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Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
so Ash, if we have multiple series'ed diodes to get upto our HV, then we should have some nH in stray incductance hanging around. if we lay up a simple RLCD || TVS then we would have scalabe module that could be tested...
you could put these modules together, pretty long. there would be 3 discrete componets, the resistor, diode and TVS, the induction and capacitence would be stray, from the PCB.
edit: i meant to put the resistor in parallel, as a volteg divider with 500k or so... ugh!
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Diode reverse recovery is quite simple. When the diode has been conducting, the junction is left full of electrons and holes. When the voltage across it reverses, it keeps on conducting in the "wrong" direction until these charge carriers are cleared out.
That is why reverse recovery behaviour is quoted as a charge, because you have to get a fixed amount of charge cleared out to turn the diode off. You can do it as a huge spike of current, or a smaller current for a longer time.
The amount of charge is not fixed though, it depends on how much forward current the diode was conducting in the few microseconds before turn-off.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Steve Conner wrote ...
Diode reverse recovery is quite simple. When the diode has been conducting, the junction is left full of electrons and holes. When the voltage across it reverses, it keeps on conducting in the "wrong" direction until these charge carriers are cleared out.
That is why reverse recovery behaviour is quoted as a charge, because you have to get a fixed amount of charge cleared out to turn the diode off. You can do it as a huge spike of current, or a smaller current for a longer time.
The amount of charge is not fixed though, it depends on how much forward current the diode was conducting in the few microseconds before turn-off.
so the reverse recovery can be treated as resistive heat loss after the blocking moment is complete?
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Patrick wrote ...
so the reverse recovery can be treated as resistive heat loss after the blocking moment is complete?
I worked this all out in a thread about a year ago, at least as far as how NPN transistors work, and a diode is a transistor with the base and emitter connected.
You have to upset the equilibrium that exists after the junction is formed by removing a few valence band electrons from the collector side of the junction before electrons will start to cross the junction, and until those valence band electrons are replaced, current can flow.
Of course, in reality, they are constantly being replaced and others are being removed, but this replacement process takes time, due to the random nature of electron movement/energy, etc.
Most electrons remain in the conduction layer, rather than dropping into the valence layer, due to energy levels, etc.
So called 'hole theory' is the mathematical model that best describes it.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Yes and no. For the first part of the recovery, the diode looks like a dead short. If the diode is supposed to be handing over conduction to a transistor, this can cause high current and resistive losses in the transistor. This is a common problem in boost converters and bridges driving loads with a leading power factor.
What happens next depends on the diode. Modern "soft recovery" ones have a smoothly damped turn-off that causes resistive losses in the diode itself. However some older fast recovery diodes would snap off pretty much instantaneously. Gold-doped signal diodes like the 1N4148 still do this, the turn-off is so fast that they can generate energy up to hundreds of MHz. This behaviour is deliberately enhanced in the "step recovery diode" which is used as a pulse generator.
Schottky diodes have no reverse recovery charge at all. They do have junction capacitance though, which can cause similar problems.
Tube rectifiers essentially behave the same as Schottkies.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
heres what i really meant before my brain stalled above...
R= 10M ohm, for voltage balancing with its seriesed peers. C= stray, keep small. L= stray, keep small. TVS= breakover at 800v? (i should have drawn this as unidirectional right?, since theres another diode. ) D= HER108 (1kV, 1 amp) or spec'ed better as needed.
EDIT: i think they raised the price on the HER108 since i bought my 750 pieces. there now 14.9 cents.
Then build these out in iterations, possible in oil or epoxy, i think 600 V per module would be reliable, and resistant to blow out.
My interest in all this is for high effciency EHV switch mode power supplies using ferrite transformers at 100-300kHz. But they would require full wave rectification.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Steve Conner wrote ...
Yes and no. For the first part of the recovery, the diode looks like a dead short. If the diode is supposed to be handing over conduction to a transistor, this can cause high current and resistive losses in the transistor. This is a common problem in boost converters and bridges driving loads with a leading power factor.
What happens next depends on the diode. Modern "soft recovery" ones have a smoothly damped turn-off that causes resistive losses in the diode itself. However some older fast recovery diodes would snap off pretty much instantaneously. Gold-doped signal diodes like the 1N4148 still do this, the turn-off is so fast that they can generate energy up to hundreds of MHz. This behaviour is deliberately enhanced in the "step recovery diode" which is used as a pulse generator.
Schottky diodes have no reverse recovery charge at all. They do have junction capacitance though, which can cause similar problems.
Tube rectifiers essentially behave the same as Schottkies.
I imagine the reason why Schottky diodes, which have junctions formed from a metal and a semi-conductor, and gold doped diodes have such short reverse recovery times is due to the metals in the junction, or more specifically, the differences in the valence band electrons associated with the metal atoms.
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Do you know any good HV diodes that perfrom well at upper KHz - lower MHz rf frequencies?
