Hi all,
After much pondering and time consuming tests..i was able to correct the problem( or at least a part of it) of instability in the circuit...But the funny thing is still i dunno the exact reason how its getting solved but it definitely works..I will be much appreciating if anyone can give me a clue..
I will try once again to state the problem
I've did a flyback converter in CCM mode using UC3842 primary inductance 3.75mH turns ratio = 10,fws=100kHz designed to operate at 50% duty with no slope compensation.
VIN 85VAC-265AC OUT=12V,2A
it was supposed to be be a text book design but it wasn't to my disappointment
1. After turning ON the circuit for testing at VIN =90VAC,Im getting only fsw<40kHz like 20kHz or some thing ..designed is 100kHz !!!.Complete problems..nothing works as expected..I'm scratching my head...
2. After some observations I reduced the feedback gain Rf from 100k to 10k then the switching frequency i got was 50 kHz..Still it was problematic..
3.I was stuck in this scenario.. couldn't move much..some times i observed oscillations at FB pins so did some tweakings at capacitors and got oscillations free.But whatever i do... i cant raise the fsw to 100kHz its stuck at 50kHz
4.I tried the diode D2 between the gate and Isense pin as mentioned in
**broken link removed**
now the problem is solved.I get clean 100 kHz gate drive and good load regulation.
I think apart from feed back oscillation problem , the CCM operation and close to 50% duty pushed the converter to instability during when the current ramp reached the threshold to ensure PWM latch.
I've still not included slope compensation..converter operates close to 50% duty..I think it may be required as im using UC3842 which is 100% duty capable....
Thanks for any comments