A Switching Regulator

A linear regulator holds the resistance of the pass transistor at just the right amount to maintain the output voltage. There is a continuous current passing through the transistor while there is a continuous voltage across it. The product of this voltage and current is power, which is manifested as heat. A large heat sink is usually required to dissipate this heat. A transformer is used to step the voltage down to a lower level before it is presented to the regulator. This reduces the power production of the transistor. This transformer is relatively large and heavy. The transformer and heat sink are, by far, the largest and heaviest parts of a linear power supply.

The following diagrams illustrate how a linear power supply produces power as heat in the pass transistor and how a switching regulator produces little power in the pass transistor.

The variable resistor above represents the pass transistor of a linear regulator. This linear regulator will have 6 volts across it and 1 amp through it and will produce 6 watts of power. This will require a large heat sink.

The switch above represents the pass transistor of a switching regulator. When the transistor is off, there are 18 volts across the transistor but essentially no current. The power produced is 18 volts multiplied by 0 amps for 0 watts.

When the transistor is switched on, there is a large current passing through the transistor, but since there is little resistance in the saturated transistor, there is little voltage across it. Let's assume that is 0 volts. In this case, there is some current multiplied by 0 volts for 0 watts.

A switching regulator continually switches the pass transistor on and off. Since the transistor spends little time in a state where there is both voltage and current present, little power is produced.

Switching regulator at nominal output This shows the switch output with a 50% duty cycle (it is on half the time and off half the time).

Switching regulator under heavy load (high current) The output voltage attempts to drop, but the switch stays in the on position longer to compensate (the duty cycle is increased).

The pass transistor is switched on and off rapidly (perhaps at 1kHz) producing a square wave. This square wave useless as a power supply output but a low-pass filter smoothes he square wave to a near-steady voltage. Current demand is compensated for by varying the duty cycle of the wave.

Switching regulator under light load (low current) The output voltage attempts to rise, but the switch stays in the on position for a shorter time to compensate (the duty cycle is decreased).

Since the switching regulator produces little heat, the input voltage does not need to be stepped down with a transformer. The regulator can be designed to work efficiently with much higher input voltages than a linear regulator

The purpose of the heavy transformer in a linear power supply is to step the input voltage down and reduce power consumption. Since the switching regulator produces much less power, it is not necessary to step the voltage down. The full receptacle voltage (165 volts peak) can be used. The heavy, expensive transformer is eliminated. The heavy heat sink is also eliminated since the pass transistor in a switching power supply produces much less power.

A switching power supply. You can see the capacitor and inductor of the filter. Note the small size. No step-down transformer or large heat sink are required.