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Modern AC "Fiddy" flybacks info and discussion

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kiat

Fri Oct 16 2015, 04:16PM

Registered Member #2115 Joined: Fri May 08 2009, 01:17PM
Location: Singapore
Posts: 46

Basically I have got a sample order of Fiddy type flybacks made, and am attempting to optimize their design for voltage output and parasitic capacitance.

I have got in touch with a seller in China (An hui) and can customize the construction to a large degree (vacuum potting in epoxy or oil, wire used, no. of turns/layers, center tap, etc).

I originally intended them to be used in a cockcroft-walton multiplier, and will soon get the seller to assemble and pot one for me.

Sales and pics at:

For a number of reasons I don't like flyback drive. You can discuss that at Fiddy's :)

A copy of the tech specs:

2 Models,
(1) 2800 turns of 31AWG double build magnet wire
(2) 1600T of 32AWG triple insulated wire (Furukawa TEX-BS ; couldn't find TEX-FS).

The potted version will not accomodate even a foil primary under the secondary -- foil is possible if you get the bare bobbin and vacuum degass in oil; however pr-sec insulation may still be too little.

With the primary on the opposite leg, coupling coeff ~ 0.9685

With the primary shorted, secondary inductances are (1) 1.65H and (2) 0.5H
This corresponds to self resonance (parallel resonance) at (1) 42kHz and (2) 82kHz

AC Resistance of the windings were measured at low frequencies, up to 10kHz. As f~SRF, parasitic capacitance affected the results when using a simple Ls-Rs model.
1600T, full Ls: 120 ohm + 50ohm/kHz
1600T, leakage Ls: 185 ohm + 20ohm/kHz ??
2800T, forgot: 340 ohm + 100ohms/kHz.

Core: UY20, material is TDK's PC40. Clip used has M3 threads. Default ungapped -- gapping spacers available but they aren't for this core.
Al ~ 3.2uH/N^2
Ve 60cc
For 200mW/cc, reasonable if forced air cooling or under oil,
f, B, fB
24khz 300mT 7200
35khz 250mT 8750
54khz 200mT 10800
91kHz 150mT 13650
For fB around 10000, 13.3V/turn

Bobbin: 8-segment, ~0.8mm insulation between segments, Segment depth ~7.5mm and width ~5?mm, OD~45mm and ID 21~23mm.

Primary: 130Turns, 14//34AWG , 11 meters, 0.28mm^2 CSA, 0.6~0.8mm OD. 2~3 layers.
SRF 68kHz , 22 ohm (Series resonant)

If there's anything that is unclear or should be elaborated above, please let me know.

Anders: The ferrite should be good to 200kHz. Above that bridge switching becomes tricky and f*B product less increment.

What topology did you use for driving to the right of SRF?

At resonance, yeah, voltage is developed across the leakage inductance. However that itself is distributed across the entire secondary winding equally (unless you gap).

So the volts/turn should still be the same? Each turn has equal Lmag and Llkg.

Interesting point about the circulating currents at resonance.
The // resonant model for the flybacks were around 20 or 30Mohm//7~9pf//8 or 27H.
Can we presume all losses (ferrite+epoxy DF+winding Rac) can be lumped into this // RLC model?
This was from the impedance analyzer readout.

Edit: The prices are 35usd (magnet wire) and 51usd (triple insulated wire) before shipping
Shipping from Singapore is 46usd/flyback and directly from China is 45USD/3 flybacks.

woodchuck

Fri Oct 23 2015, 04:33PM

Registered Member #39190 Joined: Sat Oct 26 2013, 09:15AM
Location: Boise National Forest
Posts: 65

When flyback transformers are used to generate arcs, I can imagine that one has to be concerned with voltage breakdown in the secondary, and one shouldn't operate the transformer at resonance (when voltages can spike to enormous levels).

But if a flyback is used only to drive a CW multiplier, and if a load is permanently placed across the multiplier stack, wouldn't it seem unlikely that voltage breakdown in the transformer secondary would be much of a problem even with ordinary insulation on the wire?

hen918

Fri Oct 23 2015, 09:43PM

Registered Member #11591 Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556

woodchuck wrote ...

When flyback transformers are used to generate arcs, I can imagine that one has to be concerned with voltage breakdown in the secondary, and one shouldn't operate the transformer at resonance (when voltages can spike to enormous levels).

But if a flyback is used only to drive a CW multiplier, and if a load is permanently placed across the multiplier stack, wouldn't it seem unlikely that voltage breakdown in the transformer secondary would be much of a problem even with ordinary insulation on the wire?

A constant load can be provided in the the form of a safety gap that breaks down and arcs, therefore pulling the voltage down, before the insulation gets damaged.

If the voltage total or voltage per turn is less than the breakdown voltage of the respective insulation, then there won't be a problem, the load should suppress any "voltage spikes", although it is definitely good practice to use an at least 2x safety factor.

Patrick

Tue Nov 17 2015, 07:40AM

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

You certainly have a lot of data and specs on your transformer. But parasitic, stray and coupling issues can have bizarre effects. That's why I went to so much trouble building a high voltage o-scope probe.

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