[Amps] caps across rectifier diodes

[Steve Katz]

-----Original Message-----
From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com]On
Behalf Of Ian White GM3SEK
Sent: Tuesday, March 03, 2009 8:13 AM
To: amps at contesting.com
Subject: Re: [Amps] caps across rectifier diodes


Steve Katz wrote:
>I sure agree with that.  I use high quality avalanche rectifiers and 
>never use capacitors across them.
>
>We built the HV rectifier for the targeting RADAR systems for the 
>Patriot and Patriot II missiles for many years and the silly Army spec 
>included capacitors across the rectifiers.  They served no purpose 
>whatever except to add cost and reduce reliability, so we called the 
>assembly a "rectifier compensated capacitor stack."  In the field, the 
>only things that ever failed, out of more than 30,000 assemblies 
>shipped, were the capacitors themselves.  The MTBF of the rectifiers 
>was essentially infinity, but the assembly MTBF was much smaller due to 
>the capacitors.
>

How much were you over-designing on the nominal PIV?

::The stacks were 20kV at 1.2A each.  We used 24 ea. 1000V avalanche rectifiers in series, each having a .125" round die rated 3A @ 55C with IFSM rating 300A.  The devices avalanched at ~1400V each and of course were all from the same wafer (diffusion) lot.  They *never* failed, but the capacitors would.  They were ceramic X7R barium titanate 3000V 5% tolerance capacitors, all mil-spec.  They failed anyway.  The rectifiers could handle huge reverse energy dumps (such as you get from a rapidly collapsing field, powering a magnetron), much more than the capacitors could.  Also, silicon's cheap!  -WB2WIK/6



-- 

73 from Ian GM3SEK
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