I am making an induction heater. mosfets irfz44n, c = 0.1uf, r1=r2=220ohm, l1=l2 (don’t know I made it by myself but it’s greater than the induction coil. I made it using 24awg wire, 30 turns around small transformers core), vsup is 12v, the induction coil I am using is 5 turns coil (around 3.382 micro henry, c=0.1uf and resonance frequency 158khz). The circuit runs at 158khz.
My power supply can go upto 5amps.

I used zener diodes 5V 1/4W with 1Kohm resistance. It lowers the gate voltage and mosfets heat less.

The problem is while I tested the circuit in breadboard it performed well but when I connected it to pcb one mosfet damaged (I checked the gate-source continuity and it showed that gate source are shorted).

how can I fix it?

 

"...It lowers the gate voltage and mosfets heat less" ... that doesn't sound quite right See Fig3 page 3 (https://www.digikey.com/en/htmldatasheets/production/73742/0/0/1/irfz44npbf)

Those Zeners should be 15V ... not 5V
If the Supply is less than 15V , such as your case, you can actually omit the Zeners and 1k resistors
r1 and r2 could be a little higher 470 Ohm or 1k

The diodes that you don't have labeled need to be in place

There is also a "stall" condition if the components are very close in value. Not likely, but sometimes happens. Place a 10k resistor in parallel with R1 or R2 (NOT both) to form a slight imbalance.

 

"...It lowers the gate voltage and mosfets heat less" ... that doesn't sound quite right See Fig3 page 3 (https://www.digikey.com/en/htmldatasheets/production/73742/0/0/1/irfz44npbf)

Those Zeners should be 15V ... not 5V
If the Supply is less than 15V , such as your case, you can actually omit the Zeners and 1k resistors
r1 and r2 could be a little higher 470 Ohm or 1k

The diodes that you don't have labeled need to be in place

There is also a "stall" condition if the components are very close in value. Not likely, but sometimes happens. Place a 10k resistor in parallel with R1 or R2 (NOT both) to form a slight imbalance.

Are zeners limiting something or it is fine to keep it?
And

"...It lowers the gate voltage and mosfets heat less" ... that doesn't sound quite right See Fig3 page 3 (https://www.digikey.com/en/htmldatasheets/production/73742/0/0/1/irfz44npbf)

Those Zeners should be 15V ... not 5V
If the Supply is less than 15V , such as your case, you can actually omit the Zeners and 1k resistors
r1 and r2 could be a little higher 470 Ohm or 1k

The diodes that you don't have labeled need to be in place

There is also a "stall" condition if the components are very close in value. Not likely, but sometimes happens. Place a 10k resistor in parallel with R1 or R2 (NOT both) to form a slight imbalance.

What is the purpose of increasing r1 r2 ? Is it to decrease the gate current? Or any benifit to oscillation?

 

The Zeners can add parasitic capacitance that can slow down the switching On/Off time of the mosfets (you want the mosfets to switch as fast as they can) .... increasing r1 and r2 ensures the imbalance will be greater, thus minimizing a stall condition on startup. The mosfets require so little gate current that a higher value resistor here won't be a problem. That said you want just enough so that there is a small imbalance. The imbalance ensures that one mosfet will turn on slightly sooner than the other one and initiate oscillation.