The term PULSE WIDTH MODULATION is used to
describe how much time is devoted to conduction in
the cycle. Changing the pulse width is known as
MODULATION. Pulse Width Modulation (PWM) is the
varying of the pulse width over the allowed range of a
cycle to affect the output of the machine.

MINIMUM OUTPUT

By controlling the duration of the gate signal, the IGBT
is turned on and off for different durations during a
cycle. The top drawing shows the minimum output
signal possible over a 33-microsecond time period.

The shaded portion of the signal represents the IGBT,
conducting for 2 microseconds. Since only 2
microseconds of the 33-microsecond time period is
devoted to conducting, the output power is minimized.

MAXIMUM OUTPUT

By holding the gate signals on for 31 microseconds
and allowing only 2 microseconds of off time during
the 33 microsecond cycle, the output is maximized.
The darkened area under the min. curve can be com-
pared to the darkened area under the max. curve. The
more darkened area, the more power is present.

MINIMUM OUTPUT

MAXIMUM OUTPUT

31
33

m/sec

sec

31
33

m/sec

Figure E-7