Specifications

dsPIC33F/PIC24H Family Reference Manual

43.16.4 Variable Phase PWM

The Variable Phase PWM, illustrated in Figure 43-51, constantly changes the phase shift among

PWM channels to control the flow of power, which is in contrast with most PWM circuits that vary

the duty cycle of PWM signal to control power flow. In variable phase applications, the PWM duty

cycle is often maintained at 50 percent. The phase shift value is available to all PWM modes that

use the master time base.

The variable phase PWM is used in newer power conversion topologies that are designed to

reduce switching losses. In the standard PWM methods, when a transistor switches between the

conducting state and non-conducting state (and vice versa), the transistor is exposed to the full

current and voltage condition during the time when the transistor turns ON or OFF and the power

loss (V * I * T

SW * FPWM) becomes appreciable at high frequencies.

The Zero Voltage Switching (ZVS) and Zero Current Switching (ZVC) circuit topologies attempt

to use quasi-resonant techniques that shift either the voltage or the current waveforms relative

to each other to change the value of voltage or the current to zero when the transistor turns ON

or OFF. If either the current or the voltage is zero, no switching loss occurs.

Figure 43-51: Variable Phase PWM

PWM1H

PWM1L

PWM2H

PWM2L

Variable Phase Shift

Duty Cycle

PWM1H

Period

Duty Cycle

Phase 2 (old value)

Duty Cycle

PWM2H

Phase 2 (new value)

Full-Bridge ZVT Converter

PWM1H

PWM1L

PWM2L

PWM2H

OUT