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Driving 'Fiddy's Flybacks' in flyback mode.

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Ash Small

Thu Nov 13 2014, 10:58PM

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

This is where I've got to now, using 1/2 Watt variable resistors this circuit gives me plenty of range, certainly to be going on with.

I probably don't need this whole range, but it does make it a very useful circuit. My only concern is that this layout could add extra inductance at higher frequencies. The layout still isn't finalised. This is somewhere between a schematic and a finished layout, though.

Shortest pulse is around 20uS and lowest frequency is around 1Hz. Longest pulse is around 0.05S, which is far more than I need in any realistic scenario. Highest frequency is somewhere around 500kHz, which is also far higher than I'll need for this flyback.

1415919056 3414 FT1630 556 Flyback Driver

EDIT: The flyback arrived this morning, and it looks like it's possible to get a primary underneath the secondary, rather than on the opposite leg, which is always the best place to put it. I'm now thinking of using a foil primary on the same leg as the secondary.

EDIT 2: I've started looking at MOSFET's now. First one I'm looking at is the IRF450, as it seems to be quite polular, and from first impressions looks to be acceptable, but any comments or suggestions will be welcome. Datasheet here:

EDIT 3: corrected 20nS to 20uS.

Sigurthr

Fri Nov 14 2014, 01:39AM

Registered Member #4463 Joined: Wed Apr 18 2012, 08:08AM
Location: MI's Upper Peninsula
Posts: 597

Ash Small wrote ...
EDIT 2: I've started looking at MOSFET's now. First one I'm looking at is the IRF450, as it seems to be quite polular, and from first impressions looks to be acceptable, but any comments or suggestions will be welcome. Datasheet here:

Because of how voltage is reflected back at the primary, and thus the switch, and due to the high turns ratio of these transformers, I always advocate using a VERY beefy transistor.

My go-to FET for power switching is always the FDL100N50F. I've used these in everything from DC switching to multi-kW CW inverters in the MHz range. When the internal diode is blockaded and external schottky's are used they're essentially bulletproof, even when switching capacitive loads at the Vds rating. I can put up a datasheet if you need.

Ash Small

Fri Nov 14 2014, 02:06AM

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

Looks pretty amazing, Sig.

Â£10 each from RS, but it looks worth it.

Very low RDS and 100A.

Is there any particular external Shottky that you recommend?

Patrick

Fri Nov 14 2014, 02:08AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

foil with paper or overhead projection film is a good choice for skin and proximity effect reasons.
copper foil is not found in my kitchen, and aluminum is hard/impossible to solder, so i dont know what youll do for connections. maybe crimps to copper wire?

Ash Small

Fri Nov 14 2014, 02:18AM

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

Patrick wrote ...

foil with paper or overhead projection film is a good choice for skin and proximity effect reasons.
copper foil is not found in my kitchen, and aluminum is hard/impossible to solder, so i dont know what youll do for connections. maybe crimps to copper wire?

Maybe copper foil, or maybe experiment with crimped aluminium.

Shrad

Fri Nov 14 2014, 07:55AM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780

if I recall well enough 3M sells some copper foil adhesive which could be used for this kind of thing

Dr. Slack

Fri Nov 14 2014, 09:10AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

and aluminum is hard/impossible to solder,

nah! You just have to use the right flux. I bought some 'alli solder' from farnell two decades ago, and I'm only halfway through 1m of it. The trick is to heat the foil from the back, hotter than usual, and dab the multi-core on it so the flux gets heated by the foil. If you use the iron bit on the solder side, it sizzles the flux away while the foil is still cold and doesn't do its stuff. The flux is a bit nuclear in behaviour, so take the opportuntity to clean the foil at this point. Now solder the wire onto the tinned bit.

Ash Small

Fri Nov 14 2014, 11:03AM

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

Dr. Slack wrote ...

and aluminum is hard/impossible to solder,

You can solder it using an agressive flux, but you also need the right solder. tin-zinc solders are not good. tin-lead-silver is much better. The datasheet for the 'multicore' product available from Farnell is here:

Ash Small

Fri Nov 14 2014, 12:33PM

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

Is this Shottky diode suitable for protecting the MOSFET?

600v and 114A pulsed @25C for 10mS repetitive.

Looks like it's two devices in one package, each rated for 57A pulsed, but I'm assuming it will share the load evenly between the two legs.

These are under Â£5 from RS.

Wolfram

Fri Nov 14 2014, 01:06PM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

Ash Small wrote ...

Is this Shottky diode suitable for protecting the MOSFET?

In a regular flyback topology circuit, the MOSFET reverse diode will never be forward-biased as far as I understand, so bypassing it with a schottky diode will not make any difference, aside from increasing switching losses slightly due to the increased drain-source capacitance.

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