Actual Regulation

 

               Producing more or less voltage across the load is based upon modulating the time that the control element is closed. This is accomplished by the pulse-width modulator (PWM) driven by the error amplifier. An oscillator produces the start of pulses at a constant rate, but the end of the pulse is determined by the voltage supplied by the error amplifier. The relationship of the control voltage from the error amplifier to the pulse width that turns on the switch is shown in Figure 9-4. Note the center of the figure has a line that represents a constant level of the control voltage B that is the nominal voltage level at the rated current output. The pulse width for this control voltage is shown as width C. When the demand for current increases, the pulse width increases because the ON time of the pulse is increased. More energy is stored in the inductor so that the increased current can be supplied and the voltage maintained. The integration of the current pulses by the output filter establishes the output voltage level. More ON time in the pulses produces a higher voltage, less ON time in the pulses produces a lower voltage. As shown in Figure 9-4, when minimum current is required the pulse width is narrow with a short ON time.

Likewise, when maximum current is required the pulse width is wide with a long ON time. Here is a description of the regulation in simple terms. When the load demands more current the output voltage tends to decrease. This voltage decrease is sampled and converted to an error voltage that increases the control voltage B and increases the ON time of the pulses. The increase in ON time supplies the increased current and raises the output voltage to its required value. A load that demands less current would tend to increase the output voltage. The voltage increase is sampled and converted to an error voltage that decreases the control voltage B and decreases the ON time of the pulse. The decrease in ON time of the pulses lowers the voltage and satisfies the demand for less current. Switching regulators operate at frequencies from 100 kHz to several million cycles/sec. Because of the range of frequencies and the switching action, there is some concern about RFI energy; and attention must be paid to the shielding of sensitive circuits.