Electronics > Projects, Designs, and Technical Stuff
Mosfet paralleling reliability in real life - more smaller ones vs fewer bigger
T3sl4co1l:
--- Quote from: MagicSmoker on September 21, 2019, 09:58:24 pm ---Eh? Turn-on is very lossy in a MOSFET-based hard-switched converter due to the energy stored in Coss. You really will need to either use SiC MOSFETs or else go with a ZVS or above-resonance topology.
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A note, there's very little energy stored in Coss as such, but because the opposite side capacitor is in series with the supply, that voltage stacks with its Coss energy, flipping the curve: instead of delivering little power in the high-C region, it delivers maximum power there. The super-bottom-heavy C(V) curve makes it look like a drawn-out diode recovery (it happens over 10s of volts, not ~1V), and the low side switch dissipates similar power as a result (or the switching loop inductance absorbs the energy and turns it into voltage spike, or a snubber consumes it).
Which is also a good point, with SuperJunction transistors you need to take special care of the switching loop at low current conditions. It's not hard switching into diode recovery, but it's almost as rough.
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--- Quote from: Miyuki on September 21, 2019, 12:54:22 pm ---what will do to switching losses driving mosfets off very hard with like -10V with minimal gate resistance and rely on diodes to clamp that spike
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Makes no difference with MOSFETs. Or IGBTs, for that matter. The only reason to bring the gate negative during turn-off is to prevent shoot-through from Miller capacitance of the opposite leg turning on, but that is not a consideration in the two-switch forward.
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Well, you can get more current through the driver+gate resistances -- this is basically why datasheet t_f figures are so much longer than t_r, there's less drop from Miller plateau voltage to driver output low voltage. It's not usually necessary, but it can still help you get a little bit more performance. (More often, you're burning switching speed with additional gate resistance anyway.)
Tim
MagicSmoker:
--- Quote from: T3sl4co1l on September 22, 2019, 02:44:27 am ---A note, there's very little energy stored in Coss as such...
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Yes, this is an underappreciated 2nd order effect - and, to be honest, one I hadn't considered here - but I mainly referred to Coss because the OP did. That said, whether Coss stores enough energy to be a problem very much depends on the type of MOSFET and, especially, the thermal resistance from junction to case/sink/ambient, etc. Those TO-220FP devices tend to have terrible Rth-jc (typically 3C/W), so even 4-5W of additional loss can be a real headache.
--- Quote from: T3sl4co1l on September 22, 2019, 02:44:27 am ---Well, you can get more current through the driver+gate resistances...
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Technically correct, but not remotely practical - or even desirable - in the context of what the OP wants; namely, to drive (5) switches in parallel. It would take heroic efforts just to get the transition times to 50ns, much the less the <20ns that could be achieved with a single MOSFET and an appropriately sized bipolar driver in a tight loop.
jonpaul:
Hello from Paris:
Paralleled MOSFETs are subject to parasitics inthe VHF range, use gate damper resistors and/or beads.
Best topology for high power designs nowadays is NOT FWD, etc but soft switching, resonant, etc.
You can read numerous papers and books on these topics. This greatly lowers switching loss and can run at 500k-20 MHz. Thus caps and transformers and inductors shrink.
Just the ramblings of a retired EE, worked since 1960s in analog and power design.
Bon journee,
Jon
Miyuki:
MagicSmoker: You are right, I was blindly using some table from TDK and it is way off at this frequencies :o upper limit is much lower, more like 150mT at max duty
jonpaul: Resonant converter with constant current output and wide output voltage? For constant voltage and steady currents they are nice, but for this application it little too exotic. I dont even find much papers about this use. And have no idea how it can be controlled.
I might try to use one big Si MOSFET like IXFH40N85X with high allowed gate voltage and drive it really hard
It have Ugss +-30V and 40V transients
So driving it with something like +20 and -10 can get reasonable small switching losses
And keep layout really tight to limit transients
Kleinstein:
If the power is to high for a single switch, one could consider a multi phase design. It needs a 2nd / 3 rd transformer / inductor, but could reduce parasitic inductance as smaller units are used. I am not sure if there are suitable controllers.
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