Spot modification:
Dimension of 'Switch' changed from 3 to 6 signals for n-gates added.
PWM generation for space vector modulation, as done e.g. by TI's
TMSM320P14 DSP.
A duty ratio a=1 indicates a continuously closed upper switch of
the first inverter leg. At a duty ratio a=0, the turn-on time
during each PWM period is equally distributed to the lower and
upper switch and the resulting mean value of the phase voltage u_a0
is zero. At a duty ratio a=-1, the lower switch is continuously
closed, etc.