PFC output voltage 420VDC?

[treez]

For a 90-265VAC smps, (100w), why would you make the pfc output at 420V?....i mean, if it were made to 390V then that means less switching losses.....and if it transiently falls to below 360V at high line, then so what?...it just means you need a diode acorss the boost pfc inductor and let the peak of the mains break over the diode and push current into the output cap of the pfc directly?

 

[D.A.(Tony)Stewart]

What about margin for PLT? What attenuation of power line transients do you have at switching rate, f.

 

[treez]

I see your point, but isnt 418v a bit near the 450V "common" voltage of the electrolytics at the output of the pfc?.....as you know, pfc's are slow and if suddenly no-loaded, the vout could go above 450V.

Our transient protection is via a MOV which is after the emi filter, and just before the diode bridge. We put it there because we want it to last longer, and felt the filter would sheild it somewhat. The fuse is right after the input connector as you'd expect.

 

[Dan Mills]

The better sort of PFC chip will stop switching momentarily if it sees the bus voltage > 5% or so above the setpoint, precisely to minimise the issue on a load disconnect transient.

420V is a little higher then I would go absent other constraints, but for example going to 420V might allow the use of a smaller bus cap if the following stage could then tolerate additional ripple, or you might get some more hold up time or something else that was a desirable feature.

Regards, Dan.