[Amps] HV Supplies and Diodes

[Gary Schafer]

Since all modern diodes have an avalanche region for reverse current
protection, placing resistors across diodes in a string can only make
matters worse. 
Without resistors, current will be the same anywhere in a series circuit so
no reverse current will flow in any one diode unless it flows in all. If
resistors are placed in parallel you force reverse current through all the
resistors. If one resistor happens to be a higher resistance than the
others, or it opens, you then force reverse current through that diode if
the resistance is high enough.

All diodes have capacitance and take time to turn off. In high frequency
switching this can be a problem and capacitors across each diode can help.
At 60 Hz it is not necessary.

The ARRL handbook power supply section says that diode manufacturers
recommend that resistors and capacitors not be placed across a diode string.
All diodes should be of the same type in a string.

Do not mix old and new style diodes.

73
Gary  K4FMX

> -----Original Message-----
> From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] On
> Behalf Of Jim Tonne
> Sent: Wednesday, January 02, 2008 2:20 PM
> To: Dennis W0JX; amps at contesting.com
> Subject: Re: [Amps] HV Supplies and Diodes
> 
> 
> When a string of series-connected diodes is reverse-biased
> (during part of the cycle), the voltage drop across one is
> proportional to its leakage resistance.  If the junction of the
> diode type has significant capacitance, then that enters into
> the picture too.
> 
> If you use, in parallel with each diode, a resistor whose value
> is a fraction of the leakage resistance, then those resistors will
> swamp the leakage and so tend to equalize the voltage drop
> across each diode in a string.
> 
> If you use, in parallel with each diode, a capacitor whose
> value is several times that of the junction, then those capacitors
> will swamp the junctions and again tend to equalize the voltage
> drop across each diode in a string.
> 
> During the forward-conduction part of the cycle of course
> we have no problems with any of the above.
> 
> I am not familiar with the rules on avalanche diodes.
> 
> - Jim WB6BLD
> 
> 
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