Electronic Design

Power: Pulse-Width Modulation

Sponsored by: NATIONAL SEMICONDUCTOR

Why PWM?
Switch-mode converters employ a power semiconductor switch (usually a MOSFET) to drive a magnetic element (transformer or inductor) whose rectified output produces a dc voltage. Efficiencies exceeding 90% are common, about twice that of a linear regulator.

A switch-mode converter varies its dc output current in response to load changes. One widely used approach is pulse-width modulation (PWM), which controls the power switch output power by varying its ON and OFF times. The ratio of ON time to the switching period time is the duty cycle. Figure 1 shows three different variations of the PWM duty cycle: 10%, 50%, and 90%. The higher the duty cycle, the higher the power semiconductor switch output power.

What is the configuration of a PWM circuit?
Figure 2 shows a simplified PWM controller employed in a switch-mode converter. In operation, a fraction of the dc output voltage feeds back to the error amplifier, which causes the comparator to control the PWM ON and OFF times. If the power MOSFET’s filtered output changes, the feedback adjusts the duty cycle to maintain the output voltage at the desired level.

To generate the PWM signal, the error amplifier accepts the feedback signal input and a stable voltage reference to produce an output related to the difference of the two inputs. The comparator compares the error amplifier’s output voltage with the ramp (sawtooth) from the oscillator, producing a modulated pulse width. The comparator output is applied to the switching logic, whose output goes to the output driver for the external power MOSFET. The switching logic provides the capability to enable or disable the PWM signal applied to the power MOSFET.

What is the difference between voltage- and current-mode PWM controllers?
The circuit shown in Figure 2 is a voltage-mode PWM controller in which the error amplifier output is compared to a voltage ramp from the oscillator to determine the output pulse width. A current mode PWM replaces the oscillator ramp with a ramp that is proportional to the current in the magnetic element.

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