[Amps] PIV requirement for identical, individual diodes

[Jim Thomson]

Date: Fri, 20 Apr 2012 13:39:14 -0400
From: "Gary Schafer" <garyschafer at comcast.net>
Subject: Re: [Amps] PIV requirement for identical, individual diodes
used in a bridge rectifier configuration?

The PIV across each diode in a bridge will be the peak voltage of the
transformer voltage.

In your case with a 1600 vac rms transformer the peak voltage will be 1.414
x 1600 = 2262 volts peak. Each diode in the string must withstand that
amount of reverse voltage. You need 1.5x to 2x safety margin so each diode
should be rated at 3400 volts or so. You may get by with 3000 volts per
diode but 4000 volts would be better.

Just draw a diode bridge circuit and you can easily figure out how much
voltage is across each diode. Consider that at any given time one side of
the transformer is at ground and the other side is connected to the filter
capacitor. Then look at what diodes are reversed biased.

73
Gary K4FMX

##  In the old days, they would use 2X  for a safety margin.  If you use MOVs
across the incoming 240 vac line, typ they wont fully conduct in some cases till the
peak line V is aprx double.    These days, makers of commercial diode bridge 
assys  will use  3X  for a safety factor.  Since your no load B+ will be 2262 vdc
I would use a bare min of 5 x 6A10  diodes per leg..and pref  7 x diodes per leg.  
6A10 diodes are dirt cheap anyway.   With 5-7 x 6A10 diodes  per leg, 20-28
diodes in total,  you will never blow em up.  In that FWB  config the  diodes only have
a 50 %  duty cycle anyway..even with a key down cxr.   In the above config, the bridge
is good  for 12 A  CCS,  5-7 kv piv  and a 400A surge rating. 

##  In actual testing, an individual  6A10 diode runs warm with 2A  CCS.  An individual
1N5408 runs warm with 1 A CCS.     In the FWB  config above, using  6A10’s, the bridge
is warm with 4 A CCS.    The  same brideg with 1N5408s  will run warm with 2 A  CCS.  

later... Jim  VE7RF