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PMP20289: Measuring the inductance of the high frequency transformer

Armin Jafari
Prodigy 65 points

Part Number: PMP20289

I noticed that in "PMP20289 RevA Test Report" document, on page4 it is mentioned that the inductance values for the transformer and the inductor is 67.5uH and 1.66uH respectively. The frequency of the measurement is said to be 400kHz. Here is my question:

Can you please explain about the measurement method/ device which has been used to get those uH numbers?

As far as I know at 400kHz of AC frequency, finding a measurement tool (e.g. impedance analyzer, LC meter etc.) which can give the required power for magnetizing the core, is very difficult. So I am really interested in knowing what you did in this case?

Bests,

Armin

  • 0
    TI__Expert 4270 points

    Hi Armin,

    This is Sheng-Yang, the design engineer of PMP20289. The transformer inductances were measured with HP 4275A LCR meter. That is, the measured inductance numbers are the values at low current.

    Regards,

    Sheng-Yang Yu

  • 0 in reply to Sheng-yang Yu1
    Prodigy 65 points

    Dear Sheng-Yang,

    Thank you for your kind reply, and it is my pleasure to be in touch with you!


    Do the inductance values that you measured with the LCR meter correspond with the real value while operation in the real circuit?

    (I ask this because as far as I know, when the current for measurement is low, it cannot magnetize the core, and due to variable slope of the B-H curve of the magnetic material, the mio (Magnetic Permeability) would be different from the case of a magnetized-core-measurement. Therefore, there should be an error (sometimes maybe 200% error), between the inductance measured and the real inductance when the transformer (or inductor) is in the power circuit and magnetized well.)

    You do a great favor if expain a bit more about this.

    Bests,

    Armin

  • 0 in reply to Armin Jafari
    TI__Expert 4270 points

    Hi Armin,

    The measurement results from LCR meter should be closed enough to the operational value in the real circuit. The most accurate way to measure the inductance should be applying a high voltage PWM to the transformer at both open  and short output winding and calculates the inductance from Ldi/dt=V using the current and voltage measured. 

    However, we don't usually test transformer inductance that way...

    Regards,

    Sheng-Yang Yu

  • 0 in reply to Sheng-yang Yu1
    Prodigy 65 points

    Dear Sheng-Yang,

    Thank you very much for your kind and clear answer.

    Wish you a very nice time.

    Bests,

    Armin