Electronics > Repair

300W inverter repair - problem with H-bridge

(1/4) > >>

bborisov567:
I have been trying to repair this 300w inverter. When I got it everything in H-bridge was blow. I replaced all the components from the optocouplers to the Mosfets. I wasn’t able to identify the gate protection diodes (D4&D6 in the schematic) but there were remains of glass package so I put two 1N4148s.Everything ran fine for a while (maybe less than e minute without load) and then the whole top part to the bridge including the BJTs blew. I thought the cause were 1N4148s so I replied everything and put two Schottky diodes instead. In the schematic D4 and D6 they are marked as 1N4007 but I believe some faster diodes are need for gate protection. After a while the same thing happened this time blowing 3 mosfets and the BJTs. The mosfets don’t heat up at all before they blow. Also I have looked at the Gate pulses of the top mosfets - on one of them the impulses were clean, on the other one it looked like there is some oscillation but didn’t have enough time to check the measurement. Unfortunately I don’t have a differential probe to see multiple signals. I have looked at the schematic and started wondering what goes bad first - the BJTs or the mosfets? Interesting thing is that the optocouplers are still good so that leads me to think that mosfets blow first. Mosfets and the BJTs are bought from Aliexpress so there is some probability of being fake, but at this point I want to be sure there is no other issue before go out and buy some genuine mosfets. I attach a photo of the mosfet if somebody has experience with that brand, a schematic of just the H-bridge and an old blog post explaining the whole device: https://cschirp.de/index.php/elektronik/wechselrichter-reverse-engineering.html

BennoG:
put the 300V part on a 100V current limited psu (or the highest you have)
connect the output of the BCP 817 to ground and see what the current is doing.

step2 connect a scope to the outputs of both BCP 817 and see if they are not ringing or doing hf stuff.

Benno

Microcheap:

--- Quote from: bborisov567 on October 19, 2024, 05:25:51 pm ---... Mosfets and the BJTs are bought from Aliexpress ... [/url]

--- End quote ---

That's your problem right there. I wouldn't waste another second trying to debug the thing before replacing the parts for ones from a reliable source.

I have already learned the lesson: NEVER trust components ordered from Aliexpress or eBay, especially transistors and op-amps. Always assume they are fake junk, because most of the time they are.  :horse:

There's no point, you can get good parts from LCSC for nearly the same prices and some times cheaper.

BennoG:
I missed the Aliexpress part, yes replace the from a respectful source Farnell / digi-key / mouser / RS components / tme / lcsc.

Benno

bborisov567:
I guess will get some genuine parts on monday. What about the gate protection diodes for the reverse voltage on the gates of the mosfet? Should i put back 1N4148s or leave the schottky diodes?

Navigation

[0] Message Index

[#] Next page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod