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Electronics => Beginners => Topic started by: kevinchen on November 10, 2013, 05:49:23 am

Title: Ceramic cap microphonics in a high-vibration racecar
Post by: kevinchen on November 10, 2013, 05:49:23 am

Hi all,

I've been lurking unregistered to find answers to electronics questions for awhile, but I didn't see that this one had been answered before!

I'm designing a switching power supply to power the digital logic of a racecar from the car's 12-V battery. I'm planning to use ceramic caps on the input and output, but I heard on the Amp Hour that they have problems with microphonics, especially the X7R ones.

Since there will be a lot of vibration (compared to a consumer car, ours feels like it has no suspension, and needs to drive at 60mph/100kph), I was wondering if this is serious enough to reduce the quality of the converter's output. Thanks for your help!

Kevin Chen
Columbia Univ. Formula SAE Team

Switching IC datasheet: http://www.ti.com/product/LM22672-Q1 (http://www.ti.com/product/LM22672-Q1)

Title: Re: Ceramic cap microphonics in a high-vibration racecar
Post by: tszaboo on November 10, 2013, 09:21:35 am

Check out this datasheet: http://www.linear.com/product/LT1964 (http://www.linear.com/product/LT1964) They measured the noise for certain vibration (tapping the capacitor with pencil). It was 1 mV peak to peak. Even if you multiply the noise 100 times, you are still well within the region of any digital IC.
If price is no object, AVX and Kemet has capacitors which are on lead frame, which supposed to decrease the microphonics effect, or clever mounting of the PCB can help. Or buy millitary/aerospace rated caps.

Title: Re: Ceramic cap microphonics in a high-vibration racecar
Post by: SeanB on November 10, 2013, 10:07:56 am

When you make the board glue the caps to the board, and use a flexible conformal coat ( you will need a conformal coatr in any case for automotive use)  so just choose a reenterable coat for the board. As well use some flexible mountings for the electronics boards in any case. If you are stuck go look at how VW mounts the engine management units in a VW Golf/rabbit/Polo/Seat in the top of the engine bay, and look at the flexible mounts they use, as well as how they use ceramic capacitors inside.

Scrapyard and buy some broken boxes and tear apart for tips.

Title: Re: Ceramic cap microphonics in a high-vibration racecar
Post by: Zero999 on November 10, 2013, 05:44:02 pm

Put an electrolytic and small ceramic in parallel. The electrolytic will short the noise created by the ceramic to ground and the ceramic will have a lower impedance, especially at high frequencies.

Title: Re: Ceramic cap microphonics in a high-vibration racecar
Post by: kevinchen on November 10, 2013, 05:58:53 pm

Thanks for all your advice, everyone!