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Registered Member #11591
Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556
I know they can use a special bonded ferrite powder that basically means that the "gap" is distributed across the whole core. It means that they can make the "gap" exactly the same size every time. It probably works out cheaper overall as well.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Patrick wrote ...
yeah im not sure if having or getting rid of the gap helps your guys' ZVS type circuits. i mainly use traditional flyback circuits, so for me its essential.
As ashSmall said, we often dont get a choice as there epoxied.
As I understand it, the Mazzilli ZVS circuit is a push-pull, conventional transformer circuit, and therefore doesn't want a gap (there may be exceptions, but I'm not familiar with any, other than possibly to prevent saturation), and traditional 'flyback' circuits are actually 'coupled inductors' (not transformers) and generally do require a gap, not just to prevent saturation, but also to store more energy.
Maybe someone else can shed some more light on this subject?
EDIT:
hen918 wrote ...
I know they can use a special bonded ferrite powder that basically means that the "gap" is distributed across the whole core. It means that they can make the "gap" exactly the same size every time. It probably works out cheaper overall as well.
I also have some old flybacks with three gaps in the leg that contains the windings. Also, they don't always get the 'distributed gap' ones the same every time, due to various production factors, and they are actually measured after production to determine the specification.
Registered Member #11591
Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556
Yeah, the definition of "flyback transformer" is a coupled inductor. Using it for conventional transformers leads to confusion. ZVS drivers rely on the (relatively low) inductance of the flyback transformer's primary windings to keep a reasonably high frequency. If there was no gap (real or distributed) the inductance would be too high and you would end up with ridiculously low frequencies, (usually resulting in the oscillations stalling and one of the MOSFETs blowing up) If you don't use any sort of gap, you end up with a normal transformer which you can drive with a full/half bridge, but, without the flyback effect, the secondary voltage will not be as high.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
hen918 wrote ...
Yeah, the definition of "flyback transformer" is a coupled inductor. Using it for conventional transformers leads to confusion. ZVS drivers rely on the (relatively low) inductance of the flyback transformer's primary windings to keep a reasonably high frequency. If there was no gap (real or distributed) the inductance would be too high and you would end up with ridiculously low frequencies, (usually resulting in the oscillations stalling and one of the MOSFETs blowing up) If you don't use any sort of gap, you end up with a normal transformer which you can drive with a full/half bridge, but, without the flyback effect, the secondary voltage will not be as high.
I understood that most people remove the gap when using TV flybacks with the Mazzilli ZVS circuit. I've not yet built one myself, so can't be certain, but this is the impression I got from reading threads on the subject. I suppose there would still be some distributed gap, anyway.
I do know that for flyback mode (coupled inductor) if the gap is excessive there is excessive reflected voltage on the switching transistor/fet. I was recently asked to test a driver circuit and spent hours scratching my head how I was getting over 600V reflected peaks across the DS of a 100V fet when only using 12Vds supply. I pulled the old flyback apart and found that there was a chip missing from the core right at the gap. I ground down the cores and restored the original ~1mm gap spacing and the reflected voltage went back to the expected 60V. Having those huge reflected spikes caused a tremendous amount of heating in the FET. Really surprised it didn't just blow it up, but I guess the avalanche current was low enough to prevent silicon destruction.
I.e. for a flyback mode you want a gap but don't want it to be excessive. Too small of a gap means too small of energy storage and you won't get the voltage step up you expect. IIRC it does drastically reduce current draw having next to no gap though.
For push/pull "normal transformer" mode you can certainly live without a gap, but it really depends on your drive method. If you have a drive topology that isn't going to get stalled like the famous Mazzilli ZVS then you should be just fine.
One really nice thing about these modern transformers Fiddy has is that we can easily take them apart and vary them to our needs.
I'll just caution the user to watch the output voltage; these are very high turns ratio and probably require oil submersion for EHT applications.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
According to theory at least, the ZVS needs a gap to define the inductance of the core and get a good Q in the resonant circuit. In practice it seems to work without one, it runs somewhere between a LC oscillator and a ferroresonant one where the frequency is controlled by core saturation.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Hi Fiddy, have you ordered the transformers yet / is there a place for more orders? If so, do you know what is the maximum design (peak) voltage output (insulation rating) of the xfmr?
Registered Member #8817
Joined: Mon Dec 17 2012, 05:16AM
Location: Australia
Posts: 110
Dr. Dark Current wrote ...
Hi Fiddy, have you ordered the transformers yet / is there a place for more orders? If so, do you know what is the maximum design voltage output (insulation rating) of the xfmr?
Thanks, Jan
Yes the transformers have been ordered, i ordered 25 and 22 are taken leaving 3 unsold, are you interested?
Maximum design voltage or insulation rating is 83kVpk says the manufacturer.
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Modern AC Flyback Transformers
