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Why "Fast Intrinsic Diode" MOSFETs do not have full specs in the datasheets?

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Miyuki:
I just wonder why no manufacturer has detailed characteristics of reverse recovery of Intrinsic Diode?
They all have just Qrr at rather slow 100 A/µs and just a few have even IRRM
No specification on faster rates or current and temperature dependence.

Is there a way to guess values on 500 and 1000 A/µs from just that single point? I guess not.

I need to choose transistors to synchronous buck

T3sl4co1l:
Probably not very worthwhile, or follows similar patterns to other diodes.

Though the latter seems suspicious, as the characteristics are notably different -- for one, they're slower than optimized diodes are, but the forward recovery is also suspiciously low (which seems strange given the slow t_rr).  And in my measurements it doesn't seem attributable to the fact that, if Vgs = 0, the channel conducts when Vds < Vgs(th): this should act to clamp the forward recovery voltage but not shorten or eliminate it, whereas it seems to simply be absent.

So, aside from some background information, I'm afraid I don't have much to help with the stated problem.

I can offer advice on avoiding the stated problem entirely.

The body is never forward-biased when synchronously rectifying (that is to say: there is not even any point in doing it, if Iout * Rds(on) > Vf so that body conduction occurs), so this information is only needed during commutation: when the rectifier turns off, before the opposing switch turns on.

So, simple enough: don't do that.

Set dead time to zero, so that the switches are either exact, or even slightly overlapping (negative deadtime, shoot-through).

Doesn't this fuck everything up?  Take a closer look at the system.  Where does the current flow?  "Short" current flows in the switching loop.  If we know the inductance of that loop, then the current flow is not simply "short circuit current", but a ramp at dI/dt = V/L.  If dt is small say 20ns, and L is modest say 50nH, and let's say supply is 30V, we'll have dI = 12A.  Maybe quite a lot for a small converter, but if we're doing more like 50A output, we won't really miss it, will we?

The energy absorbed by the switching loop, still needs to go somewhere, so we need some manner of clamping or loss to deal with it.  50nH is high enough that a clamp snubber is feasible; simply design it for maximum peak voltage at maximum load current plus commutation current (the dI above).  For the above case, say we're using 60V transistors and a maximum 40V input so a nominal-max 20V overshoot is acceptable.  The snubber must have less than 8nH loop inductance to the transistor (which is feasible for SMTs; TO-220 would blow this handily, however!), and an RC of < 0.4Ω and C > 0.3uF, or an RCD clamp with C >> 0.3uF.  The maximum charge (of the loop inductance) is 50 + 12A so the energy is 96uJ; at 200kHz this is 19W dissipated in the resistor.  (For the clamp case, this power can be "stirred" back into the supply using a buck-boost converter; it's not usually worth the bother, though.)

I have no idea if these values are at all representative to what you have in mind, but the relations are there in any case.


If you're using a controller, unfortunately you likely have no way to optimize its timing (dead time).  Worse still, its timing variance may be too gross to be able to do a good job with external compensation (which can be done with RCD networks to give asymmetrical delays, and external gate drivers if sharp gate edges are still required).  That is to say, maybe you can adjust one unit to an acceptable balance, but how well will that hold in production?  How far was your prototype from "typ", and what is the real variance in that spec?  (At least, the way it's usually done is, I think, +/- 3σ as a truncated normal distribution, so you're pretty unlikely to get a truly bad one.  Still, that's some minimum amount of production failures that must be discarded or reworked, and definitely not great if a single board has more than a few channels, making it more likely that at least one controller has poor timing.)

Tim

Miyuki:
I have 50V input with up to 50A output (10-20V) and would like to have a switching frequency of at least 100kHz
And use 100V transistors as they will be THT and some inductance is desired in the loop
I have a digital controller, so can adjust dead time even negative

I'm considering using something like STP310N10F7 on the low side
It might be little big for this current but has at ISD=180 A, di/dt = 100 A/µs just 200 nC and IRRM 4.7 A
If it will scale similarly as a diode (STTH8R06 600V Ultrafast Diode) then at ISD=50 A, di/dt = 1000 A/µs it can be around 15A and 600 nC
But every diode behaves significantly different some has a linear increase of IRRM and Charge, some are more like exponential  :-//

Zero deadtime solution sounds interesting, just makes me nervous, about how it will behave under different loads, as at this currents and with TO-220 or similar packages, the current has a significant effect on switching behavior

Miyuki:
Oh unexpected happened
Alpha & Omega Semiconductor have on some obsolete (not recent ones  :-//) small charts showing behavior

It is for 6.5mΩ 100V one
Not a best chart, but better than nothing

David Hess:
Keep in mind that the body diode is the collector-base junction of the parasitic bipolar transistor.  They might not bother listing its specifications at extreme dI/dT where destructive operation will occur anyway.

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