Lincoln Electric SVM129-B service manual Pulse Width Modulation, Minimum Output, Maximum Output

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E-9 E-9

THEORY OF OPERATION

FIGURE E-9 – TYPICAL FET OUTPUTS

sec	48	sec	sec
	50	sec

MINIMUM OUTPUT

24sec

2sec 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 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 group1 conducting for 1 microsecond. The negative portion is the other FET group1. 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.

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1A FET group consists of the sets of FET modules grouped onto one switch board.

INVERTEC STT