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ZVS driver's inductor requirements

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Sparrow338

Thu Jun 07 2007, 03:18AM

Registered Member #661 Joined: Sat Apr 21 2007, 03:17AM
Location: Ventura, CA
Posts: 41

Ok so I am making my first ZVS flyback driver and I wanted to know how important it is to have the right size inductor for the circuit. If I wind about 25 turns of 22 awg insulated wire around a 1 inch dia. toroid core will this probably be enough inductance for the circuit? Since I don't have an inductance meter I can't measure it myself. Is there any formula for finding inductance if you know wire/core size? Thanks for the help in advance.

If it helps, here's a picture of the inductor I'm using.

...and if anyone is interested here is a picture of my dirver. (note: capacitor and inductor arn't attached yet)

Ken M.

Thu Jun 07 2007, 04:57AM

Registered Member #618 Joined: Sat Mar 31 2007, 04:15AM
Location: Us-Great Lakes
Posts: 628

That circuit layout looks extremely nice, but isn't it possible for all those uninsulated connections to short out on the heat sink, even though it has an anodized coating?

Sparrow338

Thu Jun 07 2007, 05:06AM

Registered Member #661 Joined: Sat Apr 21 2007, 03:17AM
Location: Ventura, CA
Posts: 41

Thanks WhiteArc.
Except for the Mosfets nothting is touching the heatsink so shorting out shouldn't be a problem, unless it flashes/arcs over to it. Both the Mosfets also have a non-cunductive thermal pad and all their hardware has plastic spacers so nothing is electrically connected to the heatsink.

Tonic

Thu Jun 07 2007, 05:41AM

Registered Member #528 Joined: Fri Feb 16 2007, 10:32PM
Location: Warsaw, Poland
Posts: 166

Remember, if you will give a powerful power supply and driver, a high current would heat enough to melt or soft solder joints and.. well, many things may happen, for instance I've heard of someone, for which one leg of capacitor pulled out and killed both transistors. To prevent this, I use screw connections much as I can and when it comes to soldering, I wrap one wire around another and then solder. Just a small hint if you face with this problem, though it's less possibly :)

Inductor looks well-made and wiring seems to be enough. But I always wondered, why people suggest to use iron powdered ones instead of ferrite..

Sparrow338

Thu Jun 07 2007, 05:57AM

Registered Member #661 Joined: Sat Apr 21 2007, 03:17AM
Location: Ventura, CA
Posts: 41

Thanks Tonic.
I'm sure if I ran it hard enogh it would melt the joints. However, I was planning on using this driver for smaller flybacks and due to its Mosfets being only rated up to 150 Volts, I wasn't planning on driving it with anything over 12 Volts. Later after the fun of this one wears off I was planning on making another driver that would have a much more functional, less...nice looking..., set up that I can really drive hard and not have things over heat/melt. Probably using 900 Volt 30 Amp Mosfets.

I'm sure iron powdered toroids and ferrite ones both have their advantages and disadvantages. Maybe the iron ones have some advantage over ferrite in this use. Although I don't know what the electrical use differences between the iron and ferrite are. I have a couple of huge, 2.5 inch dia., ferrite cores that I was gonna use in a more powerful driver, when I make it.

...

Thu Jun 07 2007, 06:07AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

The exact value of the inductor isn't critical, just as long as you have enough inductance.

In this application powdered iron is preferred because it doesn't saturate as easily as ferrite does, and to tell you the truth I am not sure how people using ferrite get away with it. Yours should work, but if you have a scope I would definitely scope the voltage across the tank cap, and make sure it is a nice clean sine wave.

I use a 1" id/1.5" od by .5" thick yellow/green cores with 75 turns. Have used it at up to a kW of output power...

Adrian

Thu Jun 07 2007, 01:38PM

Registered Member #697 Joined: Thu May 10 2007, 12:28PM
Location: Australia
Posts: 22

I have tried three different powder-cored inductors and also a 300-turn air-core inductor and none of them made any noticable difference to the arc. I don't have a scope so I cant see what electrical effect they have on the circuit.

Marko

Thu Jun 07 2007, 02:26PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

EG: as it is now, your inductor is very bad.

With 25 turns it will saturate at current of 0,38A. (Assuming it is the common material 77, u = 2000 and saturation B of 0,3T.

But it has monster inductance of about 1,5mH wich is about 15 times you would need for this circuit.
That ferrite core won't work, so ditch it.

Ferrite cored inductor can work well if it's properly designed, but usually a very large core is needed to make benefit of it in comparison to material 26.

Gapped ferrite cres or ferrite rods can be used, but still aren't good like material 26.

Material 26 saturates at about 1T wich is some 4-5 times of #77 ferrite and that's the main reason why is it worth here.

Dig out a ''yellow-white'' material 26 - powdered iron toroid out of a computer power supply and wind some 30 turns on it.

Sulaiman

Thu Jun 07 2007, 04:29PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

My favorite for initial testing is to use an air-core inductor, a reel of wire.

Once your zvs inverter is working you can experiment with different wire/core/material etc.

Sparrow338

Thu Jun 07 2007, 10:28PM

Registered Member #661 Joined: Sat Apr 21 2007, 03:17AM
Location: Ventura, CA
Posts: 41

Thank you all for the help!

I think I'll go with the air cored inductor, easiest for me at the moment. I'll try using a reel of wire, but I do have some air cored coils of wire I made for another project that I can use. They have a 1/2 inch dia. core, 1 1/2 inches wide, and 1 inch tall. I used 22 Awg insulated copper wire. It's about 23 turns per layer and there are around 10 layers. Should be ~200 turns. Anyone have any ideas on what the induction value of this would be? Maybe one these will work? Again thanks for all the help guys.

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