Author Topic: How to use old SMPS Transformers and SMPS Transformer Design  (Read 3259 times)

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Offline Xieos

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How to use old SMPS Transformers and SMPS Transformer Design
« on: August 27, 2016, 10:35:39 pm »
Hi All,

Short Story :

I need to Identify old SMP transformers turns ratio and suitable operating frequency without dissembling the transformers

Long Story :

I was wondering how to check old SMPS transformers and re-use it.

I have around 10 tiny SMPS transformers and I need to Identify each on in terms of operating frequency and turns ratio without disassembling any of the transformers.

a lot of SMPS discussions I have went through but I didn't seem to find a detailed illustration of the SMPS transformer design and hack ( which I think is more important than the switching device it self )

I have measured the Inductance of the transformers coils with a LCR meter (while all terminals are open )  and measured the leakage inductance for the primary coils (while all other terminals are shorted together )

I also have the analog discovery 2 , and used its function generator to apply a frequency sweep to the input and check the output wave amplitude , (also identify the polarity and marked it on the transformers with black dots where the input and output are in phase )

the response I get when I applied a frequency sweep from 50 kHz to 500 kHz to a particular transformer is the output amplitude increases with a frequency till around 262 kHz then starts to drop from 262 kHz to 500 kHz.

so I'm guessing that  transformer operation limits from around 50 kHz to max of 262 kHz.

the change in the output amplitude puzzled me , as I was just expecting that it's simple just  (input peak to peak / output peak to peak ) = Transformer ratio.

I'm just missing something so your comments will be greatly appreciated. thanks

BTW ... I'm a mechanical Engineer ....  |O |O |O |O


Offline T3sl4co1l

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Re: How to use old SMPS Transformers and SMPS Transformer Design
« Reply #1 on: August 28, 2016, 01:29:20 am »
Consider what you're doing: testing with a function generator (traditionally 50 ohm source impedance) will give a bandwidth that depends on inductance and capacitance of the load.

The mechanical analogy is acoustic coupling to a spring-and-mass system.  You get the widest bandwidth, the best coupling, when the spring-and-mass system has the same mechanical impedance as the acoustic source medium.  Which is why a tuning fork (high mass and spring constant) couples poorly to the air (low mass and spring constant), but a speaker does (light cone, small spring constant).

A switching supply generally has a lower impedance, so it's capable of pushing around the transformer much more effectively.  Instead, other considerations limit power level and frequency -- the core can only handle so much flux*, just as a real spring can only cover so much displacement before bottoming out (or topping out, as in a coil spring stretching to fully straight I suppose).  For these reasons, a transformer might be suitable at different frequencies than you expect from small-signal measurements.

*Flux is the time-integral of voltage.  Simply: the longer you apply a voltage for, the more flux you've applied.  For a square wave, the applied voltage flips back and forth, so the flux goes up and down over time; the peak flux is proportional to the cycle period.  Flux is to voltage what position is to velocity.

By building a pulse testing fixture, like this,

you can drive the transformer into saturation, and determine how much flux it can handle.  By measuring the core dimensions, and assuming a typical material (usually MnZn ferrite, Bmax ~ 0.3T), you can even determine number of turns!

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The following users thanked this post: chickenHeadKnob, Cliff Matthews

Offline Xieos

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Re: How to use old SMPS Transformers and SMPS Transformer Design
« Reply #2 on: August 28, 2016, 02:50:48 pm »
 Tim Thanks for the illustration,
I was shorting the 50 ohm on the AWG through the jumper on the BNC connector board of the Analog discovery 2

Let me show you what I have done till now , and add some measurements  just to make sure that I'm going the right way before trying out the test fixture .

the attached photo have one actual transformer and I added the Schematic symbol after testing out the transformer with a Multi-meter for Coils continuity and polarity using the function Generator and the scope.
 I identified the input with yellow color and outputs with light green and sky blue color.

the table includes all the LCR Meter measurements (with a red column for the leakage inductance )
the Inductance Leakage measurements were taken as the following:

X0 > X 2        (Short Pins Y0,Y1,Y2,Z0,Z1, Open Pins X1)
X0 > X 1        (Short Pins Y0,Y1,Y2,Z0,Z1, Open Pins X2)
X1 > X 2        (Short Pins Y0,Y1,Y2,Z0,Z1, Open Pins X0)
Y0 > Y1         (Short Pins X0,X1,X2,Z0,Z1, Open Pins Y2)
Y0 > Y2         (Short Pins X0,X1,X2,Z0,Z1, Open Pins Y1)
Y1 > Y2         (Short Pins X0,X1,X2,Z0,Z1, Open Pins Y0)
Z0 > Z1         (Short Pins Y0,Y1,Y2,X0,X1 )

one thing also about the Test setup , will I change the frequency or change the duty cycle of the applied DC pulses....?

thanks in advance


Online Cliff Matthews

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« Last Edit: August 29, 2016, 05:54:19 pm by Cliff Matthews »

Offline Xieos

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Re: How to use old SMPS Transformers and SMPS Transformer Design
« Reply #4 on: September 04, 2016, 03:56:06 pm »
Thanks guys for the re-direction

I have also read this post
Which is really helpful,  but I just want to confirm its integrity and on the end of the post there is a calculation for the primary and secondary inductance. .. I can't actually get the numbers right, can any one explain it in details


Offline blueskull

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