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E-8
RANGER 305D
E-8
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FIGURE E.6 – IGBT OPERATION
PULSE WIDTH MODULATION:
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 MODU-
LATION. 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 sig-
nal possible over a 50-microsecond time period.
The positive portion of the signal represents one IGBT
group conducting for 2 microseconds. The dwell time
(off time) is 48 microseconds. Since only 2 microsec-
onds 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 and
allowing only 2 microseconds of dwell time (off time)
during the 50-microsecond cycle, the output is maxi-
mized. The darkened area under the top curve can be
compared to the area under the bottom curve. The
more darkened area under the curve, the more power
is present.
THEORY OF OPERATION