Why no synchronous rectification in PSU's?

[ben7]

I was wondering, everyone keeps using these big 30A diodes for rectification. Why not use synchronous rectification? Isn't it very efficient? The only drawback I see is more expense, and complicated circuit.

Any ideas?

-Ben

[Khron]

Re: Why no synchronous rectification in PSU's?

On the contrary, it *IS* more efficient, but is also indeed more expensive. Instead of cheap-ish diodes, you'd need pretty large-current MOSFETs (which cost a pretty penny), synchronous rectification controllers (ditto), plus the cost of (extra) R&D.

On the other hand, i've seen it in a 130w HP laptop power brick. And i don't quote me on this, but there's a chance some of the higher-end ATX's (think KW-range) might employ it as well...

EDIT: Yeah, it seems there ARE at least SOME that employ synchronous rectification: http://www.hardwaresecrets.com/artic...-Review/1514/5 . This one's an 850w one At the more "normal" 3-600w range, i'm not sure it justifies the extra expense / complexity...

[ben7]

Re: Why no synchronous rectification in PSU's?

EDIT: Yeah, it seems there ARE at least SOME that employ synchronous rectification: http://www.hardwaresecrets.com/artic...-Review/1514/5 . This one's an 850w one At the more "normal" 3-600w range, i'm not sure it justifies the extra expense / complexity...

Yeah, I figured some high-end ones use it.

[Behemot]

Re: Why no synchronous rectification in PSU's?

Well almsot all high-end products use it for last two years. Seasonic X (80+ gold) was first, with their resonant topology. I think you will also find them in most servers for many years. I haven't seen any, but I think this is one of the technologies they *have to* use to achieve efficiencies greatly over 90 %.

Just notice one thing, in servers, there are 90+ PSUs for quite some time. Why it is not in desktops for many years now? Well, why it should be when they can serve you 80+, 80+ bronze, 80+ silver, 80+ gold etc. every like two years, right?

But anyway, I have the feeling it is in every better UPS these days. Because most UPSes don't use fan and also more heat means less battery runtime (and overall life too).