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I'm currently looking at SRR1240-8R2M. On the datasheet, it shows us the inductance of the inductor at 100KHz is 8.2uH. But I'm trying to run my circuit at 2.2MHz.

How do I determine the inductance when I'm running at 2.2MHz? The SRF is 7.96MHz.

Thanks.

Edit: Sorry, I made a mistake. The SRF is 32MHz, not 7.96Mhz. 7.96MHz is the test frequency.

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  • \$\begingroup\$ You need to measure the inductance at 2.2 MHz with an impedance analyzer like the old HP4194 (university labs sometimes have fun toys). An impedance analyzer also gives the loss resistance, an important number for switchers. Or, make an RLC bandpass filter with a center freq near 2.2 MHz and figure out the inductance from your measurements. \$\endgroup\$
    – qrk
    Commented May 18, 2021 at 0:26

3 Answers 3

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You’ll have to derate Imax and Ipp ripple significantly @ 2MHz due to core loss. This means L may reduce 10% for a 50’C rise.

*updated with new info on SRF

L won’t change at all unless you overheat it or get within the Q BW effects at resonance. The higher the Q factor, the closer you may operate near resonance but core loss rises with f and impedance rises sharply near SRF.

For an LC resonant filter the gain rises with Q and peaks at resonance.

For an Inductor the Q drops 0 at resonance and the inductance rises like the Q in an LC filter then drops to 0 and it becomes just a resistor with DCR after which it becomes capacitive as the phase shifts rapidly from interwinding parasitics then C declines at the same rate as L rose before resonance.

*Core loss

This loss in watts increases in 3 dimensions with the cube root of (f)as a function of current or magnetic flux.

Thus when tested at 100kHz and operated away from Q=0 at 2.2MHz the core loss induced temperature rise increases by \$ 22^{0.33}=2.8\$ so current max ratings must be reduced to ~35% of 100kHz spec.

but FET losses also increase due to C effects near SRF unless using ZVS. Thermal runaway exists if core losses result when temp rise then drops in L then faster with thermal runaway. You should put a heat sensor on magnetics until the design has an acceptable margin. (Dk > 1)

Recommendation

https://www.mouser.com/datasheet/2/427/ihlp-6767gz-01-1763230.pdf

Only use parts rated for your operating frequency.

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    \$\begingroup\$ Sorry, I made a mistake. The SRF is 32MHz, not 7.96Mhz. 7.96MHz is the test frequency. \$\endgroup\$
    – cy1125
    Commented May 18, 2021 at 13:38
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With the inductance specified at 100KHz, you're likely going to operating this well outside its design intent at 2.2MHZ. You generally want to be well away from the SRF.

You should pick a different inductor technology than this one if you intend to run it at 2.2MHZ.

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  • \$\begingroup\$ Sorry, I made a mistake. The SRF is 32MHz, not 7.96Mhz. 7.96MHz is the test frequency. \$\endgroup\$
    – cy1125
    Commented May 18, 2021 at 13:37
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You typically would like to be operating at <10x below SRF. When you're at <10x SRF, the inductance is pretty much the nominal inductance. As you approach the SRF, inductance increases - but you'll have to measure what it is. It might be OK, it might not.

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  • \$\begingroup\$ Sorry, I made a mistake. The SRF is 32MHz, not 7.96Mhz. 7.96MHz is the test frequency. \$\endgroup\$
    – cy1125
    Commented May 18, 2021 at 13:38

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