Chapter 25 FlexPWM
MPC5602P Microcontroller Reference Manual, Rev. 4
Freescale Semiconductor
683
edges of different PWM signal, the signals will be phase shifted with respect to each other, as illustrated
in
. This results in certain advantages when applied to a power stage. For example, when
operating a multi-phase inverter at a low modulation index, all of the PWM switching edges from the
different phases occur at nearly the same time. This can be troublesome from a noise standpoint, especially
if ADC readings of the inverter must be scheduled near those times. Phase shifting the PWM signals can
open up timing windows between the switching edges to allow a signal to be sampled by the ADC.
However, phase shifting does not affect the duty cycle so average load voltage is not affected.
Figure 25-31. Phase-shifted outputs example
An additional benefit of phase-shifted PWMs can be seen in
. In this case, an H-bridge circuit
is driven by four PWM signals to control the voltage waveform on the primary of a transformer. Both left
and right side PWMs are configured to always generate a square wave with 50% duty cycle. This works
for the H-bridge since no narrow pulse widths are generated, reducing the high-frequency switching
requirements of the transistors. Notice that the square wave on the right side of the H-Bridge is
phase-shifted compared to the left side of the H-Bridge. As a result, the transformer primary sees the
bottom waveform across its terminals. The RMS value of this waveform is directly controlled by the
amount of phase shift of the square waves. Regardless of the phase shift, no DC component appears in the
load voltage as long as the duty cycle of each square wave remains at 50%, making this technique ideally
suited for transformer loads. As a result, this topology is frequently used in industrial welders to adjust the
amount of energy delivered to the weld arc.
VAL1 (0x0100)
VAL5
VAL3
VAL0 (0x0000)
VAL4
VAL2
INIT (0xFF00)
PWMA
PWMB