# About the primary inductance of the LT3999

In the link: www.analog.com/.../high-frequency-push-pull-dc-dc-converter.html, equation 14 and equation 15. They both have a factor "Ts/4". I wonder how the figure '4' come from? Thanks.

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• Hi,

I believe this is assuming the SW is running at 50% duty cycle. Also, the magnetizing current is AC with average value of zero. As SW off period, the voltage on magnetizing inductance is VIN-VSW. The peak of the magnetizing inductance current IM (di) should be half of the V/L*dt, as this current should have an average of zero. As a result, the Ts/4 is combining with half duty cycle and half of the total inductance current change.

Thanks,

Wenwei.

Reply
• Hi,

I believe this is assuming the SW is running at 50% duty cycle. Also, the magnetizing current is AC with average value of zero. As SW off period, the voltage on magnetizing inductance is VIN-VSW. The peak of the magnetizing inductance current IM (di) should be half of the V/L*dt, as this current should have an average of zero. As a result, the Ts/4 is combining with half duty cycle and half of the total inductance current change.

Thanks,

Wenwei.

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