- Jan 1, 1970
The RCD snubber seems like a good solution but the problem is the cost
of the diodes needed. I have three high current diodes on the way in
the post and I may be able to spare one of them for that. However
even with that I am having some difficulty getting the values right so
that the snubber can be used multiple times, for instance to have
three 100uS pulses spaced at 100uS. The snubber capacitor finds
itself still charged on the second current disconnection and will not
have much effect. However that is probably something that can be
solved with the right values.
Actually, you may want to consider commutating the snubbers them selves.
Say 10 us after turn off.
The capacitor self discharging is not an option because the pulse
widths and spacing need to settable to arbitrary values controlled by
a pic from some 50 uS all the way up to 5mS, so it is not possible to
tailor component values to fit the timings required. Also, I need the
pulses to have a very sharp cutoff, something that cannot be achieved
if one depends on the capacitor voltage decaying to below the holding
voltage to cut off.
As for current sharing I am currently not doing anything except
paralleling the emitters and collectors of all 24 transistors on two
thick copper busbars. The gates are separated into four groups of six
to reduce to some extent the interaction between the transistors on
turn off. In future I intend to have a 0.25 ohm resistor on the
collector of each transistor to help distribute the current. It will
cause some 15 volts drop at maximum load, but that is a small price to
pay if it does its job. The plans are also for each gate to have its
own optically isolated driver to completely eliminate any interaction
between the gates of the various transistors.
You actually want a few (probably no more than 4) milliohms between the
emitter terminals and the bus bar.
Wild think. The voltages and currents make FET totem pole stacks
difficult. IGBTs have speed and balance related issues. SCRs at that
power level cannot be reasonably commutated fast enough. Have you
considered radar modulator tubes? Or even more common high power UHF