An efficient method of PWM waveform generation using a microprocessor

Abstract

A Z-80 microprocessor-based control for a three-phase PWM inverter employing thyristors is discussed. This PWM inverter combines the advantages of hardware modules and software algorithms to improve the flexibility and accuracy of the operation. A floating point arithmetic unit developed in software forms a vital link of the software structure that calculates the PWM pulsewidths from the basic equation of a regular sampled PWM. During PWM waveform generation the inherent time delay involved in the execution of instructions in the interrupt subroutines results in shifting of the edges of the PWM pulses and thus introduces a certain degree of harmonic distortion. The cumulative nature of this time delay will alter the frequency of the output waveform in a continuous way. By employing a suitable hardware structure the time delay involved in the interrupt subroutine is minimized to a great extent and the cumulative effect of the time delay is nullified. To handle the interrupts arising during PWM waveform generation a simple and novel interrupt controller that is capable of handling the interrupts effectively is built around conventional TTL ICs instead of using a standard integrated interrupt controller module like 8259.