Will capacitance decrease of MLCCs help me pass the test?

by C K   Last Updated February 11, 2019 09:25 AM

The product I currently design (a fork sensor) has a metal chassis which is not connected to earth ground. Examining the product of a competitor, I see there is a large SMD capacitor between it's chassis and ground. Thanks to some folks here, I was able to understand its purpose: to prevent RF noise from coupling to sensitive analog parts of the device. (Also includes capacitor photo)

However, these devices are subject to a dielectric test in certification, which applies 500VAC RMS voltage between chassis and exposed pins. I am having a real hard time finding an SMD capacitor with 500VAC rating.

My plan is to place an MLCC type capacitor with 1000V withstand voltage. 500VAC test signal will have about 700V peak voltage which my capacitor should be able to handle. However, this time the capacitance value is a problem. The sample I have has about 60nF of capacitance. Doing the math, about 4.7W is wasted on the capacitor. (I don't know how the sample does it, or if it conforms to standard, I couldn't find it in their website)

Then I had an idea: MLCCs tend to have a capacitance drop in high DC voltages, which means lower currents will flow! This is the exact behavior I want: bypass low amplitude noise, and block high amplitude test signal.

How do MLCCs behave with low frequency/high voltage signals? Does this capacitance drop also show itself in 50 Hz?

I would also glad if you can suggest me any capacitor to use in this scenario. I found these AC Safety Capacitors as an alternative. However I don't know how long will these last with 500VAC. It is above their rating, however below their withstanding voltage.

If none of these work, I will just plug in a capacitor very low capacitance and pray it filters enough noise in the application.

Tags : capacitor

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