Re: SSTC - Q's

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: herwig.roscher-at-gmx.de 



Shaun,

 > The MOSFET's that are used on the plasmasonic have a "fast body diode"
- The IXFH13N50 I am using have a "fast intrinsic rectifier" also.

 > the internal diode takes a long time to stop conducting when the voltage on
 > the gate decreases.
- In my case the t rr of the diode is 300 ns, while td (off) of the
FET is 100 ns. Therefore I have disabled the body diode by connecting
a large Schottky diode in series with the drain and replaced it with
a very fast MUR 1560.

Some figures:

Taking the output signal of the TL 494 as a reference, the turn-on of
the FET is delayed by 30 ns and the turn-off is delayed by 200 ns. I
will run the system at about 370 kHz. Do these figures sound solid?

 > What happens is the other MOSFET turns on while this body diode is still
 > conducting and then you have a short.
- Unfortunately I did not realize the large difference between turn-
on and turn-off time (it is the first time I am using power MOSFETs).

 > The turn ons are delayed abit the the resistor and parallel shottky
 > combination.
- So the goal is to make the turn-on time more or less equal to the
turn-off time?

Reading that zener diodes connected to the gate directly might cause
oscillations, I have connected a series resistor of 15 Ohms in
between. This resistor should increase the turn-on time as well. In
order not to make the turn-off time longer, I have connected a
Schottky diode across.

BTW: Instead of two back-to-back zener diodes I have used a
bidirectional suppressor diode.

 > I thought of the voltage spikes too, so I made four snubbers to go
 > across the Source to Drain of each MOSFET.  51ohms resistor in series
 > with 1nF cap.
- I have tested your solution and found that the ringing caused by
the wire wound resistor, I used as a load, nearly disappeared! Thank
you!

 > The plasmasonic MOSFETS are 10 nF capacitance each
- C iss of my power MOSFETs is 2800 pf typ.

Regards,

Herwig



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