In a PWM inverter, the output ripple around the desired modulating function is n

Question

In a PWM inverter, the output ripple around the desired modulating function is nearly a triangle superimposed on the sinusoidal modulation. With a simple L - R load, it is possible to pick out the portion of the waveform with the worst-case ripple as if we were analyzing ripple for a buck converter. Given a PWM inverter with L - R load such that L/R = 100 mus, and given V_in = 240 V, M(t) = 170 cos(120pi t), f_switch = 50 kHz, estimate the maximum peak-to-peak ripple around the desired modulating function, as measured at the output.

Explanation / Answer

Pulse Width Modulation:

Pulse Width Modulation (PWM) uses digital signals to control power applications, as well as being fairly easy to convert back to analog with a minimum of hardware.

Analog systems, such as linear power supplies, tend to generate a lot of heat since they are basically variable resistors carrying a lot of current. Digital systems don’t generally generate as much heat. Almost all the heat generated by a switching device is during the transition (which is done quickly), while the device is neither on nor off, but in between. This is because power follows the following formula:

P = E I, or Watts = Voltage X Current

If either voltage or current is near zero then power will be near zero. PWM takes full advantage of this fact.

PWM can have many of the characteristics of an analog control system, in that the digital signal can be free wheeling. PWM does not have to capture data, although there are exceptions to this with higher end controllers.

Voltage regulation is done by averaging the PWM signal. Output voltage is represented by the following equation:

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