0
\$\begingroup\$

I'm in the process of designing a PCB for an RGB LED light source that will allow me to dim each color channel separately using PWM. To help manage the thermal output of the LEDs I'm trying to use an aluminum PCB substrate, and to try to keep costs in line I want to design a 1 layer board for the LEDs. The power, drivers and PWM generator will be on a separate board.

My concern is that the routing necessary to get everything on one layer involves a lot of parallel traces carrying PWM signals, and I'm worried I might end up generating some unwanted EMI. Each string of LEDs is going to be driven at a constant 200mA at a voltage which will vary from about 16V to 24V depending on the color. I'm shooting for a PWM frequency of 30-50khz.

The attached image is a schematic of my proposed routing. Will this cause EMI problems?

enter image description here

\$\endgroup\$
0
\$\begingroup\$

Intentionally or not, your layout works to minimize emissions. If you look at the currents through the pairs of adjacent traces - or the three in he middle where the center string is tapped off, the currents sum to zero, so as long at the traces are kept fairly close to each other, you're minimizing the area enclosed in each loop, effectively minimizing the antenna size. Especially given that you've got a conductive core to the board, this would be a good thing - a large loop would induce eddy currents, which would also help to damp the current changes in the loops.

Why the high PWM frequency? Lowering that would itself reduce the emissions, and so long as it's above a couple of hundred Hz, it won't be visible, unless the board is moving relative to the observer.

| improve this answer | |
\$\endgroup\$
  • \$\begingroup\$ Thank you, that is reassuring! The device is intended to be used with digital cameras, so shutter speeds above 1/1000th of a second are totally possible. I did a bunch of reading and it seemed like 30khz was a good frequency to target for devices that will be used with camera/video equipment. \$\endgroup\$ – flimsy Mar 4 at 21:44
1
\$\begingroup\$

You may consider applying the wire twisting technique reducing current loops:

enter image description here

But most gain in EMC will be achieved by filtering the voltage after the PWM driver before feeding it to the LED's, or in increasing the rise and fall time of the PWM signal.

| improve this answer | |
\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.