Lincoln Electric SVM159-A service manual Pulse Width Modulation, Minimum Output Maximum Output

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E-8

THEORY OF OPERATION

FIGURE E.7 — TYPICAL IGBT OUTPUTS.

sec	48	sec
	50	sec

MINIMUM OUTPUT

E-8

sec

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24 sec

2 sec 24 sec

50sec

MAXIMUM OUTPUT

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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 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

MAXIMUM OUTPUT

By holding the gate signal on for 24 microseconds each, and allowing only two 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 that is under the curve indi- cates that more power is present.

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

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By controlling the duration of the gate signal, the IGBT is turned on and off for different durations during the cycle. The top drawing in Figure E.7 shows the mini- mum output signal possible over a 50-microsecond time period.

The positive portion of the signal represents one IGBT group1 conducting for one microsecond. The negative

portion is the other IGBT group1. The dwell time (off time) is 48 microseconds (both IGBT groups off). Since only two microseconds of the 50-microsecond time period is devoted to conducting, the output power is minimized.

POWER WAVE 355/405