Solving Severe destructive reverse recovery in LLC converter

[cupoftea]

Pages 13, 14 and 15 of the following show that in typical LLC converters using typical offtheshelf LLC controller IC’s, capacitive mode destruction (due to severely ruinous reverse recovery current) simply cannot be avoided….

https://www.infineon.com/dgdl/Infin...N.pdf?fileId=5546d4625b3ca4ec015b3e58e44f1048

The way to avoid it, as they say, is to put in a current transformer, and measure the phase shift between the resonant capacitor current, and the MOSFET gate signals. As soon as the phase gets to zero, then the “next” MOSFET should not be switched ON, and the currently ON MOSFET should be switched OFF.

Another way to solve the problem of severe reverse recovery though, is simply to add low Vf diodes in series with the FETs, and SiC diodes in pllel to both fet and low Vf diode, as in the attached, would you agree?
(LTspice and JPG schem attached)

Another way woudl be to use SiC FETs....i wonder why the app note didnt suggest this?

 

[cupoftea]

The only other way to avoid “Disastrous reverse recovery” [DRR] in an LLC is to use normal FETs, but switch at fixed frequency of the upper resonant frequency, and simply use on/off control, or burst mode, to regulate the output voltage….with switching at the fixed frequency, you simply never see the sudden change in switching frequency which can result in DRR....do you agree?

 

[cupoftea]

On pages 13,14,15, they are measuring to see when the resonant current in the primary goes through zero at the same time as the FET switches ON........then they are saying that it is in capacitive mode and the next FET switch on event shoudl be stopped......talk about break-neck timing.......they are saying that when they have found out that the "Next" fet shoudl not be switched on......they have less than the dead time available to make the decision............that for me, is too tight...and not really possible......you're talking maybe 100ns to do a time measurement, assess that measurement, then act......its theoretically possible but in practice?....i think best just to use SiC FETs?

 

[dick_freebird]

Not clear to me that reverse recovery per se is the problem. Activating the parasitic BJT by drain dV/dt is another and potentially driven by control of power switch phasing. Cross conduction of not-ideally-driven devices is another.

I'd want some bench and lab failure analysis before I pinned the tail on a particular donkey.

Your SiC FETs will likely not have as "good" a BJT under them (to be held off by rBS, against Cdb*dV/dt, a foot-race) and more tolerant of heat spikes. But I believe that SiC has reliability effects still to be discovered.

 

[cupoftea]

As such....is the attached an excellent way of avoiding damage from reverse recovery in offline LLC converters?
A Low Vf, Standard, Si Diode is simply placed in series with the Si FET, so as to stop its intrinsic diode from ever conducting.......another diode (SiC) then conducts the "freewheel", backflow current as shown.
Problem solved?......even go into the capacitive region and no damage from reverse recovery will ever occur?

(LTspice and jpeg attached, should you wish)

BTW, adding the said diodes is unlikely to impact efficiency...(even if it did , it would be only 0.5% max).....but actualy it wont impact efficiency at all.....because using these diodes means you can use the lowest rds(on) FET that you can find.......this is not usually allowed with LLC converters, as its usually wanted to use a fet with high enough rds(on) such that the current flow through it puts enough reverse voltage across the intrinsic diode to sweep out its minority carriers...............but when you use the diodes, this no longer applies.

 

[Easy peasy]

This approach to solving the dv/dt issue in mosfets has been around since power mosfets were first used in aggressive switching - it is a valid approach for - as you say - using the largest mosfet you can lay your hands on - although this has now been superseded by the low RDSon of SiC.

 

[cupoftea]

[EDIT]....just seen Easy Peasy's kindly sent reply above after i posted...thankyou for this....yes, though i am still struggling to find low rdson SiC...specially at low price......with SiC......the reasonably priced ones all seem to need at least 15V Vgs, and some suffer very high VDS_on rise at the higher currents............the good low rdson SiC ones ive seen are pretty well out of our price range.....and they need the uncomfortably high vgs drive.

Also, i seem to have suffered more FET loss with Sic due to ESD damage.....i dont know if its me...but SiC FETs seem to die from ESD a lot easier....could just be me though....i think with SiC, the high Vgs, is right up by the Vgs(max), and so they dont bother putting protection zeners in the SiC FETs...and then they die from ESD... at least thats how it seems. Whereas Si FETs have vgs(max) right up at 30V, so they can safely add a zener into the FET....and solve ESD in the same shot.
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Hi,
The attached LTspice simulation shows how to totally solve reverse recovery problems in the LLC converter! It shows an LLC with a series diode and parallel SiC diode…..and as a comparison an LLC with just normal Si FETs in the leg.

The switching frequency is taken down into the capacitive region ……the “series diode LLC” suffers absolutely zero diode reverse recovery!!!.......you can see that there are high current spikes , but doing the calculation, you can see that these spikes correspond only to what you expect to see due to the sudden discharge of the FET stray capacitance….not reverse recovery.

Whereas the LLC with no series diode suffers from quite utterly horrendous reverse recovery current spiking…FET killers !!!...how many of those reverse recovery spikes does it take to kill a FET?....probably just one!.........and even if the FET doesn’t die…it will be weakened such that it will soon die.

Maybe it is thought we can simply avoid the capacitive region by setting a minimum switching frequency……but this only works if your load never deviates from maximum….eg overload or short or startup, or restart, or return from temporary brownout etc etc…..when that happens the LLC resonant frequencies change such that our “minimum frequency” is no longer well set…..and we stray into the capacitive region anyway……..i wonder does anyone yet believe that they can detect the first LLC hard switch of a series of them?…..ie, detect it before it actually it happens…so you can avoid switching on at all…..i very much doubt anyone has done this….though there is an Infineon App note where work toward that has been done...maybe they succeeded?

But anyway…you can eradicate the problem by using the series standard diode as shown here (nice low Vf) and parallel SiC diode. And as a bonus, you get to pick the lowest rds(on) FET that you like!!!...no more worrying that the rdson isn’t going to be big enough to raise vds sufficiently during ON time so as to sweep out minority carriers in the intrinsic diode……the intrinsic diode is not going to conduct because your series diode has blocked it!

You now have an LLC converter, that, as long as you have current sense transformer/comparator primary side, is just as tame as a little DCDC buck converter!

And you loose no efficiency…because you can now choose the lowest rdson FET.

 

[Easy peasy]

IXYS mosfets ( now part of littlefuse) have the required dv/dt rating to survive aggressive switching in ZVS converters that ( temporarily ) lose ZVS, adaptive gate drive can be made to serve here too - as in a softer turn on when ZVS is lost ....

generally >= 50V/nS capability for an OFF mosfet will keep one safe

 

[cupoftea]

Thanks, regarding the solution with the pair of diodes to "take out" the fet intrinsic diode..
...is the actual reverse recovery of the added-in series diode an issue at all?.....i presume it still needs to be reverse recovered fairly promptly?.
Also, i must confess i have been looking for articles on this, but found none, which surprises me.

 

[Easy peasy]

often a schottky is used in the source ( lower drop ) and an ultra-fast but soft diode used as the anti-parallel, since this diode is not ON for very long at all it can be sized down a bit ( except for real inverters ).

 

[cupoftea]

Thanks, also , as the attached LTspice shows, the voltage across the series diode can be greatly reduced by putting 100pF to 1nF across it...though i suppose its part of a capacitive divider with the fet CDS's....also, i am reluctant to put a cap across it, as it provides a bit of a conduction path for the dreaded intrinsic diode.

 

[Easy peasy]

you cannot reduce the the static on drop of a diode by putting a cap across it.

 

[cupoftea]

Thanks, yes, though my apologoes, as i was referring to the off state voltage on the diode.

 

[cupoftea]

IXYS mosfets ( now part of littlefuse) have the required dv/dt rating to survive aggressive switching in ZVS converters that ( temporarily ) lose ZVS, adaptive gate drive can be made to serve here too - as in a softer turn on when ZVS is lost ....

...Thanks, and it makes you wonder if the converter reverts to softer_turn_on just before or just_after the LLC has lost ZVS....
...Because AYK, a single instance of big reverse recovery in an LLC can kill it or severely weaken it.

generally >= 50V/nS capability for an OFF mosfet will keep one safe

...Thanks, sorry my muddled mind is reading ">=" as "Faster turn off".....which is presumably what was meant?..ie, the FET is needed to be turned off quickly/quicker...(?)

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It seems odd that no Application Note, or article, anywhere on the web, discusses the herewith_discussed use of a series diode (plus sic antiparallel diode) with a LLC converter...

https://www.st.com/resource/en/appl...t-halfbridge-converter-stmicroelectronics.pdf
https://www.infineon.com/dgdl/Appli...n.pdf?fileId=db3a30433a047ba0013a4a60e3be64a1
https://www.monolithicpower.com/en/...Note/lang/en/sku/HR1000AGS-Z/document_id/5926
https://www.onsemi.com/pub/collateral/and90061-d.pdf
https://vtechworks.lib.vt.edu/bitstream/handle/10919/28982/Ch4.pdf
https://www.power.com/sites/default/files/documents/an55.pdf
https://ww1.microchip.com/downloads...cationNotes/ApplicationNotes/LLC_Appnotes.pdf
https://www.monolithicpower.com/en/...600w-llc-converter-for-a-pc-power-supply.html
https://psim.powersimtech.com/hubfs...er for 3.3-kW On-Board EV Charger (AN003).pdf

...Nobody mentions it......which makes you wonder how they are all avoiding severe reverse recovery?

The only mention of it is Infineon, in one App Note, where they claim to have software which can detect severe reverse recovery just before its about to happen....and then allows you to not switch the fets on, so that you can totally avoid it......that, realistically , is the only other way to avoid severe reverse recovery in the LLC if you dont use the series (& antiparallel) diodes.

Seems strange its not discussed anywhere though.......specially since one single instance of severe reverse recovery can severely weaken or kill the LLC converter.
And it seems strange that Infineon aren't talking up their software detection of Reverse_recovery_imminent.....if they have really got software that does that...then it is surely the biggest revolution in SMPS since a long time....why so schtum?

 

[cupoftea]

AYK, there are many events that bring about the dreaded reverse recovery in the LLC......so thats overload, short cct output....undervoltage input.....startup, re-start-up, heavy load transients, and some more. Also, since even one single instance of the reverse recovery in an LLC can kill or seriously weaken the converter, the detection needs to detect the disaster before it happens, and then stop switching the fets so it doesnt happen..........to detect this (imminent_reverse_recovery) in that timescale is extremely challenging.......as discussed, infineon claims to have LLC circuitry and software that can do that ( i think they do?)...........but if it works, then it is truly amazing........without that software based proteciton, it needs the series diode to be sure of not getting reverse recovery based destruction.

But yes, and its literally amazing that the App Notes and semico's dont talk of this.......touting LLC controllers without mentioning it isn't a great idea...some of the LLC App Notes work through Ohms Law calcs with you......but then dont bother to tell you about the reverse recovery issue.

And if infineon really do have that avoid_reverse_recovery software....then the Nobel Prize wouldnt be enough for them!

This approach to solving the dv/dt issue in mosfets has been around since power mosfets were first used in aggressive switching -

Thanks yes...
..AYK, its an old technique, but i believe needs re-introduction and re-birthing, due to the rising popularity of the "current sloshing" converters like the LLC....AYK, "current sloshers" slosh the primary current about so that the fet diode comes on just before the fet switchs on........others in the same category, AYK, are Phase shift full bridge and the "Half_bridge_with_big_leakage_inductance_but_no_output_inductor"...aswell as one variety of the forward converter if the leakage L in the txfmr is big enough. (Active clamp forward).

AYK, the problem with "current sloshing", is that it can get "out of phase", and the diode can get sloshed on just as the opposite fet is turned ON....literally one single instance of this can kill or seriously weaken the converter....thats why its so difficult to avoid if you dont use the series diode.
...and its literally amazing that the App Notes and semico's dont talk of this.......touting LLC controllers without mentioning it isn't a great idea...some of the LLC App Notes work through Ohms Law calcs with you......but then dont bother to tell you about the reverse recovery issue.

And if infineon really do have that avoid_reverse_recovery software....then the Nobel Prize wouldnt be enough for them!
...or even a mention on BBC's "click" program!!