-----Original Message-----
From: Larry Molitor <w7iuv@nis4u.com>
To: Steve Thompson <rfamps@ic24.net>; 'amps' <amps@contesting.com>
To: <amps@contesting.com>
Date: 04 February 2000 14:36
Subject: Re: [AMPS] Power Supply Diodes
>At 03:32 PM 2/4/00 -0800, Steve Thompson wrote:
>
>>Can't claim practical experience, but I did run a Spice simulation. I
>>assumed the primary R effect doubled the secondary Rs, and I assumed about
>>4.5kV dc (didn't keep a copy of your original post).
>>
>>At .75A dc in the load, the current in the diodes is 6.5A peak. At 2A dc,
>>the diode current is 12A peak, with a longer conduction period. This is
40ms
>>after 'turn on', when the current appears to have settled to a constant
>>value.
>>
>>Steve
>
>Steve, thanks for the Spice run. Very interesting.
>
>My actual output voltage at key down was 3100, key up (ZSAC) was about
>3400. Using the graphs, I had estimated about 2.9 amps RMS current. But
>what does that really mean in this context. I know we can calculate the
>peak diode current, but few, if any, rectifier diodes have a REPETITIVE
>PEAK CURRENT spec. So what's a guy to do?
In general, over react. The problem can be quite subtle - The basic,
international ageed spec for a rectifier might have very few parameters, say
reverse breakdown voltage, forward drop at some current, and a single half
cycle surge rating. Anything exceeding these specs can legitimately be
marked with the part number. Particular manufacturers might have more more
comprehensive specs. for their parts - Philips was always good in this
respect - but you can't rely on unbranded (or other brand) devices being the
same.
Personally, I try to avoid running with the dc current more than 50% of the
diode rating, whether or not I know the repetitive peak rating. Also, keep
them cool - the PSU will benefit from airflow just as much as the rf
section.
>
>I can see a trend here. It's becoming apparent that 6 amp diodes in the
>scenario under discussion will barely provide sufficient safety margin to
>allow for variations in parts and design assumptions.
If they are 6A rated and made with modern silicon, I doubt that you would
have any problems.
73, Steve
--
FAQ on WWW: http://www.contesting.com/ampsfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|