On Thu, 3 Jul 2008 11:12:27 -0500, "Mook Johnson" <mook@[EMAIL PROTECTED]
>
wrote:
>In a DCM flyback with its high peak to average current ratio. When
sizing a
>diode do you rate thediode for the maximum peak current. I have a 2A
>average current with 12A peaks at the start of the flyback. Repeats
every
>5uS.
>
>Which current rating should I look for in the diode? 3A? 6A? 15A? All
of
>this makes a difference in the size of the diode package which I'm trying
to
>minimize. The diode datasheets don't have a repetative peak current
rating
>on the nonrepeataive rating.
>
Conventional rectifiers are rated for average forward current, as the
forward drop is relatively constant. The power loss vs average current
for a range of duty cycles is a fairly common chart provided for
design - 0.1 duty assumes a 10:1 peak to average current ratio.
In hard switching, peak current and temperature will affect stored
charge in conventional diodes, increasing losses elsewhere.
Schottkys tending to look more resistive and have RMS considerations.
Though forward losses reduce as Tj increases, increasing leakage may
be signifigant and can result in thermal runaway.
Your concern should be Tj, Rthja, power loss, and maximum ambient, in
most cases.
RL
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