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kinser 11th February 2005 04:13 PM

EE core for car SMPS
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Hi all,
I found This kinda "big" transformer in an old apple psu. its an EE transformer, I would like to use it as a transformer for a car SMPS.
but I need to know if its good, and how to calculate the nuber of winding for primary and secondry's. and most important... how much power it can supply(watts). what i would like is to get about 56Vdc+/-.
can anyone help me with this thing?


N-Channel 11th February 2005 04:52 PM

EE Core for Car SMPS

Seen your posts in some of the other Power Supply threads. Couple of questions: Is this a flyback or half-bridge topology? :confused: You can usually tell if there are two hi-voltage transistors as opposed to only one. If flyback, then you might have a hard time with respect to the leakage inductance, as the core for the transformer has a small gap. :bawling:

If I'm looking at photo 1 OK, it looks like there is no gap, so you should be OK. :D

Check out some of my other poses in the Power Supply forum (I don't know how to re-direct you there with a hyperlink, sorry) for reference material and books to use. Geo. CHryssis' book, High Frequency Switching Power Supplies (c) 1989 breaks up SMPS design inti chapters by the sections of the supply.

Chapter 6 (I think- it's been a long time since I've looked at the book), is devoted entirely to the transformer (for forward topologies), and the transfermer-choke (for flyback topologies). It shows you how to calculate the number of turns based on frequency, material type, topology, and a whole host of other parameters.

If this book is available in Israel, I would recommend picking up a copy, or borrowing it from a library.

Best of luck, and let us SMPS gearheads know how this turns out.


jackinnj 11th February 2005 05:10 PM

probably 150 - 200 watts.

before taking the transformer apart, figure out the winding ratios --

with an ohmeter determine which of the pins are connected to each other -- you can connect an audio oscillator at 10kHz to one of the sets of windings and measure the resulting voltages on the other pins to determine the turns ratio (just make sure that your oscillator doesn't get too loaded by the transformer). Be careful because there will be high voltage on one or more of the windings.

you might find that using the original primary as the new secondary is going to work fine in your application -- you will need to adjust the duty cycle of the switcher, take into consideration the voltage drops in the rectifiers and chokes etc.

kinser 11th February 2005 05:24 PM

Hi, I have done a stupid thing, earlly today I took off all of the windings, so I guss I might as well throw it away.:bawling:
but what about the transformer on an AT power supply? I have two of them still on the AT board, cauld I use both of them to get more power?
BTW the transformer had one mosfet at the primary(I think, I had a quick look at it). Also I have one LT494 and one 7500(they are the same) Is there any SMPS circuit thats based on this chip? could you post It if you have one?


jackinnj 11th February 2005 05:39 PM

email me and I will send you the ATX article from QEX -- everything you need to know about modding a supply. -- the author was a magnetics guru from some California aerospace company.


N-Channel 11th February 2005 07:05 PM


jackinnj's QEX article is a realy good one. It's very thorough and several pages long. He sent it to me a couple of weeks ago, and I'm still digesting it. :xeye:

BTW, don't throw that old core out- you can still use it, as it might not be as hard as you think to identify the core material and get specs on it. If it IS a flyback transformer-choke, there are some SMPS circuits you might be able to use it in, just not this one.

As for the two AT transformers you have, I wouldn't use them on the original AT board, but make your own pc boards (you're probably gonna do that, anuway).

There are alot of circuits for +12V center-tap push-pull topology using the TL494. You can even make separate separate power supply modules for each channel, using one core for each module, synchronizing the two '494s together to eliminate beat frequencies. This way, each channel can have its own dedicated power supply. It's actually really easy to do. Go to and look up TL494. Synchronizing is on the page 9 of the datasheet.

kinser 11th February 2005 07:15 PM

Thanks for the article,
But I what to make a 12v(car battrey) powered SMPS,
not to mod an ATX power supply, as for the AT xformers, I know that I will have to take them off and make new boards :) and what range of power will they deliver?

Keep posting!


jackinnj 11th February 2005 08:51 PM

well, you now have all the equations -- instead of 165V on the primary of the transformer you have a different number -- 12V

you can omit the "housekeeping" power winding on the transformer since it's possible to avail yourself of the 12V from the battery.

the circuit of figure 18A (the TL494 and the "base-drive-transformer") is the same, you have to change the turns ratio in the transformer -- which is T1 in figure 18B -- of course this changes the inductance etc. but you should be able to figure it out from the text.

kinser 11th February 2005 09:16 PM

thanks for making that clear. In my AT power supply there are two TO-220 2SC2335 transistors, can I use them?


I dot realy understand how to calculste the windings....:xeye:
please help me..

N-Channel 12th February 2005 12:37 AM

2SC2335 Transistors

Those are the main switching transistors, and since they will see over 320V across them, they are most likely 400V, 8-15A rated NPNs.

Not a real good choice if you want alot of current on the primary. You could change them to some good N-Channel MOSFETs rated at 60V. Any N-Channel MOSFET over a 30A drain current ratinf will do- there are many choices.

As for your AT transformer cores, if they came out of a 200-250W AT box, then they are probably good for about 60-80W continuous power, with extended peaks of 200-250W. That is a dirty little secret behind all those impressive ATX power supply ratings. 500W (continuous) from an ATX box? :whazzat: I don't think so. for a 500W rated unit, more like 120W continuous, even with forced-air cooling.

Anyway, I'm babbling again, I always do that. In all honesty, 80W CONTINUOUS power PER channel is more than plenty for your needs. And remember about using two PWM supplies in synchronicity. It will look cool, work great, impress your friends and get you all the girls! :cool:

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