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Registered Member #4394
Joined: Mon Feb 06 2012, 07:43PM
Location:
Posts: 40
Hi everyone hope your all having a nice new year!
I'm hoping that someone may be able to assist me on a project im currently working on which is a mains powered induction heater.
im using a quasi resonant arangement using a single fga25n120 igbt triggered by an adjsutable square wave generator coded onto an 8 bit chip. Its basically a clone of a very cheap style of induction hob.
my oscillator is run from a seperate 15v supply and the heater will succesfully heat object in the work coil if i bypass the rectifier and use a 16v bench top power supply.
However when i switch to a 10a variac connected to the rectifier i see a huge magnetic field in the coil which just moves the work piece but doesnt heat it and i see whopping current drain even at low voltages.
Now i'm pretty sure its something small i am overlooking as its a simple circuit! please note im running all my mains tests through an isolation transformer so i can scope it.
Ive included a quick sketch of my circuit and also a photo of the quick testing pcb i made up please dont bash the design i made it in 20 mins :]
please note the igbt has been desoldered to test if its died or not but is right over where it solders on to the board
i'd be very grateful for any help as its not something im too familiar with and its quite frustrating not not knowing why i cant get it to work
regards
also i just noticed my 5uf cap is wrong on the diagram, it goes from the negative location to after the choke on the positive rail not before! ooops
Registered Member #102
Joined: Thu Feb 09 2006, 08:15PM
Location: Raleigh, NC
Posts: 169
well, either your IGBT/mosfet is a short, or your rectifier is backwards (which might have produced a dead switch also) there's many ways to test the switch, and its fairly easy to check your rectifier.
Registered Member #4394
Joined: Mon Feb 06 2012, 07:43PM
Location:
Posts: 40
Thanks Kolas, the rectifier works as it should i have just drawn it wrong. i have also tested it with a different rectifier and a bunch of caps in parallel equaling about 20 000 uf as i had thought it might be ripple that was killing the igbt even though this circuit should be able to handle unsmoothed rectified dc. i wonder now if im getting some sort of ringing that is causing the igbt to switch out of zvs. i have noticed a change in the square wave output of the oscillator when the circuit is connected to a power supply that looks very saw tooth with distinct Sinusoidal properties! i will try get some snaps of the scope tonight
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Hello, the 100uH inductor is placed wrong, I think it's usually used only as a noise suppresion inductor before the 5uF cap. The work coil would then go from the positive end of the 5uF filter cap to the IGBT collector, and the 0.27uF cap would go in parallel to the igbt (between the collector and emitter).
Registered Member #102
Joined: Thu Feb 09 2006, 08:15PM
Location: Raleigh, NC
Posts: 169
I'm pretty sure that this circuit was closer to correct before, having the little cap be PFC for the work coil making a parallel resonant circuit. But something to realize here is that in order to achieve ZCS you need to be switching based on the tank's current via feed back. A simple 555 circuit is unlikely to be "tuned" correctly. Any work pieces inside the coil will cause the effective inductance of your work coil will change, and therefore the resonant frequency has changed. I also believe that the 100uh inductor is meant to add impedance to the tank and acts as a fail safe (it needs to be a matched inductor to your PFC) i recommend you read this page.
Registered Member #4394
Joined: Mon Feb 06 2012, 07:43PM
Location:
Posts: 40
Hi Kolas!
been through Mr Burnetts page many times haha! he does say that aslong as the frequency is on the high side of resonance it should only cause loss of power or for the whole thing to drop out of resonance but i am having catastrophic failure of the igbt? the heater will also be used to heat the same load everytime so once i find the resonance i can just leave it calibrated. under 16v dc from my bench supply the heater draws 4 amps and gently heats the load in the coil. when i hook it up to a 10a variac running through a rectifier and 20 000uf of smoothing at about 15 volts it appears to be trying to draw 40amps! im assuming for some reason or another the igbt is latching open causing the work coil to act as a huge heating element, resulting in the huge current drain.
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Induction heated nightmare
