Switching Power: Changing Voltage Without Throwing Away Heat
Change the conversion ratio and compare the heat each approach throws away
The two bars split the power each approach uses to turn the same input voltage into the same output voltage. Gold is the useful power that reaches the output; red is the loss thrown away as heat. Sweep the conversion ratio, the ratio of output to input. As the ratio shrinks the linear approach grows its red band, but switching keeps its gold almost unchanged no matter the ratio.
The linear approach burns the difference as heat
A linear regulator uses the transistor like a variable resistor to drop the excess voltage. The same current I flows from input to output, so the transistor sees the input-output gap (Vin - Vout), and the power (Vin - Vout)·I becomes heat outright. The useful power reaching the output is Vout·I, so the efficiency is η = Vout / Vin, the conversion ratio itself. Drop 12 V to 5 V and more than half vanishes as heat; drop it to 3.3 V and three quarters vanish. The bigger the step-down, the hotter and more wasteful it gets.
An ideal switch has no loss: chop, then average
A switch has only two states. On, the voltage across it is zero so V·I = 0; off, the current is zero so again V·I = 0. Either way an ideal switch dissipates almost no power. So instead of dropping voltage in a variable resistor, you flip the switch on and off quickly, chopping the input into slices. Call the fraction of each period the switch is on the duty ratio D; the average of the chopped voltage is D·Vin. When an LC filter of an inductor and a capacitor smooths and averages these slices, the output becomes Vout = D·Vin. Change only the duty ratio and the output voltage is set, with no power burned anywhere along the way.
Efficiency does not care about the ratio
The linear efficiency was the conversion ratio itself, so making 1.2 V from 12 V collapses it to ten percent. Switching is different. Its loss comes not from the input-output gap but only from the non-idealities of the switch and inductor, so however large the ratio you ask for, the efficiency holds steady and often tops ninety percent. This is exactly why almost every power supply, from a phone charger to a laptop adapter to a data-center rail, is a switching one. The price for switching fast is switching noise and the need for an inductor and capacitor, so the circuit is more complex than a linear one. Even so, the bigger the step-down, the more overwhelming switching’s advantage.
Back to the first screen
When the ratio was small, the linear bar was mostly red heat; raising it near 1 filled the bar with gold useful power. That is because the linear efficiency was the conversion ratio itself, η = Vout / Vin. The switching bar stayed almost all gold throughout. An ideal switch, zero voltage when on and zero current when off, burns no power, and an LC averages the chopped input to make Vout = D·Vin. Stepping beyond the rectification of the one-way valve that is a diode, we stand at the threshold of power electronics, where merely turning a device on and off changes voltage efficiently.
What comes next
Here we treated the switch as ideal. The final unit opens up what that switch really is. A power MOSFET is that switch, and a real switch loses a little through its on-resistance while on, and through the overlap of voltage and current in the instant of turning on and off. These conduction and switching losses set the ceiling on efficiency, and we see why faster, lower-resistance devices are chased.