coates
Member level 1
Hi guys,
I'm trying to replicate and understand an offline flyback converter and I have some questions for those with experience. The circuit I'm trying to replicate is this one. It uses the NCP1216 controller.
I'm using a variac to soft-start my line voltage so I can identify problems before components blow. The secondary has no-load, just the capacitors+bleed resistor.
When I'm ramping the voltage up to ~120VDC (minimum mains) the controller is going flat-out, i.e. 50% duty (because the FB pin is still high), the primary current gets quite high, and the voltage across the snubber capacitor greatly exceeds what one would expect (>1kV). Once the voltage ramping has reached ~120VDC the primary current falls to nothing and the voltage across the snubber falls to normal levels (~150V). Everything is OK. I'm pretty sure the primary current only falls because the controller enters "skipping" mode because the feedback pin.
As soon as I put a load on the secondary the snubber voltage ramps up beyond normal levels again and the secondary rectifier diode fails. I think this is because the reflected drain voltage exceeds the diode voltage: Np/Ns = 6.25, 1kV/6.25 = 160V > 100V. The diode fails before the transistor even though both their voltage ratings are being abused. I think this is because it's rated better for avalanche mode.
Why is the RCD snubber not clamping properly? Does this circuit really work?
Thank you.
I'm trying to replicate and understand an offline flyback converter and I have some questions for those with experience. The circuit I'm trying to replicate is this one. It uses the NCP1216 controller.
I'm using a variac to soft-start my line voltage so I can identify problems before components blow. The secondary has no-load, just the capacitors+bleed resistor.
When I'm ramping the voltage up to ~120VDC (minimum mains) the controller is going flat-out, i.e. 50% duty (because the FB pin is still high), the primary current gets quite high, and the voltage across the snubber capacitor greatly exceeds what one would expect (>1kV). Once the voltage ramping has reached ~120VDC the primary current falls to nothing and the voltage across the snubber falls to normal levels (~150V). Everything is OK. I'm pretty sure the primary current only falls because the controller enters "skipping" mode because the feedback pin.
As soon as I put a load on the secondary the snubber voltage ramps up beyond normal levels again and the secondary rectifier diode fails. I think this is because the reflected drain voltage exceeds the diode voltage: Np/Ns = 6.25, 1kV/6.25 = 160V > 100V. The diode fails before the transistor even though both their voltage ratings are being abused. I think this is because it's rated better for avalanche mode.
Why is the RCD snubber not clamping properly? Does this circuit really work?
Thank you.