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# TIDA-00443: Input cap design

Part Number: TIDA-00443

In section 4.2.6, the switching frequency used is 98 kHz instead of 100 kHz. What’s the reason for this.

If we look at the schematic, it is found that two 0.68 uF caps are used. Could you please let me know why two caps are used.

Also, the two caps are not placed in parallel between the same points. L3 comes in between the two caps. Why is this done.

Kindly explain.

Thank you.

• Hi,  Milarepa

The two capacitors not only provide high frequency ripple current,  but also act as EMI filter, two separated caps will provide two stages of differential noise filters.

Regards

Hely

• Why the switching frequency used is 98kHz instead of 100 kHz.

• that's because of chose resistance of R12.

• How to calculate the maximum ripple RMS current flowing through this input capacitor. In the reference, Iripple  = 2.75 A, but this is peak to peak.

• Hi, Milarepa

I am not the designer of this design, but you should be able to find such calculation in the design guide.

Thanks

Hely

• Hi Hely,

Actually, it is not given in the design guide. Could you please refer the thread to any other person who can help me on this design. I need it little urgently.

Thanks and regards,

• Hi, Milarepa

Since ripple current is also a SIN wave, so you can estimate it with Irms=(Imaxripple/2)/*sqrt(2).

Regards

Hely

• I think ripple current is not sinusoidal, because for switching period duration, the voltage across the inductor can be assumed to be constant voltage, and hence current ripple rise and fall will be linear. Its more a triangular waveform than sinusoidal. Therefore, the capacitor current is a triangular waveform as I had drawn before.

• Hi, Milarepa

sorry for the late replay.

See below tested waveforms on a 4kW PFC, green is inductor current. It is a sinusoidal wave in macro level, however, it is triangular in micro level.

Regards

Hely

• So, the capacitor ripple current is triangular and the max rms ripple current should be calculated assuming triangular wave shape:

Kindly let me know if this is correct.

Thank you.

• Iripple is changing from the waveform I presented, so you can't assume a constant Iripple in each AC cycle. means you still need calculate the RMS value of this sinusoidal Iripple.