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Help with ZVS driver..

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jovica

Mon Nov 10 2008, 10:17AM

Registered Member #1790 Joined: Fri Oct 31 2008, 10:10AM
Location: Sydney, Australia
Posts: 40

Myke wrote ...

For a IH, I would suggest something else besides a ZVS flyback driver.

Why ?

A ZVS flyback driver is a great starter in induction heaters, just make a 5+5 turns 5mm2 + work coil and run it off 40 volts or so of a decent power supply (10 amps or so) and you have a 500watt IH just play around with the tank cap and remember that you will need to build a mmc to support the power being pumped into the work coil. Then you can go on to building larger IH's

Arcstarter

Mon Nov 10 2008, 04:43PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

HFsstc-freak wrote ...

Arcstarter, your ZVS driver SUCKS!!!

Ohhhh, so your the guy who said that to me on youtube? Like i said to that message, *laughs*.

Can we move on to the ZVS now and enough off topic stuff about how much my ZVS SUCKS?!?!

Myke, i did not make the ZVS for the induction heater, i am just experimenting. But, if i can get it to work properly i will use it for an experimental induction heater. I am no planning on making it all good and stuff, just hot enough to be able to bend stuff, for fun.

When i get a full wave bridge rectifier that is good for like 50 volts and 35 amps, i will hook the ZVS into my mot. I have some good filter capacitors that i got from alltronics(it was the 5 pound bag of misc caps, that i got for 10 bucks). I am planning on using a 8700uf 100 volt sprague powerlytic.

Antonio

Mon Nov 10 2008, 09:07PM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

This driver is quite strange. With just the elements shown in the schematic it is not an oscillator and should not work. Indeed, a simulation of it doesn't work consistently. There is a tendency for both transistors to conduct simultaneously short-circuiting the power supply. If it works in practice, must be because of parasitic resistances, capacitances or inductances in the construction.

Dr. Dark Current

Mon Nov 10 2008, 09:32PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Antonio wrote ...

This driver is quite strange. With just the elements shown in the schematic it is not an oscillator and should not work. Indeed, a simulation of it doesn't work consistently. There is a tendency for both transistors to conduct simultaneously short-circuiting the power supply. If it works in practice, must be because of parasitic resistances, capacitances or inductances in the construction.

It works in theory too, I don't know what's wrong with it. The primary LC just keeps oscillating by itself, the transistors just "poke" the resonant circuit. It's true that there's a small region where both conduct simultaneously (or at least partially), but they cannot s/c the supply because of the dc link choke, and again the tank circuit just keeps going, reversing teh polarity, and the diode feedback takes care of opening the right transistor.

Antonio

Mon Nov 10 2008, 11:16PM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

Dr. Kilovolt wrote ...

It works in theory too, I don't know what's wrong with it. The primary LC just keeps oscillating by itself, the transistors just "poke" the resonant circuit. It's true that there's a small region where both conduct simultaneously (or at least partially), but they cannot s/c the supply because of the dc link choke, and again the tank circuit just keeps going, reversing teh polarity, and the diode feedback takes care of opening the right transistor.

In a simulation the circuit only works consistently, producing an almost sinusoidal oscillation, if some resistance is considered in series with the power supply or with the choke. Otherwise it may oscillate, but generating irregular pulses. The required value depends on the load. I am still looking at why this happens. In practice, the resistances are there, and so the circuit is expected to work, at least when not heavily loaded.
It's interesting to observe why the choke is needed. When the circuit is powered up, both transistors conduct simultaneously, drawing a quite high current pulse. The choke presents a load to both transistors, reducing both drain voltages, and eventually causing both diodes to conduct. When this happens, the circuit becomes unstable and starts oscillating. It then settles in a nonlinear oscillation filtered by the tank circuit formed by the capacitor and the two primary windings. Without the choke the diodes never conduct.
It also reduces the current at the intervals when both transistors conduct, of course.

Dr. Dark Current

Tue Nov 11 2008, 08:47AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

The choke works as a constant current source, without it the circuit could not work, because the tank circuit could not oscillate as it would theoretically draw infinite current from the supply ...

jovica

Tue Nov 11 2008, 10:47AM

Registered Member #1790 Joined: Fri Oct 31 2008, 10:10AM
Location: Sydney, Australia
Posts: 40

Dr. Kilovolt wrote ...

The choke works as a constant current source, without it the circuit could not work, because the tank circuit could not oscillate as it would theoretically draw infinite current from the supply ...

Really?

I ran a zvs without a choke an it worked but the sparks were actually more purple than white.

can you expain this ?

Dr. Dark Current

Tue Nov 11 2008, 01:35PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Tekken_LordS wrote ...

I ran a zvs without a choke an it worked but the sparks were actually more purple than white.

can you expain this ?

Maybe, if you had loose coupling in the primary (e.g. wound with thin wire) and a small resonant capacitor, it may badly work (transistors getting hot,...).

Another thing is that if you used a mains transformer just rectified and not filtered, its secondary can act as the choke. In this case the circuit would run perfectly fine without any choke.

Steve Conner

Tue Nov 11 2008, 02:44PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

It works fine in a ferroresonant version with no choke, too. You get a square wave output, and the frequency is set by core saturation like in the blocking oscillator. You can make a ghetto inverter by driving a tube filament transformer backwards with it.

All of these push-pull oscillator circuits rely on an initial imbalance, or noise amplified by positive feedback, to get them started. In a computer simulation with no thermal noise and two identical MOSFET models, it might never start.

Arcstarter

Sat Nov 15 2008, 02:34AM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Do you think maybe the irfp460's have too high Rds on? It is .22 ohms and the suggestion for Rds on, is .15 ohms. It doesn't sound like too much of a difference to me, but what do you guys think? I cannot recall anyone that used irfp460's but i am quite sure it is something else in my circuit, because i am sure someone out there has used an irfp460.

also, another thing is, i still have 20-30 turns on my inductor of thick wire, and apparently, the suggested amount is like 200.. So, is this where it went wrong? I am going to go make a new inductor and see what is gonna happen.

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