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Mosfet paralleling reliability in real life - more smaller ones vs fewer bigger

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T3sl4co1l:
Why not a pair of IGBTs?  Should be just workable at that frequency, or fine a bit lower.  If you're going for lower switching losses in a compact package (hence the higher frequency) I guess that wouldn't work out; instead I would recommend SiC MOSFETs.

Paralleling MOSFETs is generally fine, but don't forget to consider stray inductance.  In the Sam Ben-Yaakov video, he considers the current divider circuit formed by Rds(on)s.  Add stray inductances in series with these, and change the current to a step or ramp source, and you will have a representation of current sharing in the switching circuit.  Current equalizes after a few L/R time constants, so besides reducing stray inductance, it can actually help to have some additional resistance.

Note that the stray inductance is, always, the loop inductance, from one switch to its opposing switch.  For the two-switch forward, that's one transistor, and its catch diode, through the local bypass capacitor.  The other transistor and diode act in an independent loop, and that loop needs to have the correct inductance as well.  (The bypass can be shared, that's fine -- as long as both inductances work out.)

A typical layout, with transistors lined up on a heatsink, you would need to alternate transistor and diode, and put bypass caps in front of the row.

It's not much more trouble to simply use fully independent inverters.  Use a single transistor and diode per leg, and a pair of legs, with a transformer and all the support components (input bypass cap, driver, output rectifier..) per channel or phase.  Use N phases, with a phase shift between each, to reach the desired total capacity.

This is a bit annoying to do at 1500W, so I don't know that I would bother to do it (probably, I would choose a single phase, or two phase interleaved, full wave, forward converter).  It is the only practical way to scale up arbitrarily.

The fundamental problem with scaling, is what ultimately underlies this argument.  It becomes much more apparent at low voltages: where stray inductances hit so much harder.  Say you're doing a 12V to 1.0V Vcore supply on a motherboard.  You need 100A out, so you're switching pulses of about 100A / N at the input, for N phases.  A hundred amperes in a single stage is just preposterous: that's a 0.12 ohm switching load impedance to begin with, and even doing it at 100kHz, you have to contend with the reactance of even a very compact loop of 5nH being 3mohm, i.e., drawing a reactive power of 2.6% of the total power, give or take.  And that's just at Fsw; basically all the harmonics are going into it as well.  That reactive power is just going to be burned as switching loss, unless you go to lengths to conserve it (quasi-resonant snubber?), and even then, you can't snub very much of it because the snubber itself is going to have about as much stray inductance!

So you need to divide and conquer, and 10A per phase is far more manageable, indeed well enough that Fsw can be pushing 1MHz while switching losses stay quite comfortable.

Finally, the other side is this: why not just build a bunch of inverters and run them in parallel?  Why phase interleave?  The best part is, when phases are interleaved, their ripple currents interfere and partially cancel out.  You can save on total capacitance and filtering this way.

Tim

T3sl4co1l:

--- Quote from: MagicSmoker on September 21, 2019, 09:46:16 am ---The energy stored in the transformer magnetizing and leakage inductances (and any other stray inductance in the loop) is returned to the supply via the body diodes each switching cycle. Even in fast 800V-900V SuperJunction MOSFETs these diodes will have a reverse recovery time in the >300ns range, which is way too slow for operation at 200kHz.

--- End quote ---

Reverse recovery is only relevant in hard switching (CCM).

Flyback is typically operated in DCM, so this doesn't matter (for certain degrees of "matter" -- in the two-switch, the diodes act in series and one will inevitably recover before the other, leaving some recovery charge left in the other one).

Although I suppose you wouldn't be doing DCM at this power level, but you shouldn't be forcing a flyback design at this power level, either.

On a related note, in ZVS (inductive load) configurations (including resonant), body diode recovery occurs during normal load current, while the transistor is on -- this probably lengthens the recovery time (because the recovery voltage drop is small?) but it means most MOSFETs are adequate to a MHz or more, even high voltage Si being adequate at half a MHz or more.

Tim

MagicSmoker:

--- Quote from: T3sl4co1l on September 21, 2019, 10:30:45 am ---Reverse recovery is only relevant in hard switching (CCM).

Flyback is typically operated in DCM, so this doesn't matter (for certain degrees of "matter" -- in the two-switch, the diodes act in series and one will inevitably recover before the other, leaving some recovery charge left in the other one).
--- End quote ---

Err... the OP first said this is a two-switch forward, not a flyback.


--- Quote from: T3sl4co1l on September 21, 2019, 10:30:45 am ---...
On a related note, in ZVS (inductive load) configurations (including resonant), body diode recovery occurs during normal load current, while the transistor is on -- this probably lengthens the recovery time (because the recovery voltage drop is small?) but it means most MOSFETs are adequate to a MHz or more, even high voltage Si being adequate at half a MHz or more.
--- End quote ---

Well, someone hasn't had their coffee yet this morning...  Re-read my previous post.  :P

T3sl4co1l:

--- Quote from: MagicSmoker on September 21, 2019, 11:03:59 am ---Err... the OP first said this is a two-switch forward, not a flyback.

--- End quote ---

Note sure how I internalized it as flyback, but it works out the same on the primary side at least. :P

Transformer utilization is half, relative to full wave; I wouldn't call it insignificant at scale.  There may be many costs much greater than the transformer alone, but in an absolute sense, no.



--- Quote ---Well, someone hasn't had their coffee yet this morning...  Re-read my previous post.  :P

--- End quote ---

Just reiterating it in case it hasn't become crystal clear to the OP :)

Tim

MagicSmoker:

--- Quote from: T3sl4co1l on September 21, 2019, 11:27:59 am ---Note sure how I internalized it as flyback, but it works out the same on the primary side at least. :P

Transformer utilization is half, relative to full wave; I wouldn't call it insignificant at scale.  There may be many costs much greater than the transformer alone, but in an absolute sense, no.
--- End quote ---

Oh, I totally agree that the (hard-switched) two-switch forward is not even remotely the topology of choice here (despite immunity from cross-conduction failure, which is otherwise very compelling), but the penalty in power throughput at >100kHz is not nearly as bad as you might think because max flux swing has to be greatly curtailed to keep core losses under control, anyway. Practically speaking, the advantage in transformer utilization of bipolar vs. unipolar operation at fsw >100kHz is more like 20%, rather than a doubling. Still, one shouldn't be hard-switching 650V at 200kHz, anyway, so rather a moot point.

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