-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com]On
Behalf Of Ian White GM3SEK
Sent: Tuesday, March 03, 2009 8:13 AM
To: amps@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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