Hephaestus Audio

Class-D PWM Harmonics

June 24th, 2009
by Joey White

Features to note in this plot of the PWM harmonic content versus the duty cycle:

At idle (D=0.5) only odd harmonics are present, as expected from a square wave
Away from idle (D<0.5 or D>0.5) even harmonics added, as expected from PWM
Magnitudes are equal at duty cycle extremes, as expected from an impulse function

If the class-D amplifier is a hysteresis modulator with a passive 2nd order output filter:

Features to note in this new plot of the PWM harmonic content versus the duty cycle:

The harmonics increase abruptly as clipping is approached, consistent with observation
At idle the output is dominated by the fundamental (i.e. a low level sine wave at $f_{s}$ )
Designs with more stable switching frequency at duty cycle extremes are easier to filter

Switching frequency versus duty cycle relationship for a hysteresis modulator:

$f_{s}=4D\left ( 1-D \right )f_{0}$

$f_{s}$ is the switching frequency at $D$
$f_{0}$ is the switching frequency at $D=0.5$
$D$ is the duty cycle $0\leq D\leq 1$

This entry was posted on Wednesday, June 24th, 2009 at 9:03 am and is filed under Switching Amplifiers. You can leave a response, or trackback from your own site.