I have bought broken DC to AC inverters to repair and I notice the internal fuses never blow, but the DC side mosfets always short and burn out.
For example an IRF 3205 has a 110 amp rating continuous. pulsed 390, at 100*c 80 amps.
It is protected by a group of 30 amp fuses.
Why do mosfets self destruct without the fuse blowing first?
If the fuse wont do a thing, why bother with internal fuses at all?
Here is a pic of a board I am replacing mosfets on.
So you can see the board layout. there are 2 input boards.
Six 30 amp fuses, so three 30 amp fuses on each board side.
There are four input mosfets on each side, total eight per board.
4 input mosfets share 90 possible amps until some fuse supposedly blows, which they dont.
I think these mosfets are turned on in pairs, each board has two drive chips, on one side of the board, one driver gate line runs to two mosfets, the other driver gate line runs to the other 2 mosfets. So they are alternatively turned on and off by the mosfet driver.
So that tells me each pair of mosfets would be exposed to 90 amps which is well within half their max rating.
All I can think, is fuses blow too slowly to do any good.
The mosfets see a huge current spike, and they short source to drain becoming low ohm resistors which limits the DC current so the fuses stay intact, while the mosfets burn.
Yes, the inverter supposedly have internal over current protection. Which apparently does not work reliably?
This inverter board burnt out when attempting to turn on a 16000 btu heat pump.
I turned on the heat pump switch and heard some sizzling and every input mosfet on both input boards burned. The AC output mosfets are ok.
So what I will do is put a relay on the heat pump, so that if inverter is powered on, the heat pump can not run.
A heat pump like this is a massive inductive load, does that somehow affect the overload protection built into the inverter circuit? Or is somehow that circuit defective? I have made no modifications to any such circuit in this inverter.
And would a 20 amp push button thermal breaker on the inverter's AC output blow before the internal mosfets blow? This inverter has a 3000 watt AC rating.
For example an IRF 3205 has a 110 amp rating continuous. pulsed 390, at 100*c 80 amps.
It is protected by a group of 30 amp fuses.
Why do mosfets self destruct without the fuse blowing first?
If the fuse wont do a thing, why bother with internal fuses at all?
Here is a pic of a board I am replacing mosfets on.
So you can see the board layout. there are 2 input boards.
Six 30 amp fuses, so three 30 amp fuses on each board side.
There are four input mosfets on each side, total eight per board.
4 input mosfets share 90 possible amps until some fuse supposedly blows, which they dont.
I think these mosfets are turned on in pairs, each board has two drive chips, on one side of the board, one driver gate line runs to two mosfets, the other driver gate line runs to the other 2 mosfets. So they are alternatively turned on and off by the mosfet driver.
So that tells me each pair of mosfets would be exposed to 90 amps which is well within half their max rating.
All I can think, is fuses blow too slowly to do any good.
The mosfets see a huge current spike, and they short source to drain becoming low ohm resistors which limits the DC current so the fuses stay intact, while the mosfets burn.
Yes, the inverter supposedly have internal over current protection. Which apparently does not work reliably?
This inverter board burnt out when attempting to turn on a 16000 btu heat pump.
I turned on the heat pump switch and heard some sizzling and every input mosfet on both input boards burned. The AC output mosfets are ok.
So what I will do is put a relay on the heat pump, so that if inverter is powered on, the heat pump can not run.
A heat pump like this is a massive inductive load, does that somehow affect the overload protection built into the inverter circuit? Or is somehow that circuit defective? I have made no modifications to any such circuit in this inverter.
And would a 20 amp push button thermal breaker on the inverter's AC output blow before the internal mosfets blow? This inverter has a 3000 watt AC rating.