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chris2 15th February 2013 02:05 PM

Leakage Inductance
Hi everyone,

I'm building a power supply similar to the one from Carlos.

It has some snubber capacitors/resistors with std. values.

Just 4 fun, I thought I would try to optimize those values for the parts I have chosen.

I read an article here somewhere about snubbers and how to det. values for them. The article said that since the ringing of an AC/DC psu occurs when the diodes are tuning off and that this is due to the leakage inductance + stray capacitance of the transformer/diodes.

Therefore, has to det. the leakage inductance and stray capacitance and then plug those into some equations to calc them.
For example:
-snubber resistor, SR = sqrt(L/C), where L is the leakage inductance and C the stray or interwinding capacitance
-snubber capacitor, SC = 2 * Pie * sqrt(L*C) / R.

So, the first question is:
1. how to measure the leakage inductance of a transformer.
2. Do i simply short the secondaries with a good thick wire and measure the inductance of the primary, is that it?
3. Or do i need to det. all the resistances, e.g. of the short wire, my DMM when shorted, dc resistance of primaries, secondaries, turn ratio's, reflected resistance, etc.?

Sorry but I'm a little confused about how to go about this... I'm just hobbyist, so a super expensive LCR meter isn't an option for me. However I do have an oscilloscope and a function generator at my disposal.


Mark Johnson 15th February 2013 02:27 PM

If you have a sine wave generator and an oscilloscope you can use the method shown in this post

chris2 15th February 2013 05:16 PM

two equations for 2 unknowns
Thanks transistormarkj for the links!

Is the R value in the second link's diagram, the secondary dc resistance?

Also what are the two equations he mentions?


Speedskater 16th February 2013 12:53 AM

Was it?

"Calculating Optimum Snubbers"

by Jim Hagerman
Hagerman Technology LLC: Technical Articles

soundchaser001 16th February 2013 01:04 AM

You can measure it by putting a short across the secondary and measuring the primary inductance. We used to measure it that way when I worked at a transformer shop in high school. Wikipedia has a page on it too.

Leakage inductance - Wikipedia, the free encyclopedia


nigelwright7557 16th February 2013 01:15 AM

I supress oscillation by looking at the frequency of it then use:
1/2 pi r c to determine the components.
This has worked really well in fly-back SMPS.

chris2 16th February 2013 07:20 AM

Yes Speedskater,

one of the articles I read was from Jim Hagerman but also and article from Cornell Dubilier, which he references.


chris2 16th February 2013 09:05 AM


using transistormarkj's inductance measuring techniques, e.g. det. nat. freq. of secondary with and without an "extra" known capacitance, Cx using sig. generator and oscilloscope.

resonant frequency1 = 1 / (2 * pie * sqrt(Ls * Cs))
resonant frequency2 = 1 / (2 * pie * sqrt(Ls * (Cs + Cx)))

Ls = secondary inductance
Cs = secondary stray capacitance

Therefore equation 1: (using resonant freq. equation)
Ls * Cs = 1 / (4 * pie * frequency1^2)

And equation 2:
Ls * Cs + Ls * Cx = 1 / (4 * pie * frequency2^2)

Substituting equation1 into equation2 yields:
Ls = (F2 - F1) / Cx

where F2 = 1 / (4 * pie * frequency2^2) and F1 = 1 / (4 * pie * frequency1^2)

To det. Cs, just substitute just calc. Ls value into first equation, e.g.:
Cs = 1 / (4 * pie * frequency1^2 * Ls)

That's it, i hope.....

P.S. I'm still not sure what the series resistor R is about in transistormarkj's diagram. Do I need to add some known value here when doing the above measurements?

Thanx and Cheers,

Mark Johnson 16th February 2013 11:59 AM

1 Attachment(s)
Those equations look good to me.


Originally Posted by chris2 (
I'm still not sure what the series resistor R is about in transistormarkj's diagram. Do I need to add some known value here when doing the above measurements?

The external series resistor R guarantees that the signal generator always sees a well behaved, easy to drive load. Even at low frequencies, where the inductor is a very low impedance. Even at high frequencies, where the capacitor is a very low impedance. I typically use a 2Kohm, 1/4 watt resistor in this position.

Important note: You want to measure the secondary's leakage inductance, so you need to SHORT the primary. As Hagermann's white paper says, and also as member soundchaser001 says here in this thread.

You can play with the measurement technique in circuit simulation if you wish. Sweep the signal generator frequency and look for the max amplitude (or look for the phase=0 crossover). Do this with different values of Cx. Put the measured resonant frequencies into your equations. Do they yield the correct inductance and capacitance values, which you installed in your simulated circuit?

Protip: If you make Cx really really large, so large you are quite certain that Cx >> Cs, then the two resonant frequencies will be quite far apart, like maybe, f1 > (3 x f2). This will result in less numerical cancellation when you're performing the arithmetic to solve for Ls and Cs. Another way to think about it is: if (Cx >> Cs) then Cs is negligibly small compared to Cx, so (Cx + Cs) = Cx and the measured resonant frequency with known Cx gives you Ls directly. It's one equation in one unknown.

chris2 16th February 2013 03:55 PM

Thanx for the verification of my prev. post transistormarkj.

The transformer i want to measure has two primaries and two secondaries.

Both primaries are wired in series for 230v AC, so i would short the other two wires here.

For the secondaries, i would make two measurements one for each winding. I assume that the results of both measurements would be almost identical.

Also thanx for the tip on using a Cx that's a lot larger than Cs. Didn't consider that but when one looks at the equation its quite obvious.

I'll post the result here later.


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