induction heater troubleshooting help

[aes92000]

Hey all, new to forum and about to loose last hair trying to figure out issue.

i have reverse engineered the pcb from an 1800W induction heater from bangood.

i re-used all the parts EXCEPT i replaced the irfp540 with irfp4668.  it was working awesome.  48-54vdc @ ~75a.
where the 3 connectors for coil connected to pcb started to burn through. most likely from me always changing coils for work being performed.

i have since then tried to replicate same board but it will not work.

it will either turn on and pop (another hair lost) or just short power supply and shut down.

i bought new 4668 mosfet and about to try again but i have question that i am wondering if it is causing issue.
is it because of the length for coil mounts on new design???

i have included pictures of original, upgraded working/bad and new design

[ritaismyconscience]

Try checking for shorts
It's really easy for a solder bridge to form between two parts with bare copper boards

[aes92000]

thank you.
i will put on my scope so i can zoom in for bridges or other defects

will the longer copper bars for coils be an issue?

[petespaco]

What are you using for a power supply?
 When you say that the board "shorts the power supply", I wonder if the board's current draw might be causing the power supply to go into current limit mode, which can reduce the voltage enough to make the mosfets lock up.
I didn't know they used IRFP540's on those boards.  I thought they were IRFP260N's.

Pete Stanaitis
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[aes92000]

it is a rectifier module
120/240VAC input
48-54VDC output
75A max (@ 240VAC)
if i can get working again, i will get another rectifier and share load for higher amps.  they wire for load sharing.

the power LED flickers on and then off as it shuts down and you hear the rectifier fans kick in and alarm light comes on.
i always turned power on to stabilize voltage before turned on heater.
i do not have coil loaded when starting and it still shuts down so it can't be overloaded, can it?
the first board i upgraded did not have that issue. 

as you can see from pics for upgraded board, i was able to get up almost max amps for supply and i always started with unloaded coil .
it would melt 1oz copper within 20 minutes.

[petespaco]

Do you have an oscilloscope so you can check to see if the circuit is oscillating at all?
I can't see the voltage ratings of those caps.   Are any of them shorted? 
Have you checked to make certain that none of your Mosfets are shorted?  Maybe wiped out out by an earlier error in wiring or ?
You say "it would melt 1oz of copper in 20 minutes". What does that mean?  WHEN?  Original board or YOUR board?  Did your board ever work?  Did it ever start up with a low "idling current"?

 "it was working awesome.  48-54vdc @ ~75a.":  Which one?
that's double the power that the original board was designed for.  No wonder it failed.
What dimension of work coil were you using when you got those readings?

[petespaco]

It would be nice to hear from the OP.