I'm designing for a very power sensitive motor controller, and I'm concerned about the power that will be lost to the ESR of my decoupling capacitors. I have a prototype board as well, but there's no way to tell how much energy is being wasted within the capacitors.
With the capacitors, I expect the current draw into the board to resemble a square wave, because it's a 3-phase BLDC motor. However, with all my nice decoupling, it's become a reasonably sinusoidal wave at my PWM frequency.
Is there any way to determine how much power is being wasted in these capacitors? All of the literature I can find on it seems to be overly complex white papers. Are there rules of thumb or first order approximations I can use?
Could I also perhaps remove the capacitors, capture voltage and current waveforms, and add them back and check the difference? This approach would tell me the results for the current design, but doesn't really help in the design process.
Use polypropylene coupling caps. If they aren't good enough, nothing will work.
The cap conducts AC and is sized for low impedence at the switching frequency giving low ripple voltage .It is reasonable here to neglect leakage losses.Your current draw is a square wave which occurs when system inductance gives high reactance at the switching frequency.We want the AC current to go through the caps and not through the DC power leads in order to make EMC problems less likely so we just calc the RMS value of your AC square wave ripple current .Now it is just I squared R where R is the Cap ESR .The power wasted in the caps will be small compared to the motor power but it can heat up the caps reducing thier lifetime .Remember that a 10 degree temp rise will roughly halve the life of an electrolitic cap .In most applications the cap life is adequate when there is no additional heating due to ripple current .If you find that your ballpark calcs show significant losses that will heat the caps by say 30 degrees C then your reliability could be in jepordy .Placing more caps in parallel is an easy way out of this problem .