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There is a small isolation transformer 'L1' of 1:1 ratio as shown in the image. A 0-24V, 75kHz square wave is applied from push-pull mosfets stage at the 'V_SW' input of the circuit. I want to find out the approx inductance value of the isolation transformer using calculation. Is there some way to do this calculation?

enter image description here

New Edit:

I have found this transformer and it is actually not a transformer but a Common mode choke for EMI suppression filter. But in the above circuit it is used for isolation purpose as it is just 1:1 turns ratio.

My question is that this choke is available from 33 mH to 0.5 mH values. Should I use lower Inductance value or higher or some middle value? How will it effect the working of this circuit?

The exact part is found at this link: http://www.mycoiltech.com/emi-suppression-filter-common-mode-choke-coil-uu9-for-ac-power-lines_p75.html

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3 Answers 3

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There's no way to do it from calculation alone, you need some measurements.

Take all the secondary loading off, and measure the current waveform as you apply your 24v 75k square wave. Then you'll have enough data to compute primary inductance, via V = Lpri * dI/dt

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  • \$\begingroup\$ Is it possible that the L1(pri) and C2||C3 form LC series resonant circuit at 75kHz input frequency? \$\endgroup\$
    – scico111
    Commented Feb 22, 2019 at 7:03
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    \$\begingroup\$ @scico111 In this case no. It’s not resonant. \$\endgroup\$
    – winny
    Commented Feb 22, 2019 at 7:14
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    \$\begingroup\$ Pin 2 of the transformer goes to whatever Vcc is. The capacitors are just power supply decoupling, low ESR, etc. \$\endgroup\$
    – user105652
    Commented Feb 22, 2019 at 7:16
  • \$\begingroup\$ @Sparky256 It means the capacitors can be put on Pin 1 or Pin 2 of transformer and they will give same results. \$\endgroup\$
    – scico111
    Commented Feb 22, 2019 at 7:20
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C1 is after the diode rectifier, so it will be a DC voltage with a small voltage ripple of 150 kHz.

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If you know the winding turn number \$N\$ and the core geometry and core material you can calculate the inductance as

\$L_m = \frac{N^2}{R_m}\$

with

\$R_m = \frac{l_m}{\mu_0 \cdot \mu_r \cdot A_q}\$

where at 75 kHz the core is usually ferrite. \$l_m\$ is the average length of the flux line in the core and \$A_q\$ is the (center leg) cross section.

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