PROBLEM WITH PFC DIODE...... kEEPS BLOWING UP

[sekhar]

I have made a pfc for the front end of my smps ........
It works great for 450W in 230V . But when ever i go down below 130V in the input the the pfc diode HFA08TB60 becomes very hot and tends to blow it self.
I thought due to high current the diode was blowing up but when I changed the diode to HFA25TB60 the pfc lost its power factor and the inductor MAkes strange noise.....
Any idea why this is happening......
Does this have to do any thing with reverse recover issue of the bigger diode....
I am completely lost any help would be great ....

i am using UC3854 as my pfc controller and the basic Schmatic from the datasheet.....



regards
sekhar

[sawreyrw]

I don't know the exact details of your power section, but the diode needs to be mounted on a heat sink. If it's not, it will most likely overheat and be damaged. The thermal resistance from juntion to ambient is 80 K/W, so 2 watts will raise the temperature beyond the maximum rated temperature.

[johansen]

did you look at the current and voltage waveform with a scope?

[sekhar]

i have a big enough heatsink................
And yes checked the voltage wave forms they r a perfectly square ....... and i get a overshoot and undershoot of say +-50V. Cant measure current waveform dont have current probe.


regards
sekhar

[benb]

Voltage waveforms exactly where? That's a lot of overshoot AND undershoot ... that could be from a too-long probe ground connection.

And yes, reverse recovery could very well be a problem.

For reference, the typical application schematic is on page 8 of the datasheet:
http://www.ti.com/lit/ds/symlink/uc3854.pdf

Is this on a PCB you've made? Can you post a pic of the layout?

Offhand I'd want to be sure whatever diode you use has at least as good or better specs of the UHV806 diode shown. Maybe you can post links to datasheets for these three diodes (the one in the schematic and the two you've used).

[luka]

I think this diode is good, as long as it is around room temp.
what about Peak Rate of Fall of Recovery Current? below 200A/us should be good

[sekhar]

here is the the pcb layout..............it also has a half bridge smps in attached to the pfc section........................

have a look ................



regards
sekhar

[RJM1]

"inductor MAkes strange noise..... "

It sounds like your inductor is saturating at low input voltages (high current). Did you upgrade the 1mH inductor when you went from a 250W design to a 430W design? For 250W your inductor should be rated at 4 to 5 amps and at 430W it should be rated at 8 to 10 amps.

[sekhar]

@RJM1 yes i upgraded the inductor from 1mh to 400uh . And from etd34 to etd39 and the inductor does nt make sound with hfa08tb60 it only makes sound with hfa25tb60............


regards
sekhar

[RJM1]

Adds picture.