SSTC - Qs

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


List,

I am in the middle of building my first SSTC. It will be very similar
to the PlasmaSonic II of Dan McCauley (Dan, thank you very much for
providing all the details and especially for your step-by-step-set-up
manual).

Scoping the waveform at the gate of the power MOSFETs confused me
somehow. Trying to understand it, I read AN-937 of International
Rectifier. On page 7 it is stated: "....to short out the dc bus. For
this reason it may be necessary to slow down the turn-on of the
HEXFET while leaving the turn-off as fast as practical.....".

IMO any slow down, whether on the turn-on or on the turn-off slope,
will increase the losses. Fast turn-off will create huge ringing if
the load is inductive. These very large spikes at the drain could
cause a dv/dt induced turn-on.

Q: Why it is not recommended to slow down turn-off instead of turn-
on?

Another means to avoid shot-through could be sensing the current in
the source of the low-side MOSFETs. After pulse shaping this signal
with a Schmitt trigger it could be used to enable the driver of the
opposite bridge branch.

Q: Did somebody try this?

Pulse transformers are said to be used for duty cycles between 35 and
65 % best, because of the Volt-seconds problem. Low duty cycles will
cause problems therefore.

Q: Has somebody used high-side drivers e.g. optocouplers?

BTW: I have wound two different GDTs using the same type of toroid
material. One was made of a very close twisted trifilar winding of
AWG 27 and the other was made of a trifilar winding of PVC insulated
stranded AWG 23 wires. Connecting each of the secondary windings to a
dummy load of a 1 kOhm resistor in parallel with a 3300 pF capacitor
I didn`t find any distinct difference in waveshape or peak voltage.
Because of the large voltage difference between the windings I am
using the PVC insulated wire now.

Lastly my driver chips TC 4422 are getting hot quickly and I solved
this problem by using a heatsink.

Thank you for your attention.

Herwig