N-channel MOSFET Safe Operating Area anomaly?

Sorry they state that they test their L2 mosfets for linear operation I dont know if this applies to typical switching fets or not.

Maybe not.

Oh no, I've already started building resistor banks to work with saturated MOSFETs -- no heatsinks on the FETs, but a 120mm fan to cool the resistors instead ;)

I bought 23 of these FETs cheap, 50c each, a while back. Their one off price is $13.28 today:

No thanks ;^) I acquired enough various injuries over the years without risking my sight.

Grant.

Besides, every time I looked at a datasheet for IGBT, they're too big, too expensive, or MOSFETs seem a better fit for what I was planning.

Grant.

Sure, and I agree with you. In fact the best linear loading solution seems to be using a bank of 7805s or LM317s ;) Cheap, and built in thermal protection. I got some LM317AHVs that I want to try in a bench supply soon (everything soon).

Grant.

I think FETS are the best for that purpose. You just have to do what you always have to do derate. Namely stay in the middle of the SOA.

If your willing to shell out the bucks for IXYS's L2 fets they are at least tested and designed for linear operation. I'm like you I got my fets at about a buck a pop so that's what I'm using 6 of.

I plan on limiting it to 250Vdc in; 300W power limiting. They should be able to handle that if not I guess I'll find out. I have lots.

It's a slow process I have other things on the go but I finished the power supply for the control board hopefully next week I can start assembly. I'll post pics of my success or failure.;-)

And sometime soonish I hope to have a hybrid power DAC going, I'm ready to wire up and program a PIC uC to control ADC and run an LCD module. Discover how well I can take measurements with matrix card construction, trying for 16 bit, need 15 bit resolution.

Spent most of last week going over the design trying to work out how to get the ADC clock at the right frequency. I'm constrained by having to take an integer number of 20ms (50Hz) samples to average ripple voltage / current. 19.6608MHz does that, and also perfect for serial baud rate, so I must be on the right track! :o)

Grant.

But they're great when you _need_ a "too big" transistor, or anywhere you're switching a lot of voltage not too quickly.

Just one pair of those FGH80N60's should be able to handle my 10kW induction heater, given enough cooling (i.e., heat spreaders). As is, I'm using four, which spreads out the heat well enough not to need spreaders. (The heatsink is water cooled, in case you're wondering.) I've ran them at 80A peak output (sinusoidal), continuously. The only thing that's burned out so far is one GBPC3508, which went after ten minutes, quite understandable as it was past ratings (obviously, a 10kW induction heater needs two in parallel, a problem which has since been, if you will, rectified).

And I spent all of $20 on those transistors. You can't even find a module rated the same (200A 600V, "fast" switching, with co-pack diodes) for under $100.

IGBTs suck below 300V, and they don't tolerate funny voltages. So they aren't great for 120VAC stuff, where conduction losses are higher, or boost/flyback converters or motor drives necessarily, where voltages can get peaky unless you add a lot of protection. These are all things that MOSFETs excel at. You can run a 500V MOSFET in a 400VDC output PFC boost circuit, and it simply works, with high efficiency over a wide range of voltage and current.

IGBTs are intrinsically slower than MOSFETs, limited by charge effects, so they aren't good for HF and RF type roles, where you can drive the piss out of a MOSFET and just have it work. They are fast enough for SMPS use, at least if you drive them well.

I've never seen an IGBT rated for avalanche. I don't know why they can't make them rated for it. I do know they love to fail if the peak voltage goes near ratings. So you use Vce(max) about twice the supply voltage, which makes 600V transistors best for 240VAC, or 1200V transistors for

A typical 5N60 FET,

This IGBT, using a conservative 20W rating (figure passive heatsink, sil-pad, and the inherent limitations of the TO-220), could do 10A average easily enough, which is 3.2kW for a pair in a half bridge forward converter. Half bridges are great for IGBTs, since you can bypass the rails tightly.

This IGBT is also available in D/D2PAK, which would reach ratings (~2W on PCB) at about 1A average, which is still 320W. Not bad for SMT.

IGBTs are astonishingly good at peak current. There's a GT8G132 in my camera's flash. It's a teeny SO-8, rated for 400V 150A (Vce(sat) typ.

Tim

Thank you, another info packed post marked 'keep' :)

Grant.