Gate drive resistor question.

Hi Rob,

it certainly could be. if you look at the peak-pulse-power rating of

the IRFBE30 Qg is about 60nC at 12V, so looks like about 5nF. at 47R thats 235ns time constant, which is a lot slower than the controller rise time. so the peak current is pretty much +12V/47R = 0.25A, and

thats not really very much; an 0805 can eat about 1W no problems, so I wouldnt expect this to be a problem. were the gate resistance more like

part of the problem, of course, is that when the FET goes to the great silicon plant in the sky, it shorts D-G and then snots Rg and the controller.....making diagnosis that little bit trickier.

Panasonic make some AWESOME 0603, 0805, 1206 surge rated resistors (they make MMA0204 and MMB0207 look puny). although they are more expensive, they fit in the same footprint, and can be used to remove suspicion from Rg

BTW a 235ns gate time constant is pretty slow, and miller capacitance might be winning the fight against Rg, thereby stretching out the switching time even further, exacerbating switching losses. look at the gate waveform, at the plateau that occurs around Vth. This should be short (< 50ns). If it is not short, the gate drive is too weak. true for both turn on and turn off (although life is easy at turn-on for DCM).

if its a DCM smps, check it has a soft-start, as DCM supplies ALWAYS power up in CCM until the output cap is charged. A useful test is to set up some time delay relays to turn a unit on for, say 10s (enough for it to start running) then off for long enough for all the caps to discharge. repeat ad nauseum. this will pretty soon let you know if there is a start-up issue.

Cheers Terry

Hi there Terry. There is something I forgot to add. There is a reverse diode across the resistor to decreace turn off time. On turn on the Miller capacitance is definately winning. In fact the gate voltage starts to fall for about 2V , never mind level out!! It seems that the gate resistor may well be a potential problem. I'm going to see if I can increace it to a 2010 footprint. As you have mentioned , once things have gone "pear shaped" it's difficult to find the cause of the failure :0( I'll check out if the supply has a decent soft start as well. Thanks for the help. Cheers Robin

Hi Robin,

the diode halves the number of 3W pulses seen by the resistor, and in so doing halves the duty cycle, making life a bit easier for the resistor.

in that case, I'd definitely:

a) look at the panasonic parts (digikey has them) - easier than a PCB re-spin.

(an alternative would be to parallel 5 x 220R 0805, which will happily suck up 3W PPP ad infinitum.)

b) look carefully at the turn-on switching loss. if the gate drive is losing the fight with miller, the turn-on loss (if CCM) will be a lot higher than necessary. if its DCM, then the turn-on loss is

Cheers Terry

Rg

Hi Terry. I'll see if I can improve the turn on time. The fet is not getting to hot though. Even at full power (36W about) it's not getting much above 50deg c , so I'm not sure if it is that serious. There are some other changes that need to be made so a board redesign is probably on the cards anyway. I'm not sure I understand your turn-on loss equasion. Is (0.5*Cstray*Vpk^2*Fsmps) the power dissapated in the FET during switch-on? There is no reference to Rds or current. I think I'm missing the boat here :0(

Cheers Rob.

Oh, OK.

if its DCM, then the current at turn-on is (by definition) zero. BUT when the FET turns on, the D-S capacitance (and xfmr capacitance and any other strays) have to be discharged, by the FET. so the FET dissipates

in CCM, in addition to this capacitive loss, there is 0.5*Vpk*Imin*Tfall energy dumped into FET, where Imin is the CCM current value at turn-on and Tfall is the drain voltage fall time - this assumes the FET current ramps up to Imin before Vds begins to fall. so this switching loss is

while the FET is on, Rdson comes into play, and you get Irms^2*Rdson conduction loss.

and just because the average FET temperature is 50C doesnt mean the junction is OK - ESPECIALLY during the turn-on (and turn-off) transients. Fairchild give thermal models for their FETs (eg FDP047AN08) use one of those and plug the swiching loss into it, you might be (unpleasantly) surprised at how high Tj can get in a short period of time - during switching edges, you can treat the IGBT die as if it were adiabatic, so all the switching energy causes the junction alone to heat, no heat flows into the tab. see "electro-thermal modelling of multi-megawatt power electronic applications using PSPICE" from

Cheers Terry

I'm using the NCP1200A, similar to the 1207.

You may want to check your waveforms at turn off and compare them to page 12 and 13 of this datasheet.

They do suggest using a diode between Vcc and the HV pin in the 1207 datasheet but maybe whoever designed the SMPS was too cheap.

a écrit dans le message de news: snipped-for-privacy@f10g2000hsf.googlegroups.com...

After the 47R resistor, just at the gate, you could shunt the gate to ground with a small high value resistor like 100K. That resistor won't affect the driving in any way. If the 47R fails, at least the gate will be shunted to ground instead of having it floating.

Stranger

nice in theory. in practice kind of a waste of time, as it doesnt take much effort to win an argument against 100k.

you could, however, use 10k + diode + pnp, set up as an active shunt. I'll leave the exact details as an exercise (hint: motorola used to make a chip that did this, back when SMPS controllers could only source current)

Cheers Terry

test your theory by shorting the gate to ground so you overstress the gate resistor....run it like that for a day or two and see if you can cause the resistor to blow open the way you think....if a few samples don't open with a 10x or similar overtest like this, then I would look elsewhere for your problem.

How much margin do you have in the FET dissipation and voltage and current ratings, how hot does it get?

Mark

Thanks for all the tips guys. I'll try running the resistor into a short for a while and see what happens. The fet is not getting to hot at all. 50deg c at full load.

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Interesting , I'll look into this. Could quite easely be an issue. Cheers Rob

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OK,

how about the Drain Souce voltage derating? gate voltage derating? Drain current derating?

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Hi there. Fet's rated to DS-800V Drain voltage never goes above 600V. Ringing is nicely snubbed. GS voltage is the normal +-20V. Never goes above 15V. DS current is 4.1A. Does not go much above 1A.

Cheers Rob