Random pulse width modulation for improving the performance of electronic power converters

Description:

A variety of algorithms for implementing random pulse width modulation (RPWM) in motor-drive power inverters have been developed. Usage of motor drives onboard the automobile is increasing rapidly with the introduction of electric, hybrid, and fuel-cell vehicles. The major benefits of RPWM relate to acoustic and electromagnetic noise, both of which are major design criteria for electric propulsion or accessory drives in an automotive application.

Problem
Operation of most power electronic converters, dc-to-ac inverters in particular, is based on the principle of pulse width modulation (PWM). However, unwanted byproducts of the operation of the inverter generally include: (a) annoying tonal acoustic noise, (b) susceptibility to vibration if the switching frequency coincides with one of the natural resonance frequencies of the drive system, and (c) electro-magnetic interference (EMI) concentrated in narrow frequency bands at multiples of the switching frequency.

Solution
The Random Pulse Width Modulation Method differs from the traditional PWM by altering the switching intervals. As a result, the whining tonal sound becomes a shushed “static,” susceptibility to vibration disappears, and the EMI spectrum becomes flat, allowing easier compliance with the existing electromagnetic compatibility standards.

The Random Pulse Width Modulation Device is a digital modulator. Although switching periods are varied in a random manner, the sampling frequency remains constant and equal to the reciprocal of the average switching period. This allows an easy replacement of the traditional PWM with the random PWM (RPWM) in any existing inverter. The same RPWM method and device can easily be adapted for control of any other pulse-width modulated power electronic converters, such as the dc-to-dc switching power supplies or ac-to-dc rectifiers.

Applications