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1n5ane

Thu Oct 07 2010, 06:14AM

Registered Member #3283 Joined: Wed Oct 06 2010, 11:03PM
Location: Toronto, Canada
Posts: 29

Hey guys,

I've been a long time lurker here (been lurking for the last 2-3 years). Unfortunately, I was preoccupied with other projects and didn't start dabbling in HV stuff until recently. I have been acquiring various components over the last few weeks (flybacks, iggys, mosfets, etc) and I'm looking forward to starting my first HV project. I've done quite a bit of research/reading on various flyback/ignition coil drivers but I still have some questions.

From what I gather, the most frequently suggested flyback driver is the common ZVS/mazzilli driver. I've heard that these drivers eventually "kill" the flyback and/or fry mosfets and diodes. Is there a simple way to improve the reliability of these types of drivers? I'm more of a "reliability/durability" oriented DIYer so killing and frying parts parts doesn't really sit well with me.

Secondly, what would be a conservative voltage to drive an ignition coil at? I'd like to be able to run it for more than just a few seconds (preferably a few minutes) so hooking it up to mains probably won't work for long. This is the circuit I have in mind:

Any and all advice is appreciated!

bgsniper

Thu Oct 07 2010, 08:26AM

Registered Member #3200 Joined: Wed Sept 15 2010, 06:57PM
Location: Sofia, Bulgaria
Posts: 5

for zvs don-t use high power transistors and don't run it for long. I use irf530 - 79W power transistor. High power transistors will fry the flyback transformer

Proud Mary

Thu Oct 07 2010, 08:28AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

As drawn and annotated, the 7812 regulator won't work properly. The 7812 has a 2V drop out voltage, so you will need to feed it a minimum of 14V , if regulated 12V is to come out of it.

So the supply should be +14 - 30V.

1n5ane

Thu Oct 07 2010, 06:00PM

Registered Member #3283 Joined: Wed Oct 06 2010, 11:03PM
Location: Toronto, Canada
Posts: 29

Thanks for the replies! What kind of modifications/component changes are required to run different mosfets on the zvs driver?

As for the ignition coil driver, I am planning to power it off a 12 v supply so I'll probably omit the regulator. I am more worried about frying the mosfet though; is a protection diode necessary?

Proud Mary

Thu Oct 07 2010, 06:24PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

1n5ane wrote ...

As for the ignition coil driver, I am planning to power it off a 12 v supply so I'll probably omit the regulator.

Then you should decouple the supply for the 555 and the buffer/driver from the final stage in some other way.

1n5ane

Fri Oct 08 2010, 12:39PM

Registered Member #3283 Joined: Wed Oct 06 2010, 11:03PM
Location: Toronto, Canada
Posts: 29

Is there any particular reason why the supplies should be decoupled?

Proud Mary

Fri Oct 08 2010, 12:50PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

1n5ane wrote ...

Is there any particular reason why the supplies should be decoupled?

Power MOSFETs like IRFP460 will draw a great deal of current, so unless you have a very stiff supply, it will pull down the voltage every time it conducts, which in turn will affect the operation of the timer and buffer/driver, perhaps causing erratic operation and instability in a way that is not easy to predict.

1n5ane

Fri Oct 08 2010, 03:15PM

Registered Member #3283 Joined: Wed Oct 06 2010, 11:03PM
Location: Toronto, Canada
Posts: 29

My 12v linear supply is rated for 25A continuous and 30A surge; is it stiff enough to power the circuit without problems?

Proud Mary

Fri Oct 08 2010, 03:29PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

I still think it would be best practice to separate the supplies in some way, to reduce the chances of interaction.

klugesmith

Fri Oct 08 2010, 05:40PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

1n5ane wrote ...

My 12v linear supply is rated for 25A continuous and 30A surge; is it stiff enough to power the circuit without problems?

It could be God's own power supply, and you'd still get voltage disturbances on the supply rail in your circuit, due to the resistance and -inductance- of the wires feeding it. Not a question of If, but How Big? Maybe OK, maybe not. An example where it works fine is the ignition system of an engine.

If there's a problem, it can be mitigated with a large capacitor between the DC rails, as close as possible to the switched-current loops. And/or by filtering the power to control circuit, for example with a simple RC network.

Good luck. This is a case where you can cut corners to start with, and fix where necessary.

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