Monday, 24 August 2015

mosfet - Induction heater circuit problem


I am trying to build induction heater with the help of the following schematics(taken from this page):
enter image description here


So I am using 220:10Vrms(15Vpeak) transformer with fullbridge rectifier and a capacitor to get 15V DC. If I disconnect everything from this simple "PSU", I do get 15V DC.


But, when I actually connect the circuit and measure my voltage rails I get this strange behavior on my oscilloscope, as you can see it shows 5V with some ripples.
enter image description here


The circuit draws 0.27 Amps on the primary and 1.84 on secondary of a transformer, which I measure with the voltage clamp.

enter image description here enter image description here


My coil is 1.2 uH (measured with LCR meter). I use 0.47 uF 250V Polypropylene capacitor. This should result in approximately 220 kHz resonant frequency.
enter image description here


I also use the choke of 2.2 mH on center tap.
My MOSFETS are IRFP460A.
So, questions:
1) The circuit doesn't work for me, i.e. if I probe the coil there is nothing on it, zero. If I put metal object inside(screwdriver) nothing changes. Also the overall current consumption is not affected(looking at clamp meter) if metal part is introduced.
I already tried to:
Re-wire everything from scratch, making sure everything is connected properly.
I checked the mosfets with simple testing circuit - they both work fine.

I tried to measure MOSFET output on the Drains without coil or cap in the circuit - it also shows zero on both mosfets.
2) Why if I measure voltage on my 15V rail it shows 5V with ripples? It shows perfect 15V DC if nothing is connected to it. Is this some kind of feedback? I have a choke to eliminate high frequency feedback, but overall it doesn't make sense, since the circuit does nothing to cause feedback as I think. The capacitor after full-bridge rectifier is 1000uF, seems enough too.




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