Induction heater mosfet problem again

[Joepage2008x2]

Hi everyone, a long time ago i started building an induction heater and kept having problems with my mosfets, i sorted that problem but gave up with the induction heater as i didnt have an oscilloscope. I started it again a few days ago after i got an oscilloscope and got it to work but my mosfets rated at about 55A kept blowing and can only manage 3A, i now only have a 10A IRF740 left. When i use my inverter circuit to power a bulb the mosfets dont heat up and have no problem but when i use it to power my tank circuit they get really hot and blow, i think its because its an inductive load and my diode isnt fast enough. What does everyone else think ?

If it is because of this, whats a good protection diode, i will be running it off 320V at between 1-5KW (when i scale it up, i dont want to buy 2 lots of diodes), 40-60KHz

Ignore the red numbers-it was done on multisim. Also i have a huge 250W 6Ω variable resistor in series with the power supply to adjust the current flowing.
This is a simplified version but is basically the same, also ive been using a 555 as the square wave input but i cannot get a 50% duty cycle, does anyone know a good VCO with 50% duty cycle, that can manage between 40-60KHz.

Thanks

 

[shortbus=]

Hi, while I'm by no means an expert, go to; FET Current Ratings -- Chuck's Robotics Notebook
what will tell you a lot about practical use of mosfets.

Data sheets basically lie! It takes a lot more than what they publish on the front page of the data sheet to use a mosfet and chose the right one.

The link I posted is the best one I've found, at explaining how to chose one in every day ,non-electrical engineer language.

Hope this helps you, Cary

 

[chemelec]

Hi everyone, a long time ago i started building an induction heater and kept having problems with my mosfets, i sorted that problem but gave up with the induction heater as i didnt have an oscilloscope. I started it again a few days ago after i got an oscilloscope and got it to work but my mosfets rated at about 55A kept blowing and can only manage 3A, i now only have a 10A IRF740 left. When i use my inverter circuit to power a bulb the mosfets dont heat up and have no problem but when i use it to power my tank circuit they get really hot and blow, i think its because its an inductive load and my diode isnt fast enough. What does everyone else think ?

If it is because of this, whats a good protection diode, i will be running it off 320V at between 1-5KW (when i scale it up, i dont want to buy 2 lots of diodes), 40-60KHz
View attachment 35537
Ignore the red numbers-it was done on multisim. Also i have a huge 250W 6Ω variable resistor in series with the power supply to adjust the current flowing.
This is a simplified version but is basically the same, also ive been using a 555 as the square wave input but i cannot get a 50% duty cycle, does anyone know a good VCO with 50% duty cycle, that can manage between 40-60KHz.

Thanks

A 3.5uH Coil with such a LARGE 3uf Cap in parallel. DOESN'T Sound Right.
A Dead Short to a 50Khz signal.

 

[Joepage2008x2]

surely a 55A mosfet can handle 3A though, also the IRF740 that is rated at 10A can handle 5A in my circuit. I dont think that its an error with the Mosfet because the 55A ones came from a 250W inverter which is basically about 21A through the mosfet. I think that the back emf is not getting caught by the diode and killing the mosfet, but im no expert at mosfets either so could do with an expert on mosfets, thanks anyway. The mosfet has no problem with bulbs and does no even heat up so i think this backs up the emf theory and the diode. I just dont want to blow up a lot of moneys worth of mosfets and want my induction heater to not be on fire.

 

[Joepage2008x2]

@ chemelec i have a huge variable resistor to limit the current, an inductor will be used to limit the current in the final version. I have only run it at 3A anyway. Thats how a induction heater works, when its input frequency is too low/high it should pull a low current, when you insert a piece of metal into the coil the inductance changes and therefore changes the resonance, if the input frequency matches the resonance frequency a large current should flow and heat the workpiece.

 

[nigelwright7557]

@ chemelec i have a huge variable resistor to limit the current, an inductor will be used to limit the current in the final version. I have only run it at 3A anyway. Thats how a induction heater works, when its input frequency is too low/high it should pull a low current, when you insert a piece of metal into the coil the inductance changes and therefore changes the resonance, if the input frequency matches the resonance frequency a large current should flow and heat the workpiece.

If your hitting the resonant frequency then the LC circuit becomes a rejector circuit and so should take very little current.

You could sweep the input frequency using a sig gen and see where the output is biggest and that will give you the resonant frequency.

Depending on the Q of the LC it might be quite hard to get the same resonant frequency every time.

 

[mneary]

The transformer driving the gate could be trouble. If there's any leakage inductance, the gate drive isn't clean. It could be ringing or rising above or below the gate's limits. If it rings, you spend a lot of time in transition!

Try loading the transformer secondary with a hundred ohms or so.