Lincoln Electric SVM140-A service manual Pulse Width Modulation, Minimum Output, Maximum Output

E-9 E-9

THEORY OF OPERATION

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PULSE WIDTH MODULATION

The term PULSE WIDTH MODULATION describes how much time is devoted to conduction in the posi- tive 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 below shows the minimum output signal possible over a 50-microsecond time period.

The positive portion of the signal represents one IGBT group1 conducting for 1 microsecond. The negative portion is the other IGBT group1. 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 22 microseconds each and allowing only 3 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.

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1An IGBT group consists of two IGBT modules feeding one transformer primary winding.

FIGURE E.8 – TYPICAL IGBT OUTPUTS

sec	48	sec	sec
	50	sec

MINIMUM OUTPUT

22sec

3 sec ￿ 22 sec

50sec

MAXIMUM OUTPUT

PRO-CUT 55