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GLENTEK Inc. 208 Standard Street, El Segundo, California 90245, U.S.A. (310) 322-3026
SMA7215 MANUAL
2.7 Pulse-Width-Modulation (PWM):
Pulse-width-modulation is the technique used for switching opposite pairs of output
transistors ON and OFF to control the motor drive current. When zero current is commanded
to the current loop, the opposite pairs of transistor are turned ON and OFF as shown in figure
2.5. Note that since the pulse widths are equal, the net DC current in the motor is equal to
zero.
When a non-zero current is commanded to the current loop, the transistor switching
waveform is as shown in figure 2.6A. Since there is a non-zero current command, the output
transistor pulse widths will change and the motor will see a net DC current flowing from A
through C.
Figure 2.5
Transistor switching
waveform at zero
current
Figure 2.6A
Transistor switching
waveform when current
flows from A through C
If the input to the current loop had been changed in polarity, the output transistor
switching waveform would be as shown in figure 2.6B.
Figure 2.6B
Transistor switching
waveform when current
flows from B through D
If a larger current of the same polarity was commanded to the output transistor (see
figure 2.6B) the ON-time widths of B and D would automatically increase to provide more
current.
From the previous examples it is easy to understand why this output transistor switching
technique is referred to as pulse-width-modulation.
To change the magnitude and polarity of the current flow in the motor, the pulse widths of
the opposite pairs of transistors are modulated. The frequency at which these output
transistors are switched ON and OFF is referred to as the ‘carrier frequency’.
