RE: Disabling the freewheling diode

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

Original poster: "Jan Wagner" <jwagner-at-cc.hut.fi> 

Hi!

 > In a SSTC H-bridge, the internal freewheeling diode of the
 > MOSFET is disabled by a clever arrangement of external
 > diodes. A schottkey diode above the drain keeps the internal
 > diode from getting forward biased, so that an ultrafast
 > recovery external diode can provide the path of any reverse
 > EMF from the load. Any current passing trough the MOSFET must
 > pass trough the schottkey diode.
 >
 > In a typical design, I see MOSFETS capable of handling 20A
 > continous, with a 3 A schottkey diode on top.

(the diode can be at the source or drain pin, symmetric...)

 > Why is the schottkey diode not dimentioned for a similar 20A?

Because luckily it conducts only for a short period. If the SSTC is in-tune 
and the drive signals have ~50% duty, the diode ideally
conducts no current at all. When the SSTC is really grossly out of tune 
i.e. pri current draw and the drive voltage are highly out
of phase, almost no current will be flowing into the primary, and also the 
diode conducts very little current. When slightly out of
tune, the diode will conduct the (sine shaped) current close to where the 
sine waveform crosses zero, so, again, it's not very hard
on the diodes as the average current is quite low.

Only when you're doing PWM, i.e. changing the duty cycle of the drive 
signals so that the duty t.ex. averages at 25%, the diodes
will conduct a large current - in the worst case, the peak primary current 
- and have to be rated the same as the mosfets (or, hmm,
probably only half the current rating of mosfets... (?))

cheers,
  - Jan

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