protecting mosfets in UcD

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Z

zilog

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Joined 2005
gnuffel.googlepages.com
A few minutes ago I killed the output devices in my DIY UcD. They died during a swift burst of high output current (10-12A rms sine into resistive load), the mosfets being FDP3682. They often die for me during high power bursts, and I wonder what beside high Ids that might have killed them (I believe my deadtime still is on the high side, and the mosfets turn on slow). My gate drivers dont have any protection against negative or too high Vgs, should I add a zener of 15-20V and an antiparallel schottky to window Vgs?
 
M

MOER

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Joined 2003
FDP3682

Do not use these Mosfets as their tr at 46usec is slow. You need massive dead time and the addition of about 10 ohm gate resistors to protect them. Compare the tr of the IRFB4212 at 7.7usec or the STP14NF10 at 16usec and that is why these fets work much better.
 
Eva

Eva

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Joined 2003
Note that you can't make direct comparisons between switching times of different MOSFET because they are strongly dependent on the gate resistor chosen for the test and not all datasheets follow the same proportion of resistance to gate capacitance.

For example:

FDP3682 is rated with Rg=16 ohms for Cin=1250pF
STP14NF10 is rated with Rg=4.7ohms for Cin=400pF
IRFB4212 is rated with Rg=2.5ohms for Cin=550pF

The latter is obviously going to show the shortest times, and the former the longer. In my opinion the FDP is the most suited part for cass D and the STP the least (taking body diodes into account).
 
Baldin

Baldin

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Joined 2005
sensibleaudio.dk/
It's 46 nano seconds (nS), and can't at all be considered as masive. Think these MOSFETs are ok for a class-d application. Don't think thats the problem.

Are you using a fast anti parallel diodes over DS on the fets? (like ES2D)
Are you using snubbers on the mosfets? (like the 10 Ohm-100pF in the app note)
I think that it could be caused by high spikes.

Is it running hot at idle?

Could it be the drivers not dilivering enough current at high output, and therefore not making a clean switching of the fets, causing through shoot?
 
Eva

Eva

Member
Joined 2003
RX5 said:
Eva,

can you name -other- important parameters in choosing MOSFETs for Class D operation??

Im sure many would benefit from that....:angel:

Regards,
Raff
Click to expand...

Gate drive requirements (total gate charge), body diode reverse recovery charge (sometimes hard to compare due to different test conditions), body diode maximum recovery dV/dt (immediate failure when exceeded), conduction losses...
 
RX5

RX5

Member
Joined 2004
beamx5.solarbotics.net/index.htm
Eva said:


Gate drive requirements (total gate charge), body diode reverse recovery charge (sometimes hard to compare due to different test conditions), body diode maximum recovery dV/dt (immediate failure when exceeded), conduction losses...
Click to expand...


Ah I see....:) I have -them- in my XLS mosfet selections.. thanks...:angel:

one thing I do NOT really understand though is this dV/dt... uhmmmm could you explain this-more-... in laymans term.. I mean, HOw does a device fail?? care to give a common scenario?

:cannotbe:

once again, here it is...
http://www.diyaudio.com/forums/showthread.php?s=&threadid=82447&perpage=10&pagenumber=21

post #208...

thanks :smash:

:edit:- had to find some more info about dV/dt.. found this:

http://www.irf.com/technical-info/appnotes/mosfet.pdf
 
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