toroid for car amp smps

[wrenchone]

To clarify my last post, Ae is the effective core cross section in cm^2. Ae is a basic parameter for any core, and should be found in the core specification. The power handling capability of the core depends on the copper cross-sectional area and winding resistance. This will depend on frequency, as the number of turns varies inversely with frequency. The power handling capability goes up with frequency until core losses and copper losses (from skin effect) dominate. It is in ones's interest to minimize the number of turns on the core (within reason) to reduce the winding resistance. For 12V input, 50kHz operating frequency, an Ae of 1cm^2, Bp of 2000 Gauss, the number of primary turns is
10^8 * 12/2000*1*5*10^4 = 12 turns. Since a push pull converter is used, one winds two bifilar windings of 12 turns. The twowindings should be applied simultaneuosly and be spaced close together to maximize coupling and ensure that they are identical, hence the bifilar winding. The secondary windings are scaled from the primary.

[Eva]

tennisballg :

The first image shows the right way to do the windings since in the second image you are maximizing inter-winding capacitance and this causes huge amounts of ringing

In the other hand, this core is absolutly useless as a transformer since its color code [yellow body with white side] means 'iron powder material' [usable only as a power inductor]

You must get ferrite cores, independently of the format [toroidal, EE, EI, etc...]. If you only have access to PC PSU components then your *only* choice is the main EE or EI transformer, these usually work at 30Khz and use 7 primary turns for 12V so at 50Khz you can also use 4 turns per primary. Use moderate heat to carefully soften the glue and dissasemble the core. Don't apply excessive force or you will crack the core. Count the numbers of turns when disassembling and take it into account. To know the real operating frequency of the unit look at Rt and Ct from the TL494 IC that most units use [see the datasheet and divide by 2]

[blmn]

Actually the maximum power you can get from a transformer depends on the rise of the temperature on it.

There is a book called Designing Magnetic Components for High frequency DC-DC Converters, from McLyman, with lots of information and many design examples.

I made many transformers based on that book and all of them worked fine.

Regards,

[sss]

Originally posted by blmn
Actually the maximum power you can get from a transformer depends on the rise of the temperature on it.
Regards,

and how to calculate that?
anyways
most of us using those ferrite cores without calculating the proper core size but afaik its ok to use an oversized core

[tennisballg]

Yes, both the toroid and wires are useless for a push pull SMPS, it's one of my extra toroids, I just grabbed one to wind as an example. See my earlier post for the stats on the one I want to use.

By the way, I think the temperature rise is only a byproduct of core saturation, which is really the point that we're looking for(I guess).

[wrenchone]

No, the temperature rise is due to copper loss and core loss. If you are saturating, you are in trouble, unless you designed the transformer that way. A car inverter transformer wilkl be copper loss limited due to the high primary currents. Transformer capacitance will be of little concern due to the low voltage. An off-line switcher transformer can be an entirely different affair. BTW, I don't recall seeing the power range desired.....

[Pabo]

Blmn

Actually the maximum power you can get from a transformer depends on the rise of the temperature on it.

Absolutely correct, and the temperature rise is directly proportional (if no linearly) to the flux density which is decided by the time-voltage-area. The lower the switching frequency and the higher the voltage, the higher flux density.

Therefore it is correct to say that the amount of power that can be withdrawn from a core is decided by the amount of copper that can be fit onto the core.

In principle you could take out 1000W out of a 10mm diameter core if you only could find those supra conductors laying around.

[blmn]

I agree with Wrenchone and Pabo,

The temperature rise of the transformer depends on basically the core and cooper losses on it. It is the effect of the power losses of the transformer and the root causes are as they defined.

Regards,

[sss]

what is the max switching freq for ferrite cores?

[gmarsh]

Originally posted by sss
what is the max switching freq for ferrite cores?

We've used ETD ferrites at 500KHz here at work. You have to wind them with Litz wire at these frequencies though, otherwise copper losses will go way up. Also, the switching loss in your MOSFETs will rise dramatically.

For a car amplifier SMPS, I'd stay in the 50-75KHz range.

If you need a lot of output power, you can use multiple transformers... You could wind two transformers with the primary and secondary windings in series, or you could get creative and use two identical transformers with single primary windings.