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Mazzilli ZVS Driver driven from more than 40V (how many times has this come up?)

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uzzors2k

Fri Apr 06 2007, 08:58PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

I thought it would be fun to play with the good ole' ZVS driver again, and as opposed to last time I now have an understanding of electronics, hordes of components and a 50V PSU. I got it working fine at 12V, played with some capacitor and inductor values until I got satisfying arcs. However when I tested it at 50V it failed to oscillate, and both IRFP450s were steaming in seconds. After an inspection they turned out to be dead. This is how I hooked it up.

Is this how its supposed to be done? Or did you just leave the circuit as is?

Marko

Fri Apr 06 2007, 10:57PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

I'm just diong some shots of the circuit I just built... I'l post everything in my thread after I'm done.

PS.
My circuit used to stop oscillating when I used unbalanced number of turns (1 less).
In that case it rolls a dice with each power-up and promptly stops oscillating if it feels like so, leaving one mosfet shorted. I first tought it had something with diodes, but it works fine even with 1n4007's.

But, I'm not sure if your case is similar because if oscillation stops for any reason, only one mosfet will conduct and eventually die. Other one may die only after first goes open!

Gate zeners are still a must regardless if you are using a separate low voltage gate supply.

I don't know what much else can be wrong if you done it by schematic... what did you use for the inductor?

...

Sat Apr 07 2007, 07:07AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

I would recommend adding more turns to your inductor, or adding another in series. Also, if you put a fuse in series with it (after any filter caps) you can probably prevent from blowing the fets when it latches up.

uzzors2k

Sat Apr 07 2007, 07:20AM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

I'm using a yellow ATX inductor with all the turns series up, but I'll put another one in series then. One of the mosfets survived actually, I didn't notice until now, I'll inspect that side of the driver and look for flaws. I've got short-circuit protection on my PSU, but I thought it was tripping for no reason as it sometimes does, so I bypassed it.

Oh well, one dead IRFP450 out of 300 doesn't make much of an impact, hehe. The primary is 4.5+4.5 turns, and seems to be completely even. Is that enough turns for 50 volts? Should I try cutting the regulated 12V supply and just feed it with 50V?

EDIT: I just ran it directly from 50V, and it worked. I had to parallel two IRFP450s, and the short circuit protection still trips. Its set for 25A, do you think its actually drawing that much, or just tripping due to transients? The power supply drops to 23V, at 25A thats 575W! The lights actually dimmed for a moment when the short circuit protection went from pulling an arc!

If it happens agian I'll have to make a video clip.

Steve Conner

Sat Apr 07 2007, 08:56AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

4.5 turns seems way too little for 50v. I'd use something like 12+12.

uzzors2k

Sat Apr 07 2007, 11:15AM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

I upgraded it to 8+8, since I can't fit any more than that on the small core. The PSU still trips after I extend the arc more than a few cm. Either the arc causes a current surge upon quenching, or the current builds up while pulling an arc until the PSU trips and quenches it early. Should I assume the short circuit protection is tripping on transients and just bypass it? Here is some info about it, I've upgraded to a 35A rectifier bridge since then. Current sense resistor is set for 25A.

How much current is the ZVS drawing? My multimeter is toast, so we'll have to estimate, but based on what I've read in the archives 15-20A seems normal. So unless the mosfets are latching it should be safe to bypass the protection. However I'll wait for an expert opinion. (that means anyone here)

Sulaiman

Sat Apr 07 2007, 06:12PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

From my little experience with this kind of circuit;

1)When using separate supplies for power and gate-drive
the gate-drive supply should be connected first, then the main power.

2)The ferrite core must not saturate, how to calculate that?
2.1) Integral of V.dt = Vpk.(PI/8.F) = Vdc.PI.PI/8.F (V.s)
2.2) Peak magnetic flux = Phi = B.Ac (Wb)
2.3) (Integral of V.dt)/N = Phi = B.Ac
Vdc.PI.PI/8.N.F = B.Ac
so
N= (Vdc.PI.PI)/(8.F.B.Ac)

e.g. Vdc=50V, F= 20 kHz, B= 0.2T, Ac= 200x10-6 (core area, 200sq.mm)
so N= 77 turns! (38.5+38.5 turns)
you could push it to 0.3T peak flux density for 26+26 turns
or operate at a higher frequency, 50kHz= 30 turns.
or use a larger core...etc.etc.

If you use less turns than calculated using the above, then the core will saturate
for very little extra output.

uzzors2k

Sat Apr 07 2007, 06:16PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

Thanks, that was really useful. I'll up the frequency and see how many more turns will fit on. However, since the supply voltage drops to ~23V, shouldn't the calculations be done from there? Or say 30V since current draw won't be as high.

Sulaiman

Sat Apr 07 2007, 08:15PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

I was just giving the basic principles;
actual core-size, frequency etc. are your choice.

Incidentally, for the flybacks that I have 'played' with,
the secondary has a definate resonant frequency, different but similar range for most types.
If you have a signal-generator apply it to your primary and monitor the output
(dc multimeter with a capacitor across the rectified flyback output )
and try to find the resonant frequency, then adjust your
Primary capacitor and/or number of primary turns and/or core airgap
to operate at or near this frequency, somewhere around 40kHz would be a good start.

Dr. Dark Current

Sat Apr 07 2007, 08:31PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

uzzors wrote ...

I'm using a yellow ATX inductor with all the turns series up, but I'll put another one in series then.

I think the problem is your "yellow" inductor, because these are meant to filter frequencies at some mhz, not khz. Use some black ferrite, old pc psu transformer or similar works well, fit at least 20-30turns on it.

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