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Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Herr Zapp wrote ... .
Also, can anyone provide a clear description of the function of Q702 and its associated components?
Regards, Herr zapp
I think Q702 is configured as a flyback or snubber diode. I'd guess that all those parallel groups of resistors of equal value are there either because it's cheaper to have two or three in parallel to achieve a given value of one single one of higher wattage, or to distribute the heat over a larger area of the board.
Registered Member #480
Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644
Harry -
As noted in the pdf document at Ander's link, this "appears" to be used as a some sort of a freewheeling circuit. But IGBT Q702 has an integral body diode, which would appear to be all that's required for a freewheeling function. I should have more clearly asked, is IGBT Q702 actually being "turned on" periodically through its gate? If so, exactly what happens when it conducts? And, when does Q702's body diode conduct?
It would interesting to know what the turn-on voltage is for the series-connected Zeners ZD703 & ZD704.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Herr Zapp wrote ...
Harry -
As noted in the pdf document at Ander's link, this "appears" to be used as a some sort of a freewheeling circuit. But IGBT Q702 has an integral body diode, which would appear to be all that's required for a freewheeling function. I should have more clearly asked, is IGBT Q702 actually being "turned on" periodically through its gate? If so, exactly what happens when it conducts? And, when does Q702's body diode conduct?
It would interesting to know what the turn-on voltage is for the series-connected Zeners ZD703 & ZD704.
Regards, Herr Zapp
Hr Zapp: As with the paralled resistors being used for, I speculate, the sake of some economy, to achieve a lower value of higher wattage, I think the series zeners are in series simply to achieve some higher (more expensive) value. Considering these cheese-paring economy measures ("made to a price") it may be that the body diode is a completely irrelevant, redundant circuit element, there only because the diode came with the cheapest MOSFET that Panasonic could buy in bulk.
However, we have already seen that there are errors of notation in the circuit diagram, and it is not impossible that the 'series' zeners are in fact zeners-back-to-back - where we know of one error, there are probably others that we havn't yet spotted.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
I discussed this with Anders a while back. I believe the topology is basically a single-switch converter, and the top IGBT is some kind of lossless snubber.
The MO inverter is so optimized for its application that I'd personally be scared to try and use it for anything else, like running a valve PA. I remember Ralph Hartwell built a high-voltage, high power SMPS to power a Heathkit 1kW linear amp, and I'd make something based on his design from scratch, sooner than try to modify a MO inverter.
The only thing I might change about Ralph's design is to use a SLR or current-fed topology, to do away with the high-voltage output filter choke and its troublesome resonances.
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Microwave Oven Inverter
