30-11-2016, 12:34 PM
If you put 2 perfect identical diodes in series the PIV would double. But diodes are neither perfect nor identical. The voltage will distribute between them depending on the leakge current. This is minuscule for most silicon diodes but can't be ignored. The less leaky diode will take more of the voltage which in turn will cause it to leak more. I don't know if this process reaches a safe equilibrium or not. As a conservative rule of thumb I'd take the PIV of 2 diodes in series as 50% higher than a single one.
Of course you can fit high value resistors across each diode to swamp the leakage. This may or may not be acceptable, especially as you'd be using high voltage rated parts such as VR37 series. Easier and cheaper to stick a third 1N4007 in series to be sure.
Not sure what happens as the diodes switch off. This is a relatively slow process in diodes like 1N4007 and might cause poor voltage sharing on a transient basis. No idea if this matters.
Air gaps are a totally different thing, especially as the breakdown voltage is ill defined anyway.
Of course you can fit high value resistors across each diode to swamp the leakage. This may or may not be acceptable, especially as you'd be using high voltage rated parts such as VR37 series. Easier and cheaper to stick a third 1N4007 in series to be sure.
Not sure what happens as the diodes switch off. This is a relatively slow process in diodes like 1N4007 and might cause poor voltage sharing on a transient basis. No idea if this matters.
Air gaps are a totally different thing, especially as the breakdown voltage is ill defined anyway.
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
