PWM Frequency Considerations

Often, it is beneficial to pass PWM waves through driver circuits rather than applying the signal directly to the target. Each of these driver circuits is made up of components that have to switch off and on in order to repeat the original signal, and it takes a small amount of time for that switching to take place. Manufacturers will report how long these delays are, often separated into turn on and turn off times as they can be different. These manufacturer’s values can often be a range of delay times, and may be presented with minimum and maximum values. These ranges of delay times, and the differences between turn on and turn off times, can limit the maximum PWM frequency that can be effectively used with the circuit, and they determine the minimum useful duty cycle for a given frequency. On the plus side, for low power applications (less than 2000W) components can be found which have switching times in the tens of nanoseconds with nearly equal turn on and turn off delays, which means that PWM frequencies of less than a few hundred kHz are usually safe to use.

However, for components that can handle more power, two things increase with that capacity. First, the turn on and turn off times both increase in higher power rating components, and second, the turn on and turn off times get further apart, with the turn off times getting much larger than the turn on times. These characteristics necessarily limit maximum PWM frequencies, a fact which usually isn’t really a big problem because most power circuits output lower frequencies anyway. However, some power supplies need higher frequency switching, for instance, variable frequency drives that output SPWM. For these, one solution is to use multiple faster low power components in parallel. Doing this still adds delay, but with the right component selections, it can prevent the turn on delay and turn off delay from drifting apart which can permit the use of higher PWM frequencies.

In addition to the component delays, additional delays may occur due to PCB trace or wire geometry. When designing a circuit to distribute a PWM signal, it is beneficial to try to get all of the distances from the split point to the targets to be nearly the same, with the same trace thickness, and with nearly the same capacitance with respect to ground. This is important because the traces have switching delay times which depend upon their series resistance and shunt capacitance. Fortunately, the turn on/turn off delay times for traces are usually the same.

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