I'm building a single switched AC/DC Forward Converter and have reached to the point where I have steady o/p voltages and currents.
However, I am not able to remove noise that goes back into the AC i/p line. A multi-meter(DMM) placed across the Line and Neutral at the power line input jumps around wildly once the current starts flowing in the MOSFET. The o/p voltages and currents remain steady, however.
I tried reading the AC with an analog meter, but not much difference. As long as the load current is small, it shows correct AC voltage. But as the load current increases, the meter starts showing much higher voltage (roughly 400VAC)across the input. I have tried various Y capacitors between i/p phase to Earth, Neutral to Earth, from the MOSFET source to Earth, but to no avail.
I have tried one commercial EMI filter also, but nope. I have tried with a common mode filter that I rigged up, but either my design is wrong, or I haven't got the concept right, because it did not make any difference. Some pointers/URLs for a design procedure to correctly design EMI/RFI filters will be appreciated.
My configuration is a single switch Forward converter with 230VAC Power Line Input, a reset winding (Treset=Tprimary), and a P6KE350 diode as protection from Drain to +ve bus after rectifier and capacitor. The controller is a Micro. Switching frequency is 50KHz. The MOSFET is a 6NB80, 5N90 or IRFBE30, depending on availability.
Your suggestions,recommendations and advice sought. Thanks in advance Edit: Added Schematic. My Measurement equipment are small DMMs, an old 25MHz analog scope, and not much else. I also have a Rigol-1052 50MHz DSO, but not used that for this project yet.
2nd Edit: The Micro is also being supplied from AC, with a smaller 1N4007 rectifer, and a TNY278 circuit on the same PCB. I had thought the combination of the TNY278 ckt and the Fwd converter running together could be the cause of this noise(beat frequency?) so I disconnected the track supplying the ac to the 278 part of the ckt, and supplied the micro from a 12v7AH battery, just to eliminate this as a possible cause. Again, no difference.