37.5.1.3 Phase Shifted PWMs
In the previous sections, the benefits of signed mode of operation were discussed in the
context of simplifying the required software calculations by eliminating the requirement
to bias up signed variables before applying them to the module. However, if numerical
biases are applied to the turn on and turn off edges of different PWM signal, the signals
will be phase shifted with respect to each other, as the following figure shows. 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.
INIT ($FF00)
PWM_B
PWM_A
VAL3
VAL4
. VAL1 ($0100)
VAL5
VAL0 ($0000)
VAL2
Figure 37-226. Phase Shifted Outputs Example
An additional benefit of phase shifted PWMs appears in
. In this case, an
H-Bridge circuit is driven by 4 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 out nicely for the H-Bridge
since no narrow pulse widths are generated reducing the high frequency switching
Chapter 37 Pulse Width Modulator A (PWMA/eFlexPWM)
KV4x Reference Manual, Rev. 2, 02/2015
Freescale Semiconductor, Inc.
Preliminary
829
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