+ t1d
You asked for comments about the design. Does that include areas that may be a problem?
Absolutely. That's why I posted the project... More heads are better than one.
That is a good positive view and the way I see constructive and valid comments about my designs too. But I always feel a tad guilty about finding problems with other people's work, especially when they have put a lot of effort into them, as you obviously have.
Thanks for defining your target output voltage, current, and power specification. That has been a big help, especially as I thought you were aiming at much higher figures. Just to reiterate:
I am hopeful that
- the Single-MOSFET test board will handle 30v/2a = 60w and
- the Two-MOSFET final model 30v/4a = 120w.
As everyone, no doubt, knows, the trouble with active loads, just like lab power supplies, is that you never know what is going to be connected to them. It's also well known that, ultimately there are two limiting areas, output power transistor maximum junction temperature (TJmax) and safe operating area (SOA).
So out of interest, I had a look at these two areas on your active load.
IRFP250M NMOSFET RELEVANT CHARACTERISTICS
The IRFP250M is a beefy NMOSFET: Case type= TO247, VDSmax= 200V, IDmax=30A, ThR J/C= 0.7DegC/W, Tjmax= 175DegC.
TEMPERATURE BUDGET
The total thermal resistance junction to ambient (TTRJA) = TRJC(0.7degC/W) + TRCH(1DegC/W)(mica washer) + TRHSA (3DegC/W) (assumed) = 4.7DdegC/W
The required dissipation = 60W, so the temperature difference, J/ambient = 60 * 4.7 = 282degC
Assuming that the ambient temperature inside the equipment case is 70 degC, the junction temperature would be, 282+70 = 352degC, and there, afraid to say, is the problem!
The actual allowable IRFP250M dissipation with your heat sinking arrangement is, (TJmax- Tamb)/TRtot = 105/4.7= 22W, which agrees with the general rule of thumb that you can't safely dissipate more than around 20W in a TO247 (or TO3) case.
In the above, I have assumed that your heatsinks have a thermal resistance of 3 degC/W but, going by their size, their thermal resistance may be higher. Aggressive fan cooling will bring the thermal resistance down, but not much lower than 2 degC/W, I would guess.
SAFE OPERATING AREA (SOA)
As there are no SOA lines for 100ms or DC, it would seem that the IRFP250M is intended mainly for pulse amplification, whereas for an active load the signal will include DC: battery load testing being an example. So, by extrapolation, I have added orange lines for 100ms and DC, as shown on the attached image. But these lines are not necessarily accurate: they are just a guide.
The nice thing about the SOA graph is that it applies at the maximum junction temperature of 175 degC, which is excellent.
The good news is that the SOA for the IRFP250M looks OK for your application- but you already knew that
Apologies for being the bringer of bad news about the junction temperature: I will get my coat.