Switching transistors

[udir]

Hi Friends,
I have a problem that I would like you to address.
I have an analog board which produces a 1 Mhz signal (close to sine wave).I used a Mosfet push pull terminology using a 24v DC input to generate this signal by switching the DC input using these Fets over a planar transformer.
The problem is that one of the Mosfets (or both) are burning out (you can clearly see that high currents burnt it or it became so hot that the tin welding it to the PCB has reached its boiling point and caused it to bend!) when the AC to dc power supply which delivers the 24 vdc to the switching transistors is fed by 110V AC/60 HZ (and according to spec it suppose to withstand it with no problem as it multi power supply by meanwell).

Whats strange is that when the same AC to dc power supply is fed by 220VAC/50HZ there is no problem what so ever.....which puzzles me...
The same cooling fan operates on these 2 Mosftes in both cases, so I just can get it,,,,

any advise?

 

[crutschow]

I can't clearly see anything. Please post a circuit diagram of your setup.

 

[chuckey]

I would think that your layout is unstable and the FETs are oscillating in the UHF region and changing the length of leads/decoupling capacitors is stopping the oscillations. Try ferrite beads/10 ohm resistors on the FET gate leads.
Frank

 

[udir]

already placed resistors at the gate. doesnt help.
very strange it happens only under 110 V AC/ 60hz......

 

[FvM]

I would expect a difference in the 24V DC level, or possibly superimposed ripple in a first order.
Switching frequent interferences from the power supply would be another guess.

Or something supernatural...

 

[GUMY]

First measure the curent through transistors, to see what kind of current is causing overheating. Nothing else could cause it, but current. It could be some HF oscilations as allready suggested, or some DC/60Hz grounding current.