Datasheet

dsPIC30F1010/202X
DS70178C-page 140 Preliminary © 2006 Microchip Technology Inc.
12.35 METHODS TO REDUCE EMI
The goal is to move the PWM edges around in time to
spread the EMI energy over a range of frequencies to
reduce the peak energy at any given frequency during
the EMI measurement process, which measures long
term averages.
The EMI measurement process integrates the EMI
energy into 9 kHz wide frequency bins. Assuming that
the carrier (PWM) frequency is 150 kHz, a 6% dither
will yield a 9 kHz wide dither.
12.35.1 METHOD #1: PROGRAMMABLE FRC
DITHER
This method dithers all of the PWM outputs and the
system clock. The advantage of this method is that no
CPU resources are required. It is automatic once it is
setup. The user can periodically update these values
to simulate a more random frequency pattern.
12.35.2 METHOD #2: SOFTWARE CONTROLLED
DITHER
This method uses software to dither individual PWM
channels by scaling the duty cycle and period. This
method consumes CPU resources:
Assume:
4 PWM channels updated @ 150 kHz rate:
600 kHz x (5 clocks (2 mul, 1 tblrdl, 1 mov))
= 3 MIPS additional work load
12.35.3 METHOD #3: SOFTWARE SCALING OF
TIME BASE PERIOD
This method used software to scale just the time base
period. Assuming that the dither rate is relatively slow
(about 250 Hz), the application control loop should be
able to compensate for the changes in PWM period
and adjust the duty cycle accordingly.
12.35.4 METHOD #4: FREQUENCY MODULATION
This method varies the frequency at which the PWM
cycle is varied (dithered). The frequency modulation
process is similar (mathematically speaking) to Phase
Modulation when analyzed over a small time window.
The PWM module has the capability to phase modu-
late the PWM signals via the phase offset registers.
Phase modulation has the advantage that the software
is simpler and faster because multiple multiply opera-
tions (used for dithering frequency by scaling period
and duty cycles) are replaced with fewer additions or
simple updates of phase offset
values into the phase registers.
This method also has these advantages:
1. Multi-phase and variable phase PWM modes
could still be created.
2. The PWM generators can still use the common
time base, which simplifies determining when a
“quiet time” is available for measuring current.
This method has one disadvantage: the phase modu-
lation has to be at a relatively high update rate to
achieve usable frequency spreading.
12.35.5 INDEPENDENT PWM CHANNEL
DITHERING ISSUES:
Issues for multi-phase or variable phase designs using
independent output dithering must consider these
issues:
1. The phases are no longer phase aligned.
2. Control of current sharing among phases is
more difficult.