EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: canadaboy25 on August 10, 2021, 06:30:33 pm
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Hey everyone,
I am working on a plasma TV that had some shorted MOSFETs in the Z-Sustain driver module. I replaced the shorted FETs with some Panasonic 5N2307 FETs that I pulled off a scrap board from another TV. The TV powered up and worked perfectly. However, shortly after powering it up, both FETs shorted again. I assume either the voltage or current ratings were not high enough but it is hard to tell as there is no datasheet available for these.
I got some FDB12N50TM FETs that are 500V 11.5A. The TV powers up and stays on without shorting the FETs. However, the image is very poor with lots of ghosting. I saved a waveform of the Z-Sus output with my scope and then reinstalled two more Panasonic FETs and powered up the TV long enough to capture another trace with my scope.
Comparing the two waveforms, I can see that the Panasonic FETs provide a nice clean switching output, but the FDB12N50TM FETs have a large voltage spike after each switching event. I have attached a few examples of these voltage spikes. The blue trace is the good waveform with the Panasonic FETs and the red trace is the problematic waveform with the FDB12N50TM FETs. The two traces are a bit out of sync with each other but it is still clear that the red trace has large voltage spikes that should not be there.
I have also attached a schematic of the switching module that I reverse engineered from the PCB in case that is helpful for anyone.
What parameter of the FETs could be causing these voltage spikes? The original FETs had no markings so I am trying to find a suitable replacement for them. However, I do not know what specification is not correct to be causing these large voltage spikes on the output.
If anyone has any ideas, please let me know!
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Are you absolutely certain they are MOFETs and not IGBTs . I'm by no means an expert on plasma TVs but the last one I encountered (Samsung) had IGBTs on the driver boards . IGBTs can handle much higher voltages , current , lower duty cycles , low switching losses at high voltage and lower frequencies .
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Are you absolutely certain they are MOFETs and not IGBTs . I'm by no means an expert on plasma TVs but the last one I encountered (Samsung) had IGBTs on the driver boards . IGBTs can handle much higher voltages , current , lower duty cycles , low switching losses at high voltage and lower frequencies .
I'm fairly certain that you are correct. It seems they were originally IGBTs. There is a diode across both transistors that should not be necessary if they were originally MOSFETs.
I found this youtube video just before you replied: https://www.youtube.com/watch?v=CARdm0o0FyM&t=7s (https://www.youtube.com/watch?v=CARdm0o0FyM&t=7s)
The guy is repairing one of these modules and it seems he made the same mistake as I did. He used a FGD4536 PDP Trench IGBT as a replacement.
The FGD4536 is no longer stocked on digikey or mouser and I am having trouble finding a Trench IGBT in the D2PAK package on mouser or digikey. I ordered some FGD4536 IGBTs on ebay from China for a few bucks. It will likely be a month before they are here. Hopefully they solve the issue.