We had that discussion before and there is a reason the commercial electronic loads usually use quite large shuns, like quite a large (e.g. maybe bare wire shunt extending well above the board. So the resistor size for the 1 FET version is not that bad.

The position of the shunt's, shape of the ground plane is also not really good, especially for the left channel there is no clear, direct ground connection from the shunt to the OP. For a precision circuit a ground plane can be tricky, as it gets difficult to see where the current actually flowing. So a ground pour is a poor ground if there are cuts from other traces.

I don't understand the desire to make an electronic load so small - with power electronics is helps to have the heat sources a little apart. For a prototype is also helps if there is at least space for the probes and space for possibly needed bodges.  A 7812 for the voltage regulation is large enough. If really on the squeeze one could add some  resistance between the rectifier and cap to improve on the power factor and this way reduce the raw voltage and get away with less heat from an LDO.

[t1d]

I just love your insights, Kleinstein. Excellent, always excellent. I wish you were my next door neighbor... LOL.

I have changed the v ref component, on the schematic. The new one is better, for this application, IMHO. Cheaper, I think, and less complicated. This changed the component reference numbers.

The supply for the circuit does not need much current. So I don't see a real need for a high current regulator.

I was thinking to try to use the 12v positive regulator, for the fan, too. But, this would put quiet different loads on the positive and negative regulators and that might not be a good thing?

The 2 MOSFET version uses relative low power Shunts, so this would mean a rather low drop at the shunt and thus not very precise regulation.

Well, I had wanted to use a higher wattage, single resistor, just like the single-MOSFET version, only slightly larger. From your earlier reply, about that, I thought you said that this wasn’t a great idea. Did I misunderstand? If so, what would needs to be done?

If a single shunt is difficult, or costly, I think, IIRC, that the cost of the first single shunt was not too bad. So, I would use two of those, instead of the SMDs.

The Shunts are also rather close to the MOSFETs, so there would be extra heat in this area. Those SMD resistors only get there nominal heat resistance with a relatively large copper area - so 2 SMD resistors close together only provide a slightly higher power rating, not twice the value.  So the size for the SMD shunts is misleading - it's not just the small chip, but should include 1-2 cm² or copper on the board.

We had that discussion before and there is a reason the commercial electronic loads usually use quite large shuns, like quite a large (e.g. maybe bare wire shunt extending well above the board. So the resistor size for the 1 FET version is not that bad.

Designing to have a tight board stemmed from the op amps requiring the shortest trace runs possible, the habit to limit board cost and the dimensions of the (cheap) flat-rate 100mm x 100mm boards. But, you are exactly correct. I will spread things out, a bit.

The shunts were moved to the FETs, because I thought it would improve accuracy. Does their location matter, other than providing them with enough copper and keeping them away from other heat sources? These two concerns are why the one-FET version had the shunt placed away from the FET. What about the long traces, for them?

The position of the shunt's, shape of the ground plane is also not really good, especially for the left channel there is no clear, direct ground connection from the shunt to the OP. For a precision circuit a ground plane can be tricky, as it gets difficult to see where the current actually flowing. So a ground pour is a poor ground if there are cuts from other traces.

Agreed. I moved the ground plane to the bottom, to try to improve directional access, but it did not help. I really don’t know what else to do. A multi-layered board is just too expensive, for me, for a hobby project.

I don't understand the desire to make an electronic load so small - with power electronics is helps to have the heat sources a little apart. For a prototype is also helps if there is at least space for the probes and space for possibly needed bodges.

Agreed, per above.

A 7812 for the voltage regulation is large enough. If really on the squeeze one could add some  resistance between the rectifier and cap to improve on the power factor and this way reduce the raw voltage and get away with less heat from an LDO.

I do not know this trick. Is there a name, for the technique, so that I could look it up, to study it?

Thinking on cost... As usual, DIY (especially DIY development) is not necessarily cheaper than a manufactured unit. But, I do it, for fun and education. From that perspective, it is a good value...

As always, I am s-o-o grateful. Thank you.