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PULSE WIDTH MODULATION
The term PULSE WIDTH MODULATION is used to
describe how much time is devoted to conduction in
the positive and negative portions of 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 above shows the minimum out-
put signal possible over a 50-microsecond time period.
The positive portion of the signal represents one
IGBT group conducting for 1 microsecond. The nega-
tive portion is the other IGBT group. The dwell time
(off time) is 48 microseconds (both IGBT groups off).
Since only 2 microseconds of the 50-microsecond time
period is devoted to conducting, the output power is
minimized.
MAXIMUM OUTPUT
By holding the gate signals on for 48 microseconds
each and allowing only 2 microseconds of dwell time
(off time) during the 50-microsecond cycle, the output is
maximized. The darkened area under the top curve
can be compared to the area under the bottom curve.
The more dark area under the curve, the more power is
present.
THEORY OF OPERATION
E-10 E-10
POWER MIG 350MP
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FIGURE E.9 – TYPICAL IGBT OUTPUTS
MINIMUM OUTPUT
MAXIMUM OUTPUT
24
50
24
2
48
50
sec
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