[Amps] PIV requirement for identical, individual diodes

[Jim Thomson]

-----Original Message----- 
From: Gary Schafer
Sent: Monday, April 23, 2012 7:36 AM
To: 'Jim Thomson' ; amps at contesting.com
Subject: RE: [Amps] PIV requirement for identical, individual diodes



> -----Original Message-----
> From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com]
> On Behalf Of Jim Thomson
> Sent: Monday, April 23, 2012 6:48 AM
> To: amps at contesting.com
> Subject: [Amps] PIV requirement for identical, individual diodes
>
> 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.
>

> later... Jim  VE7RF

Hi Jim,

But the filter capacitor is ALWAYS connected across the transformer with a
full wave bridge. Any spike from the power line will always be shunted by
the large filter capacitor. No way for a large spike to reverse bias any of
the diodes.

73
Gary  K4FMX

##  good point.  A buddy at work looked after a broadcast station in the 
70's.  Every
couple of years, they would get this big transient-spike in off the 
commercial AC power
line.  It would take out the tubes in the PA !    So it got past the filter 
caps.  At one of the telco's
I worked at up north, we had the commercial AC  3 phase power go screwy  on 
us on day.
It went off-on-off-on-off-on..and all in just 5 secs.    After the event, 
and all power back on,
it took out  about 8 x big dc to dc power supplies, and fried all sorts of 
other stuff.  And we had
really good Josyln giant MOVs  from line to neutral on the main panel.

##  al those dc-dc supplies ran off UPS too.  The strings of batteries are 
all -52 vdc.   The dc-dc
inverters  were -52vdc on the input at 7A...and  put out + 5vdc @ 40A.   The 
strings of batteries  were
charged up with 3 phase rectifiers.  The batteries were fine, and so was the 
large rectifiers.  The DC-DC
supplies at the tail end of all this mess didn't survive. All 8 of em had 
the 40A breaker on the output side
trip open...go figure.

##  One would think that the  filter cap in a HV supply would kill any 
transient..and real quick like, even
with low values of filter C.  Perhaps if the rise time of the  transient or 
spike is fast enough, the cap won't catch it.
Sometimes the hv cap will catch it, and sometimes it wont.   On paper, the 
cap should kill it asap.

Jim  VE7RF