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I am working on designing an EMI input filter, and I was wondering how I should go about choosing a value (capacitance) for the X-capacitor. I found a post (linked below) which talked about calculating the power factor, and then choosing a cap that gave you a decent power factor (without being too greedy.)

Is this the only way to do it, or are there any other design guidelines I should be aware of?

I know that eventually we would test our board to make sure it is in compliance, but I am more so wondering about guidelines for the initial component selection.

X capacitor selection for SMPS power supply

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  • \$\begingroup\$ RGB Engineer, are you done with this question now or do you still need some help? \$\endgroup\$
    – Andy aka
    Jan 27, 2021 at 9:13
  • \$\begingroup\$ Hello, I am still formulating my next question in my head. \$\endgroup\$ Jan 27, 2021 at 9:25

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I found a post (linked below) which talked about calculating the power factor, and then choosing a cap that gave you a decent power factor (without being too greedy).

The best line capacitor to maximize power factor to a value of 1 is when you don't use a line capacitor. Any line capacitor will degrade the power factor of an SMPS. I'll quote what you linked: -

the minimum value is based on the actual conducted EMI and what levels are acceptable for your application.

In other words, you choose the X capacitor based on keeping conducted emissions low but, you don't choose a value that is obscenely high because that will wreck your power factor: -

For example, a 0.47µF X capacitor across the line with 240VAC input at 60Hz (I know 50Hz would be more common, but lets make things worst case) will draw roughly 10W of apparent power at all times. If your SMPS is a 500W ATX power supply, then your power factor is 0.98. Great! If it is a 50W laptop power brick however, your power factor is now 0.8. Not so good.

How Do I Choose a Value for an X-Capacitor?

You choose it to significantly reduce your problems of conducted emissions as per this earlier answer I gave you entitled EMI Input Filter Design. You have to address conducted emissions first and foremost and, live with the slightly degraded power factor that the X capacitor brings.

Again, a reminder from the answer you linked: -

It really just comes down to how noisy is your power supply, how much noise from the mains you want the supply to tolerate, and how good of a power factor you want. Power factor is almost always going to be lower priority though, since most countries require you to meet EMI standards above anything else.

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