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Simple Flyback Driver Questions

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Ash Small

Wed Dec 22 2010, 07:27PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I'm planning to knock up a simple flyback driver for a little project this christmas using odds and ends I have lying around.

Initially planning to drive a transistor from a square wave signal generator, switching a 12V 4A battery charger driven flyback.

I have other options, bur thought I'd start with this setup as the charger has various protection circuits built in.

I want to include some extra protection, so was planning to put a capacitor in series with the signal generator and base of a 2N3055 (to start with), to protect the signal generator in case I blow the transistor, and a freewheeling diode across the primary of the flyback to protect the transistor.

Should I also put a diode across the transistor? Should I include any more protection capacitors? should I include any resistors in the circuit?

(I've a few other transistors and power supplies I could try, but thought I'd start as simple as possible)

While I have quite a few capacitors and diodes lying around, I'm planning initially to use a 1.16 microfarad 2100V MO capacitor with built in 10 Meggohm parallel resistor to protect the sig. gen. and a 50 nanosecond recovery 10000V 10V max forward drop 30 amp max surge current 0.5 A continuous MO diode across the primary. Will these do the job? If not, why not?

Thanks in advance.

(BTW, I'm going this route as I don't have any suitable zeners lying around for a ZVS circuit)

EDIT: Also, how many turns should I start with on the primary?

ubuntupokemoninc

Wed Dec 22 2010, 10:39PM

Banned on 1/22/2011 for repeated rule violations after multiple warnings.
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go with this driver

JM built this driver, its pretty good and the diode protects the components.

Ash Small

Wed Dec 22 2010, 11:04PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

You've hit the nail on the head there, is it better to put the diode across the primary or the transistor?....or both?....

The diode I'm thinking of using doesn't 'kick in' until ten volts (as I understand it) is this ok?

And I'm better off putting a cap across the diode?

(I want to drive the transistor from the sig. gen. so I can play about with frequency. I've built single transistor self switching circuits before, but with no protecting diodes or caps.....Would this circuit be better with a diode (and maybe a cap) across the primary as well?

(I'm planning to push it with more power eventually)

william L

Wed Dec 22 2010, 11:38PM

Registered Member #3093 Joined: Mon Aug 09 2010, 11:40PM
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You've hit the nail on the head there, is it better to put the diode across the primary or the transistor?....or both?....

I don't think you are supposed to put a diode across the primary when driving in flyback mode (right?)
the diode probably protects from overvoltage spikes damaging the transistor.

Ash Small

Thu Dec 23 2010, 12:10AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I've not seen a circuit that I recall that has a diode across the primary in flyback mode, but the field still collapses.

This is one of the main points I'm trying to address in this thread.

Can anyone explain why a diode isn't required across the primary in flyback mode?....Or would it help to include a diode across the primary?

(At the moment all the advice I've received says I should put the diode (and cap) across the transistor, I'll go with this advice unless anyone suggests otherwise, but I'd like to know why a diode isn't required across the primary in flyback mode)

Adam Munich

Thu Dec 23 2010, 12:13AM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
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Ash Small wrote ...
The diode I'm thinking of using doesn't 'kick in' until ten volts (as I understand it) is this ok?

You'd want 2 12V zener diodes in anti-series like so: â€•âˆ«â—„â€•â€•â–ºâˆ«â€•

Ash Small

Thu Dec 23 2010, 12:33AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I did post previously that I don't have any zeners lying around, so I'm not building a ZVS circuit, Grenadier, but surely I'd only need one diode across the primary, if I was to put one there?

Adam Munich

Thu Dec 23 2010, 12:44AM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
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Oh, ell zeners are very cheap, I recommend getting some.

There's 2 zeners there in antiseries, and under 12 volts nothing conducts either way. After 12V it's just like a wire, shorting any inductive kickback.

Just one zener or one diode could work too provided it's reverse biased. But if you ant to run the flyback at more than 1khz, I'd get some ultrafasts.

I don't think you'll need the diode across the primary though.

I don't think the diode is necessary though.

Ash Small

Thu Dec 23 2010, 12:54AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I accept your point, I'll put some zeners on my christmas list.......but in the meantime......I'm trying to knock up a light bulb plasma globe for the missus cos she keeps giving me shit about all the crap lying about the place......

Adam Munich

Thu Dec 23 2010, 12:56AM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

Ash Small wrote ...
os she keeps giving me shit about all the crap lying about the place......

If you're going to get into hobby electronics, she better get used to it.

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