The Boost Regulator

The boost regulator is a second type of fundamental regulator circuit for the switch-mode power supply. The electronic diagram and waveforms for this type of converter are shown in Fig. 1. From this diagram note that the transistor has been moved to a point after the inductor and it's now connected directly across the positive and negative lines of the output. The freewheeling diode is connected in series and reverse bias with the inductor. The capacitor remains in parallel with the output voltage terminals to provide filtering. It's important to remember that this circuit is also connected directly to the secondary windings of the power transformer, just like the buck regulator.

Above: Fig. 1: Electrical schematic and waveforms for a boost regulator for a switch-mode power supply.

When the transistor is switched on, current flows in this circuit and builds up in the inductor. When the transistor is turned off, the voltage that was built up across the inductor due to the stored current is returned to the circuit because it's reverse bias to the applied voltage. Since this voltage is reverse bias, it will be allowed to pass through the diode to the load when it builds to a level that is larger than the applied voltage. When the diode goes into conduction, it will pass the power stored in the inductor along with the supply voltage. This means that the output voltage of the boost converter will always be larger than the input voltage. hence, the name boost. The amount of voltage at the output will be regulated by adjusting the duty cycle of the circuit.