Capacitor selection for RC power filtering

Question

I want to design an RC filter to create a constant current supply from a PWM current supply.

The background of where I got here is...

I'm using a constant current driver LDB-600L to provide constant 600mA of current to a power off brake (https://sepac.com/products/view/thin-spring-brake/). That is what is needed to get it to click over from engaged to disengaged. Once the brake is disengaged, the required hold current is about 1/2 that much. So I'm using the dim functionality of the LDB-600L to make reduce the current. But the output is then PWM, so I decided to use a capacitor to filter the output current and provide the brake with a constant current.

For the brake, R=38.7Ohm PWM frequency is ~576Hz I figure the output RC filter corner frequency 1/(2 * pi * R.brake * C) should be much lower than the PWM frequency. So, I chose 15uF for a corner frequency of 274Hz.

But now, how do I choose a capacitor (or array of capacitors) to best fit this application. I feel like low ESR will be important since we'll be seeing some real ripple current in the capacitor. Are there any suggestions on selecting this component like type, product line, or even alternative solutions to using an RC filter to drive the brake? Would higher capacitance lower the corner frequency and lower the ripple current? Would a parallel array of capacitors lower the ESR and heating? I'm a newbie, so anything will help!

Answer 1

"Would a parallel array of capacitors lower the ESR and heating?"

Yes, it will.

It's a common thing putting in parallel, for example, 2 electrolytic capacitors to lower the overall ESR and the temperature.

Answer 2

Solution 1:

Increase the PWM frequency on whatever generates it, see if it fixes the problem. If your electromagnet is a coil, it should be able to smooth current...

Solution 2:

Replace your on/off constant current LED driver with one that supports analog dimming, for example this one or this one. Give it a control voltage, you get the current you want.

If the thing is mains-powered you can even use a mains-powered constant current dimmable led driver.

Solution 3:

So, I chose 15uF for a corner frequency of 274Hz. But now, how do I choose a capacitor (or array of capacitors) to best fit this application.

Ripple current in a cap (ie, repetitive charging/discharging current) causes heating due to resistive losses in its ESR. The manufacturer will give a spec called "maximum ripple current" in the datasheet that is meant to allow you to pick a cap that will not die of overheating.

So just get a cap with a ripple current rating higher than your ripple current (600mA) which means unless you pick special exotic cap types, it will have a lot more than 15µF. This is absolutely fine, more µF will just smooth the voltage better. If it's in a hot place, pick a 105°C cap. This is going to cost $2, so don't overthink it too much.