Re: Synchronous rectifier


<supermos@xxxxxxxxx> wrote in message
news:1140413647.305047.7280@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Hi, I am working on a integrated synchronous rectifier, in which a
comparator sense the Vds of a MOSFET to control the turning on and off
of the MOSFET. So the comparator is a zero-crossing detector. In the
design of the comparator, the offset is well controled to enhance the
efficiency. But in very light load/zero load, there is some
oscillation. Thus my question is: is anyone of you once meet this
instability problem and could anyone give some advice on this?

Thanks...
Eric


Yes, it am lot complicated. Wiv synchronous rectification, if your converter
am allow it, then you am end up with continuous current in the output
inductor even at light/zero load currents. It am mean your inductor current
am also go negative and you am have possibility of returning power from
output to input, what am called 'regen'. If your converter am not allow for
this then it am get upset about things and overvoltage some of its bits.
This am special problem wiv off line stuff which am have transformerer and
leakage inductorance stuff lieing about.

I am before watch people design such stuff and they am see weirdo stuff
happen then am explane why plus think am cool new feature :-) Then get hot
and blow up :-(

If you am not have this problem then it am should work but problem might be
elsewhere. You am rely on drain of catch mosfet to fall to zero and it am
driven by current in inductor. If current in inductor am not enuff to driv
drain to zero then do not happen so am go 'burp'.

I reccomend you am get LTspice and do model to prod about wiv what am going
on. I am have done this myself and am learned lot about it wivout thing
going 'BANG'.

Unfort you am got Gmail ac*** plus post from Gugle.

Sigh

DNA


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