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THEORY OF OPERATION
FIGURE E.10 – TYPICAL IGBT OUTPUTS
25
sec 
sec
sec 
50
sec 
sec
MINIMUM OUTPUT
24
sec
1sec 
24
sec 
1 sec 50
sec 
MAXIMUM OUTPUT
PULSE WIDTH MODULATION
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The term PULSE WIDTH MODULATION (PWM) is used to describe how much time is devoted to conduc- tion in the positive and negative portions of the cycle. Changing the pulse width is known as MODULATION. Pulse Width Modulation 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 below shows the minimum out- put signal possible over a
The shaded portion of the signal represents one IGBT group1, conducting for 1 microsecond. The negative por- tion is the other IGBT group. The dwell time (off time) is 48 microseconds (both IGBT groups off). Since only 2 microseconds of the
MAXIMUM OUTPUT
By holding the gate signals on for 24 microseconds each and allowing only 2 microseconds of dwell or off time (one microsecond during each half cycle) during the 50 microsecond cycle, the output is maximized. The dark- ened area under the minimum output curve can be com- pared to the area under the maximum output curve. The more darkened area, the more power is present.
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1An IGBT group consists of the sets of IGBT modules grouped onto one switch board.
