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.
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/ina260I 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.
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
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