Author Topic: Another electronic DC load - 60V / 20A / 300W  (Read 14738 times)

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Offline free_electron

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Re: Another electronic DC load - 60V / 20A / 300W
« Reply #25 on: March 22, 2017, 03:57:46 pm »
the problem with such loads is that they are heavily dependent on the sense resistor to set the minimum burden voltage ...

let's say i want to draw 20 ampere from a 1 volt supply ... even with the mosfets completely in conduction your sense resistor is 0.2 ohms. you can only draw 5 ampere ...

in other words : you need a much smaller sense resistor. in the milliohms range. like 1 milliohm or below. there are specialised IC's that have a trimmed sense resistor and a precision amplifier in one. use that. or use a sensefet ( mosfet with two source terminals. one is 1/100 or 1/1000 of the main current. you can stick a sense resistor there. )

another thing you may want to do is 'float' your output. make a galvanic isolation between your main processor and the actual load system. ( over a n rs232 link thru optocouplers or digital couplers..
the load is not always 'ground referenced ... what if you need to load a negative supply ?
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Offline kaktus

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Re: Another electronic DC load - 60V / 20A / 300W
« Reply #26 on: March 25, 2017, 09:30:11 pm »
I have great experience with IXTH75N10L2. The good thing is that you really a have peace of mind since it was designed primarily for linear loads. I remember I did some tests with standard FETs and even though some of them lasted for hours (!), they failed after that.

About the current sense resistor, I use WSL3637 from Vishay. Since it is 4-terminal, the sensing is very stable. Just don't solder power and sense terminals together, solder has a horrible temperature coefficient and the resistor will get hot. Another possibility is LVK12 and LVK24 from Ohmite if you don't need a high current. But they are not much cheaper than WSL3637, just smaller. Keep the resistance small and use an amplifier for current. Every milliohm counts at high currents.

You can see the documentation for my electronic load here: https://github.com/kaktus85/MightyWattR3, maybe it will inspire you :-)
 

Offline electricarTopic starter

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Re: Another electronic DC load - 60V / 20A / 300W
« Reply #27 on: March 27, 2017, 01:54:38 pm »
I just placed an order with two IXTH75N10L2 and some different opamps and am looking forward to test them :)

@Kleinstein:
I managed to have a look at your simulation and I got it stable indeed, but 100nF for C2 is just too big. The set current shape is filtered out too much. And if I reduce C2 I get over- and undershoots again.
I will do some tests with the new FET and opamps.

@free_electron:
the problem with such loads is that they are heavily dependent on the sense resistor to set the minimum burden voltage ...

let's say i want to draw 20 ampere from a 1 volt supply ... even with the mosfets completely in conduction your sense resistor is 0.2 ohms. you can only draw 5 ampere ...

Yes, that’s why I will do my further tests with 0,05R or something smaller.

Quote
in other words : you need a much smaller sense resistor. in the milliohms range. like 1 milliohm or below. there are specialised IC's that have a trimmed sense resistor and a precision amplifier in one. use that. or use a sensefet ( mosfet with two source terminals. one is 1/100 or 1/1000 of the main current. you can stick a sense resistor there. )

Do you mean something like the INA260?
http://www.ti.com/product/ina260
I couldn’t find a current sense amplifier with an integrated shunt and also an analog output.
I need the analog output as feedback for the FET controlling opamp. The digital solution with e.g. the INA260 would be too slow.

Quote
another thing you may want to do is 'float' your output. make a galvanic isolation between your main processor and the actual load system. ( over a n rs232 link thru optocouplers or digital couplers..
the load is not always 'ground referenced ... what if you need to load a negative supply ?

My design is approaching an isolated/floating DUT input. I’m using some Si8651 for that.

I have great experience with IXTH75N10L2. The good thing is that you really a have peace of mind since it was designed primarily for linear loads. I remember I did some tests with standard FETs and even though some of them lasted for hours (!), they failed after that.

About the current sense resistor, I use WSL3637 from Vishay. Since it is 4-terminal, the sensing is very stable. Just don't solder power and sense terminals together, solder has a horrible temperature coefficient and the resistor will get hot. Another possibility is LVK12 and LVK24 from Ohmite if you don't need a high current. But they are not much cheaper than WSL3637, just smaller. Keep the resistance small and use an amplifier for current. Every milliohm counts at high currents.

You can see the documentation for my electronic load here: https://github.com/kaktus85/MightyWattR3, maybe it will inspire you :-)

Thank you for your feedback! The last days I read almost everything on your blog about your electronic load design and I have to thank you for sharing this amount of useful information! Very interesting and helpful experiments! :)
I have to admit that I got the idea of switching between CC and CV from Spikee:
https://www.eevblog.com/forum/projects/modular-dummyload-~180w-20-40a/15/
But now I realize that he got this idea from your design :D
I’m really interested in the stability of the CV mode. Did you make some stability tests in CV mode?
And what I’m even more interested in is the stability of the CC mode when making steps in the set current. Do you have over-/undershoots or is the load stable?

Thank you very much in advance!

Kind regards
 

Offline Kleinstein

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Re: Another electronic DC load - 60V / 20A / 300W
« Reply #28 on: March 27, 2017, 03:32:02 pm »
The filtering of the set-point signal with C2 should not be a major problem. A smaller resistor (R14) should solve that point.

The problem with using the inductors to improve stability is that this would also slow down the response to changes in the set current. It might not be a problem in CC mode, but is would make is really hard to get a CV mode operating in a kind of cascading loops. So the inductance (the intentionally added one) is the much bigger limitation than the R14/C2 filter. It gets more difficult without the extra inductor, but one can still get a stable. though slower CC mode loop.

The current dependent response of MOSFETs makes it hard to get a fast and stable response with a small shunt (the inductor with parallel resistor is a kind of replacement for this). The problem might get even worse when using an even larger MOSFET. Already the IRFP250 might profit from a lower impedance gate driver. There is a problem with increasing noise and drift, if the shunt is chosen very small - so something like 50-100 mOhms for a FET of the size of the IRFP250 is reasonable. One usually also has an extra fuse that might add another 50 mOhms or so. With 200 mOhms of total resistance this still allows up to 5 A at 1 V. As the shunt is only a smaller part of the resistance it won't help much reducing it. So instead of an ever smaller shunt, the more viable way would be a lower current for the FET or more channels in parallel. Another problem with a very small shunt is, that parasitic inductance and inductive coupling get more important. Already with 50 mOhms one can usually not neglect the inductance.
Finally there should be an under-voltage limitation for the CC mode: it the FET is driven all the way to saturation, this could allow excessive current spikes when the voltage is going up again. The most common case would be enabling the load first and only than connect the voltage source under test. So this protection is really needed, not just an nice to have. One way to implement it, would be to limit the gate voltage to a certain value (e.g. 5 V), which would in result increase the effective on resistance of the FET used. So the minimum resistance per FET tends to be even higher: thus for the IRFP250 its not the 85 mOhms R_On for 10 V gate voltage, but more like 150 mOhms for a maybe 4-5 V gate drive.
 
As the shunt is towards GND, there is no need for good common mode rejection for the amplification of the voltage signal. So one might not need an instrumentation amplifier and can use a normal OP, with more choice of fast types.

 

Offline kaktus

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Re: Another electronic DC load - 60V / 20A / 300W
« Reply #29 on: March 27, 2017, 06:55:37 pm »
Stability was, at least for me, the most challenging part of the design.
I started with some web-collected ideas and also did some LTSpice simulations but the final values were determined largely by trial and error. I found the stability to depend a lot on the nature of the source. The hardest was a switching power supply Manson HCS-3602 (32V/30A) I especially bought for testing of MightyWatt.

So, with the present values (revision 3.1), the stability is exceptional. Settling time
from 0 to 24 A is about 1 millisecond without overshoot. Shorter settling time was possible but it
did overshoot so I chose the fastest stable settling time without overshoot in this scenario.

Settling time from 5 A to 24 A is about 0.4 ms but there is some overshoot.
Turn-off time from 24 A to zero is about 0.04 ms with some ringing (I guess the inductance). Same behaviour from 24 A to 5 A. There is a TVS which takes care of voltage spikes. I measured it with a thermal camera and it's not even warm.

I have a built-in statistics in my Windows control program and I usually get like 0.001% of relative standard deviation for current (also confirmed by my Keysight 34461A). I don't see any high-frequency oscillation on my scope either.

CV mode turned out to be much more oscillation-prone and I had to go into 20 ms settling time (30 V to 2 V) to get the same 0.001% RSD as in CC mode. So the CV mode is not that great for transient testing in MightyWatt but since it is primarily a DC load, I traded settling time for stability.

The compensation networks for CC and CV have different values of the components so I use a triple SPDT switch (MAX4619) to select not only CC or CV but also the compensation network.

I am not sure this essay I just wrote will help you but that is the current situation with MightyWatt R3. I'd say there are always tradeoffs and my priority is stability and DC accuracy.
 

Offline Crumble

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Re: Another electronic DC load - 60V / 20A / 300W
« Reply #30 on: March 28, 2017, 11:57:40 am »
Hi all,

Unfortunately, I do not have the time to fully analyse the design of this load, but it seems to be an ambitious project! There is a number of issues about using MOSFETs which I found while analysing their use in audio amplifiers.
  • SOA
    You already noticed the forward SOA of MOSFETs is quite poorly documented, and I would generally recommend against it unless they are severely underrun. Some time ago I found an interesting article about it. It is rather awkwardly written: it starts out with someone justifying HEXFETs for linear use, but is followed by a rather extensive rectification in the footnote with a rather indepth analysis of the issue. Especially the footnote might be of special interest for you. They also mention the use of lateral FETS, which have inherently better current sharing capability when paralleling them up without individual drives (which can simplify your system quite a bit). Please do note the references on the very bottom of the article. These might be more relevant than the rest of the article! I also unearthed that the max voltage degraded when using the MOSFET linear in one of the better documented Ixys devices. This seems show the current/heat spreading capability of a die has a quite well-defined, device dependent limit.
  • Gate capacitance
    When using MOSFETs you are more likely to find instability issues in it because the drive circuit is usually just an opamp. This is logical, because the static current draw of a MOSFET is negligible. Its dynamic performance might however be impacted due to the reverse transfer capacitance of the MOSFET together with the output resistance of the driver for it. Depending on the load driven the voltage amplification of this system might get quite high and the reverse transfer capacitance may cause the phase shift in the output system to increase quite significantly. This is especially the case with inductive loads or ones with a higher resistance are driven (likely on the higher voltages). It might help to use a higher current opamp or a seperate buffer stage made out of a pair of discrete transistors. I think the difference you found between opamps might very well be explained by their current capability rather than their speed. You may also test what happens if a small Miller capacitor is placed between the drain of the MOSFET and the opamp driving it, but this effect should ideally be calculated using control theory. When doing so you will find you will have to account for lead (and load) inductance, and it will be impossible to make it stable for every load.
Hopefully this helps you a little bit! I made a simple dummy load once (~100W). It uses 2 HUF75345P MOSFETs because they at least had a reasonably, but not fully, documented datasheet. Its SOA was quite a bit wider than other MOSFETs I had, and so far I've gotten away with using it up to 30-40V. I just made it really slow because I did not feel up to the task to making all the stability calculations needed to get it to work stably. It was mainly made to be simple and usable. Most of the active circuitry is under the heat sink (the stuff to the left is the fan regulator), so current stability is not great, but hey, it works... :D

Regards,
Crumble

 


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