RE: Diode Strings

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

Original poster: "Malcolm Watts" <m.j.watts-at-massey.ac.nz> 

Perhaps I might be permitted to expand on this slightly:

On 20 Aug 2004, at 7:42, Tesla list wrote:

 > Original poster: "Steve Conner" <steve.conner-at-optosci-dot-com>
 >
 >  >Yes, the PIV is 40 KV.  But it will be distributed across
 >  >two legs of a bridge
 >
 > I have seen quite a few people mention this now so I couldn't let it
 > pass. it's wrong!
 >
 > The reverse voltage seen by any diode (or diode stack) in a bridge
 > rectifier is equal to the DC output voltage, ie 1.4 times the RMS AC
 > input voltage. The reason is that when one diode is conducting, the
 > voltage across it is "negligible" therefore the whole DC output
 > voltage must appear across the other diode on that side of the bridge.
 > (If you draw the diagram you can see that the two diodes are in series
 > across the DC bus.)
 >
 > So if you're making a rectifier to have 40kV DC output, your diodes
 > must be rated at least 40kV PIV each. Preferably 1.5 to 2 times more
 > for safety.
 >
 > Steve C.

It actually depends on whether the bridge is feeding a capacitor or
not which, if it is, the above arguments rigidly apply since the
reversed diodes have to hold off the voltage across the capacitor
plus the peak voltage of the transformer.

Malcolm