Guru
Joined: 21/06/2018
Location: AustraliaPosts: 1106
| Posted: 11:50pm 08 Mar 2024 |
Fixing the Power Board:
In the event of FET failure it it is wise to remove the Bulk capacitor boards and use your "spare" capacitor boards fitted with a 220/330/470uF cap each, I dont usually even solder them in, a small tweak to the legs if they are at all loose when inserted "snapped in" to make them tight usually suffices for low current testing after replacing FETs.
In my failures to date usually a high side FET meets its maker and goes shorted, of course when the upper FETs should be off and the lower FETs turn on "Surprise M..F.." and takes them all out in an instant. How do you find a dead FET with 4 in parallel, there is a way that has worked for me.
With a current limited supply set to 1A and a few volts apply the voltage between the high side supply and the transformer connection node. This is between the Drains and Sources of the bank with a dead FET. With an accurate DVM measure each FET (Drain to Source) at the wide shoulders as close to the FET as possible usually 3 FETs will be a couple of millivolts but all exactly the same, the faulty FET will be slightly less, that is the one to desolder before checking if the others are now OK. If a difference was hard to pick turn the current up to 2 or 3A and measure again.
I have never replaced the other 3 when only one has died, despite the theory that others may have suffered I have not found that. The lower FETs though are very unlikely for any to have survived, so usually all have to be replaced.
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If anyone has any questions or comments please feel free to post away - I have been looking at this for so long now I probably take things for granted and leave important bits out.
Edited 2024-03-09 09:52 by wiseguy