Why a standard-recovery diode?

Posted by: tgi on 10/08/2008

Both the datasheet and the appnote for the linkswitch-lp are quite explicit suggesting a standard recovery (1n400x-ish) diode.

I have done my fair share of switching supplies and I know how it will behave; I expect it to be pretty hot (well not too hot at these low voltages, I guess). Frankly I know it will not be much of a diode at these speeds, it will be on most if not all of the time.

What is the motivation behind this choice? Is it supposed to absorb something during the time a fast recovery diode would already have switched off? If so, I would like to know what... If not, then it could simply be replaced with a fast recovery diode allright, something like a MUR110 or 140 etc. (just naming workhorses I am used to).

Thanks,
Didi

Comments

Hi Didi,

At low powers in specific designs standard recovery diodes are used for the clamp blocking diode. This allows some of the clamp energy to be recovered for high efficiency. As these diodes are not forced to reverse recover while a forward current is flowing they do not need to be fast or ultra fast (although using these types are not an issue). Using standard recovery diodes on the output is not acceptable - you are correct they do get very hot (enough to melt the solder holding them into the board).

Cheers

PI-Chekov

Hi PI-Chekov,
thanks for the explanation.
I have yet to figure out how the clamp energy gets recovered with a slow diode,
but my immediate issue is solved - being whether a fast diode can be used there, which seems to be the case. I am thinking a plain 4148 actually, I don't think it will ever see >100V (20V rectified voltage at this, bias/feedback winding).
Thanks again,
Didi