Control By Pulse - Width Modulation

The pulse-width modulator controls the semiconductor switch and is a complex part of a switched regulator. Nowadays, switched regulator uses a pulse-width modulator integrator circuit. The principle of control by pulse-width modulation is illustrated in Fig. 19. The simplified functional diagram of a typical pulse-width modulator is shown in Fig.19a, whereas the waveforms in Fig. 19b explain the operation.

The pulse-width modulator circuit consists of a saw-tooth generator, an error amplifier, and a comparator. The frequency of saw-tooth generator can usually be set by choosing proper values of an RC network. The error amplifier compares the reference voltage and the feedback signal. The feedback signal is obtained using a voltage divider network across the output of the SMPS circuit. For example, let the feedback signal be V_f and the reference voltage be V_ref. Then

The output of the error amplifier is compared with the saw-tooth waveform and when this voltage is greater than the output of sawtooth generator, the output of the comparator would be at logic '1'. When the output of comparator is at logic '1', the switch in the SMPS circuit can be kept in the ON state. When the comparator is at logic '0', the switch in the SMPS circuit can be kept in the OFF state.

If the output voltage tends to be greater than that indicated by equation (32), the output voltage of the error amplifier would fall and the duration for which the output of comparator remains at logic '1' would decrease. Thus the duty cycle of the switch reduces and the output of the SMPS would fall, according to equation (10). Thus it can be seen that the negative feedback control maintains the output at the desired value. For negative feedback control, the feedback signal should be applied to the inverting input of the error amplifier.

When control by frequency modulation is desired, the ON-period is kept constant, but the frequency is varied in order to bring about regulation. Such a technique is necessary if the load on the regulator tends to be come very low. It is difficult to make the ON-period below a certain time duration and when this limit is reached, control by pulse width modulation becomes impossible. Then the duty cycle is reduced by keeping the ON period fixed and increasing the cycle period. The value of minimum ON period depends on the transistor switch.

 
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