Principle of switching regulator

A switching regulators uses a pulse width modulator to produce a pulse train whose duty cycle is automatically adjusted. The basic circuit is shown in figure above. When drive pulse is high Q1 is saturated and maximum current flows through it. When the drive pulse is low, Q1 cuts off and no current flows through it. Thus the current through Q1 is series pulse having same duty cycle as the modulator output.

If the load voltage V0 tends to fall then the output of the operational amplifiers increases and a larger voltage is applied to the pulse width modulator. Therefore it produces a pulse train having a large duty cycle. The pulse train switches Q1 on and off with a greater duty cycle. So the dc value of the current through Q1 is increased. The increased current raises the load voltage to compensate for its initial decreases.

This note is derived from MSC physics, Nepal.