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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 FET
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 FET
group
1
conducting for 1 microsecond. The negative
portion is the other FET group
1
. The dwell time (off
time) is 48 microseconds (both FET groups off). Since
only 2 microseconds of the 50-microsecond time peri-
od is devoted to conducting, the output power is mini-
mized.
MAXIMUM OUTPUT
By holding the gate signals on for 24 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-9 E-9
INVERTEC STT
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MINIMUM OUTPUT
MAXIMUM OUTPUT
24
50
2
48
50
sec
sec
sec
sec
sec
sec
sec
24
sec
FIGURE E-9 – TYPICAL FET OUTPUTS
1
A FET group consists of the sets of FET modules grouped onto
one switch board.