Why "Fast Intrinsic Diode" MOSFETs do not have full specs in the datasheets? - Page 1
Forum section: electronics / projects

author: Miyuki
author_rank: Frequent Contributor
timestamp: October 24, 2021, 02:20:59 pm
content: 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 IRRMNo 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

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 24, 2021, 03:26:17 pm
content: 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

author: Miyuki
author_rank: Frequent Contributor
timestamp: October 24, 2021, 06:00:40 pm
content: I have 50V input with up to 50A output (10-20V) and would like to have a switching frequency of at least 100kHzAnd use 100V transistors as they will be THT and some inductance is desired in the loopI have a digital controller, so can adjust dead time even negativeI'm considering using something like STP310N10F7 on the low sideIt might be little big for this current but has at ISD=180 A, di/dt = 100 A/µs just 200 nC and IRRM4.7 AIf 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 nCBut every diode behaves significantly different some has a linear increase of IRRMand Charge, some are more like exponentialZero 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

author: Miyuki
author_rank: Frequent Contributor
timestamp: October 25, 2021, 04:38:36 am
content: Oh unexpected happenedAlpha & Omega Semiconductor have on some obsolete (not recent ones) small charts showing behaviorIt is for 6.5mΩ 100V oneNot a best chart, but better than nothing

author: David Hess
author_rank: Super Contributor
timestamp: October 25, 2021, 11:08:38 pm
content: 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.

author: Miyuki
author_rank: Frequent Contributor
timestamp: October 26, 2021, 06:48:06 am
content: Quote from: David Hess on October 25, 2021, 11:08:38 pmWith high voltage superjunction it is an issue but with low voltage ones advertised for synchronous rectifier? Parasitics won't let them beyond destructive speeds.

author: David Hess
author_rank: Super Contributor
timestamp: October 27, 2021, 04:01:27 am
content: Quote from: Miyuki on October 26, 2021, 06:48:06 amAs far as I know it is still an issue, but modern MOSFETs have high enough dI/dT and dV/dT ratings that it would be an unusual circuit that had problems so it is not something I would worry about except in the highest performance applications. Synchronous rectifier applications also have two additional related failure modes.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 27, 2021, 09:47:55 am
content: If the diode is slow and the dead time in which the diode is actually used is short (especially shorter than trr) it does not really matter how fast or slow the diode is. The current that flows in forward direction is stored in minority carriers and when reversed nearly all the charnge can also flow back. For a short time the diode would more behave like a slighlty leaky capacitor, with this time the leakage part would be the good part.No matter how slow the diode, you don't get more reverse charge than you had forward charge flowing before.At such high current I would consider at least a 2 phase system. It needs more control effort and slightly more magnetics, but can save on the capacitors. The parasitics also get easier with smaller parts.

author: Miyuki
author_rank: Frequent Contributor
timestamp: October 27, 2021, 07:19:21 pm
content: Quote from: Kleinstein on October 27, 2021, 09:47:55 amThat is a good point. Taking it as a capacitor makes it "simple".I plan to use film capacitors so they are not an issueBut might add the second phase for more current capability as used magnetics is at its limits.Quote from: David Hess on October 27, 2021, 04:01:27 amAs far as I know, they enter avalanche mode at reverse recovery so energy and maximal avalanche current can be an issue. When staying within limits there shall be no issue.I looked at AOS papers where they state cannot reach dV/dT breakdown even at 8000 A/us when recovery reached 40 V/ns with 100V transistor. The only limiting factor is maximal voltage.

author: David Hess
author_rank: Super Contributor
timestamp: October 27, 2021, 09:22:54 pm
content: Quote from: Miyuki on October 27, 2021, 07:19:21 pmThere are different mechanisms at work, but the main danger is secondary breakdown of the parasitic bipolar transistor included in the MOSFET structure, of which the collector-base junction is the body diode. Forward biasing the shorted base-emitter junction is the obvious one which will turn the transistor on, but the collector-base junction can also be forward biased causing reverse current gain through the emitter and secondary breakdown in reverse, which is where forward biasing the body diode during synchronous rectification can become a problem. Presumably no modern power MOSFETs suffer from this but I would not count on it in a sufficiently extreme application.The Siliconix MOSPOWER Applications book has detailed descriptions of the 4 different failure modes they originally faced and how they ameliorated them.