In order to control a 3-phase inverter bridge, we need three PWM pairs with complementary outputs. In addition, each complementary pair of PWMs needs dead time in between any OFF and ON switch events to avoid a DC bus short circuit.The PIC16F72 does not have these features in the hardware.However, this can be easily implemented using a general purpose timer and six output pins as shown in Figure 9.
Generating Software PWMs
The scheme shown here gives a fixed PWM frequency of approximately 7.9 kHz. Timer1 (1:4 prescale) is counted up from 00 to 634. At the beginning of the cycle (when Timer1 = 00), the PWMs controlling the upper switches (i.e., PWM1, PWM3 and PWM5) are turned ON. Based on the individual PWM duty cycle, the corresponding PWM output is turned OFF.
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