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ZVS transformer core material

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Camel

Tue Mar 24 2009, 07:18AM

Registered Member #1694 Joined: Sat Sept 13 2008, 09:13AM
Location: Australia
Posts: 108

Hi,

I'm building a capacitor charger using a ZVS circuit. I made myself a transformer out of a yellow and white ferrite toroid.

After testing it out, I've found the MOSFETs get very hot and there is no output voltage. Everything else stays cool except for the inductor and transformer which also get quite warm. I've checked a billion times and can't spot a problem with the actual circuit so I think perhaps the transformer core might be to blame . .

Are the yellow and white toroids not permeable enough for ZVS circuits? Or must I use a flyback transformer core?

Cheers

Dr. Dark Current

Tue Mar 24 2009, 07:37AM

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

Powdered iron cores (the blue, yellow and white ones) are not for power transmission, use ferrite core with a little air gap for your zvs circuit. You can keep the series inductor as long as it has enough inductance.

Camel

Tue Mar 24 2009, 07:49AM

Registered Member #1694 Joined: Sat Sept 13 2008, 09:13AM
Location: Australia
Posts: 108

Oh, I see. What do you mean a little air gap? Do you mean as little as possible, or is an air gap somehow beneficial?

101111

Tue Mar 24 2009, 08:17AM

Registered Member #575 Joined: Sun Mar 11 2007, 04:00AM
Location: Norway
Posts: 263

Jan, blue and white ones are not necessarily powdered iron cores.

Samuel, with round cores it can be rather difficult to make a air gap, and by saying difficult I mean almost impossible.
You should go for another type of core, that has two sections, then you can put some thin paper where the core meets, this will give the core the function to hold some of it the energy while switching. (I can't exactly explain why this is good/necessary but I can confirm that this will maybe help on efficiency and performance)

For cores, you can use a old LOPT core, or a old fly back core used in a CRT TV or monitor, these cores have built in air gaps.

Camel

Tue Mar 24 2009, 08:30AM

Registered Member #1694 Joined: Sat Sept 13 2008, 09:13AM
Location: Australia
Posts: 108

I've started winding another transformer on a core from a TV. One half of the core had cracked in half so I glued it back together.
Thanks for the info.

Camel

Wed Mar 25 2009, 07:43AM

Registered Member #1694 Joined: Sat Sept 13 2008, 09:13AM
Location: Australia
Posts: 108

Ok, I the transformers primary is 8 turns with a center tap and around 250 turns for the secondary. Its wound on a core from TV. The inductor is 35 turns on a yellow and white core. The resonant cap is 2.35uF. The voltage of the supply is 26V.

I turned it on and the voltage of the caps being charged quickly rose to 26V. The MOSFETs still got really hot really quickly, but I could hear a quiet hum unlike last time.
I also tried it out at 13V and the voltage of the caps rose to 125V, which I found interesting.

Does anyone reckon they know whats going on?

jonny5

Wed Mar 25 2009, 10:34AM

Registered Member #1807 Joined: Tue Nov 11 2008, 07:36AM
Location: San Luis Obispo, California
Posts: 19

Is the transformer core gapped? The Mazzilli utilizes a parallel LC tank circuit...your C is the resonant capacitor, and the L is the center tapped primary. The catch is if your transformer were to be perfectly coupled (no air gap, K=1) then your transformer couldn't store any energy. No energy storage, no resonance. Adding an air gap (even a really small one) lowers the effective core permeability and primary inductance...and stores energy for the L part of the LC tank.

Are you are rectifying the output of a variac for your 26/13V supply? If so, then you might be hearing 120Hz ripple. If not, and it's higher frequency (and your core isn't gapped) then your resonant frequency might be in the audible frequency range. One bummer there is that the integral of volt x seconds will be huge, and your transformer saturated! (does the core get hot?) If you have access to a scope, then a waveform of the mosfet drain-source voltage could shed a lot of light on the situation.

Just curious, but do you have zeners/TVS on the mosfet gates? 26V would be cutting in close for VGS...only asking because I forgot and added a few more mosfets to my silicon graveyard!

Good luck, and sounds like a cool project.

Camel

Wed Mar 25 2009, 10:57AM

Registered Member #1694 Joined: Sat Sept 13 2008, 09:13AM
Location: Australia
Posts: 108

jonny5 wrote ...

Is the transformer core gapped? The Mazzilli utilizes a parallel LC tank circuit...your C is the resonant capacitor, and the L is the center tapped primary. The catch is if your transformer were to be perfectly coupled (no air gap, K=1) then your transformer couldn't store any energy. No energy storage, no resonance. Adding an air gap (even a really small one) lowers the effective core permeability and primary inductance...and stores energy for the L part of the LC tank.

Hi,

Yeap, there is a gap measuring around 2mm.

jonny5 wrote ...

Are you are rectifying the output of a variac for your 26/13V supply? If so, then you might be hearing 120Hz ripple. If not, and it's higher frequency (and your core isn't gapped) then your resonant frequency might be in the audible frequency range. One bummer there is that the integral of volt x seconds will be huge, and your transformer saturated! (does the core get hot?) If you have access to a scope, then a waveform of the mosfet drain-source voltage could shed a lot of light on the situation.

No, the power supply is a bunch of AA NiMH cells. I guess the sound would be better described as a high pitched squeel. The pitch of the squeel gradually gets higher over a period of a few seconds until I can't hear it. Also the core stays quite cool.

I wish I had a scope. I have access to one, but I'm afraid I might kill it - the owner would not be impressed.

jonny5 wrote ...

Just curious, but do you have zeners/TVS on the mosfet gates? 26V would be cutting in close for VGS...only asking because I forgot and added a few more mosfets to my silicon graveyard!

Good luck, and sounds like a cool project.

Yup, there are a couple of 12V zeners on the gates. The circuit I'm trying to make is this one

except I'm using IRFP450s instead of IRFP250s.

Cheers :)

jonny5

Wed Mar 25 2009, 07:49PM

Registered Member #1807 Joined: Tue Nov 11 2008, 07:36AM
Location: San Luis Obispo, California
Posts: 19

With the IRFP450's, I wonder if the fets Rds might be causing a problem. The Vds of the fet that's ON is essentially I*Rds. An IRFP450 is spec'd at 0.33 ohms at 25C, about 0.5 ohms at 70C, and 0.66 ohms at 110C (approximations with 10Vgs). Also, the fet's threshold voltage begins at about 2V. What's the scenario? Let's say one of the fets is conducting 3A at 70C. With a 0.5 ohm rds, its drain voltage is 1.5V. The Mazzilli driver uses the cross-coupled diodes between the gate/drain to turn off the opposite fet, so the gate voltage of the fet that's off is 1.5V + diode drop (0.6V). That's 2.1V...barely threshold, but if it is then the fet that's off...actually isn't (just barely turned on). Problem is, this might be thermal feedback. (Off) Fet gets a little hotter (from barely turning on during off time) and, next 1/2 cycle when it's ON, its Rds is a little higher, increasing the opposite (Off-sorta ON)fet's Vgs...etc. Do you think this is even plausible? I've never seen it happen, just brain-stormin. Anyway, if you happen to have some lower Rds fets around, maybe they would be worth a try? Good luck!

uzzors2k

Wed Mar 25 2009, 10:12PM

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

The problem here is most likely magnetizing current being too high. I never took note of the air gap size I used, but I'm sure it was smaller than 2mm. Try reducing the gap spacing to less than 1mm and see if that helps. IIRC I only used one or two of the spacers that come with the cores found in flybacks.

The only thing that I disagree with is inductors requiring an airgap to store energy. Some (or a lot) is indeed stored in a gap, but there's also energy stored in the magnetic field around the core. After all, an inductor's potential energy is 0.5*L*I^2. Resonance can still occur without an air gap.

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