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Andre

Mon Oct 22 2012, 07:40PM

Registered Member #6921 Joined: Wed Sept 26 2012, 07:47PM
Location:
Posts: 109

I have 2 transformer both have 10 turns on the primary, and one of them has 2000 turns in the secondary and the other has 4000 turns, I'm running at 45Khz 24V, with the one with 2000 turns in the secondary it works fine but with the one with 4000 turns it blows up the Mosfet, should I set the frequency to 20Khz?

Dr. Brownout

Mon Oct 22 2012, 08:20PM

Registered Member #2405 Joined: Fri Oct 02 2009, 12:59AM
Location: Wisconsin
Posts: 140

We really dont have enough information. What are the transformers driving are they driving mosfets or are the mosfets driving the transformer? What is the application and if you would post a schematic that would be helpful.

Sulaiman

Mon Oct 22 2012, 08:37PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Not enough data for an accurate estimate;
primary voltage, core cross-sectional area .....
assuming that the two transformers use similar cores and the windings are of similar volume and that the problem with the 4000 turn coil is due to it's self-resonant frequency being below 45 kHz
then the 4000 turn coil would probably run better at a lower frequency
but at a lower frequency the core may saturate
so the primary voltage would have to be reduced
or the primary number of turns increased
either of which would reduce the output voltage of the 4000 turn secondary.

ALSO, there may be one or more shorted turns in the 4000 turn secondary.

Since I have the kit to do it I would measure the resonant frequencies of the 2000 and 4000 turn secondaries and measure the Q of the two transformers.

If you can't do this then try a lower frequency for the 4000 turn secondary,
I'd start around 30 to 35 kHz.

Andre

Mon Oct 22 2012, 08:55PM

Registered Member #6921 Joined: Wed Sept 26 2012, 07:47PM
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Posts: 109

Dr. Brownout. 2 mosfet driving the transformer similar to this circuit

Thank Sulaiman, how does the kit looks like?, I will like to measure the resonant frequencies of the secondary.

Dr. Brownout

Tue Oct 23 2012, 01:03AM

Registered Member #2405 Joined: Fri Oct 02 2009, 12:59AM
Location: Wisconsin
Posts: 140

I would try what Sulaiman suggested and lower the frequency of the 4000 turn transformer. Unless you know the core material its going to be an experimental process because it could be as sulaimian suggests the transformer is probably resonating below 45Khz. If you can play with the frequency at a reduced voltage you will probably find the sweet spot for that transformer (providing its not shorted somewhere).

Andre

Tue Oct 23 2012, 03:31AM

Registered Member #6921 Joined: Wed Sept 26 2012, 07:47PM
Location:
Posts: 109

Dr. Brownout wrote ...

I would try what Sulaiman suggested and lower the frequency of the 4000 turn transformer. Unless you know the core material its going to be an experimental process because it could be as sulaimian suggests the transformer is probably resonating below 45Khz. If you can play with the frequency at a reduced voltage you will probably find the sweet spot for that transformer (providing its not shorted somewhere).

is there another way to find out the correct frequency than trial and error?

Sulaiman

Tue Oct 23 2012, 12:37PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

By 'kit' I meant equipment ... sig.gen. 'scope and an LCR meter that also displays 'Q' (or 'D' = 1/Q)
Do you have a sinewave signal generator?
and a 'scope?

Andre

Tue Oct 23 2012, 03:14PM

Registered Member #6921 Joined: Wed Sept 26 2012, 07:47PM
Location:
Posts: 109

Sulaiman wrote ...

By 'kit' I meant equipment ... sig.gen. 'scope and an LCR meter that also displays 'Q' (or 'D' = 1/Q)
Do you have a sinewave signal generator?
and a 'scope?

yes have all 3 , so what values I should look for? or reference site that you can point me to.
thank you!

Ash Small

Tue Oct 23 2012, 04:40PM

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

What I would do, and others may have better ideas, is to connect a 100 Ohm resistor in series with the 4000 turn secondary, then connect these to the sig. gen.

then connect the scope across the 4000 turn secondary, then sdjust the frequency until you get maximum peak to peak. This will be the resonant frequency of the coil.

(I usually use this method to find the resonant frequency of an inductor/capacitor tank circuit, using a known capacitance, so I can calculate the inductance of the coil, but I assume it will work in this case too.)

Andre

Thu Oct 25 2012, 03:44PM

Registered Member #6921 Joined: Wed Sept 26 2012, 07:47PM
Location:
Posts: 109

Ash Small wrote ...

What I would do, and others may have better ideas, is to connect a 100 Ohm resistor in series with the 4000 turn secondary, then connect these to the sig. gen.

then connect the scope across the 4000 turn secondary, then sdjust the frequency until you get maximum peak to peak. This will be the resonant frequency of the coil.

(I usually use this method to find the resonant frequency of an inductor/capacitor tank circuit, using a known capacitance, so I can calculate the inductance of the coil, but I assume it will work in this case too.)

what do you usually see with this test? this is what I'm seeing on both transformers. at 30hz, 300hz and 3khz, the scope prove 2 is the secondary and 1 is the primary, the pulse generator is on the secondary in parallel with a 100ohms resistor

29ylax4

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