Driving a forward converter with a flyback IC (didn't work)(but still fixed it) - Page 1
Forum section: electronics / projects

author: GelatinCapacitor
author_rank: Contributor
timestamp: June 22, 2023, 05:38:08 am
content: I've got a ~150W power supply, presented in the following simplified diagram:As you can see, it's a half-bridge forward on a voltage doubler. Also the driver IC is dead.I have a bunch of flyback power supplies: you know; from phone chargers, from DVD players, from laptop chargers... the typical topology:What if I were to take a random primary circuitry out these flyback supplies, and turn the previous circuit into something like this:Turning it into a simpler forward converter.So, what could possibly go wrong?Of course I'll try to find an IC that drives a transistor, has a dedicated pin for the optocoupler and does not require an auxiliary winding.I'm not an expert, but I know this is not a great idea. What are my chances of success?

author: Hiemal
author_rank: Regular Contributor
timestamp: June 23, 2023, 05:19:57 am
content: Well. A standard single ended forward converter uses an additional winding to achieve transformer reset, whereas in the original design (a half bridge you said), transformer reset is done by the other transistor as it switches back and forth between both. Without the additional winding, the transformer will be driven into saturation and ...essentially won't work anymore. Not really something you can swap in unfortunately. Best solution would be to replace the original switcher, or, use a different power supply altogether without swapping components.

author: GelatinCapacitor
author_rank: Contributor
timestamp: June 24, 2023, 12:03:50 am
content: Thank you.Yes, I could replace this psu no problem, but I wanna try fixing it somehow.I would love to get a replacement IC of some sort but unfortunately they are hard to get 'round here.What if I were to drive the core somewhat like this?:I know I may risk killing the transistors, trying to find some alternative.I will try something this weekend; I'll let you know what blows up.

author: GelatinCapacitor
author_rank: Contributor
timestamp: August 24, 2023, 04:32:27 am
content: The device I was trying to fix is a SONY CRT TV model KV-32FV16. The power supply was dead; specifically the MCR5102 controller IC.Here are some pictures: schematic, psboard (removed IC), and the IC on heat sink.My first attempt was to use this what assume is a fake HP psu:The FET was dead. I replaced it. The new FET and its snubber resistor blew up. Maybe it's still fixable, but left it for later.So I grabbed this instead:And... it was kinda working? The supply reached the desired voltage but in pulses. Every second a pulse of the expected 135V came out of the supply. Nice that these IC's are hard to kill. Didn't get stable power, don't know exactly why; I wish I had an oscilloscope.I don't remember if I tried increasing the output load; maybe I did, maybe didn't. Dumb me if I forgot about that. Because of that now I want to repeat the experiment.I tried many arrangements of demagnitizing circuits for the transformer, but adding nothing at all is what worked best.I tried reducing the value of the IC's current sensing resistor, little by little expecting some change. This happened:Maybe that psu still works if I replaced the FET. But left that aside as well.So then I grabbed this:It has a strange IC: MIP2E4DMY. Repeated all the same steps above, got same results as before: power in pulses, and idiot me doesn't remember if tried having a proper load on the output.Nothing blew up though.Then I grabbed an old CRT TV model KV-21R22/5. It has an interesting psu:So I removed the main transformer, replaced the transistors with 13009, added 18k on the TL431 divider, and the KV-32FV16 is working again.I don't know. Maybe it'll die again soon. Obviously I hope not.