Switching Power: Changing Voltage Without Throwing Away Heat
Change the conversion ratio and compare the heat the two methods waste
Each bar splits the power a method uses when converting the same input voltage to the same output voltage. Gold is the useful power that goes to the output, and red is the loss thrown away as heat. As you sweep the conversion ratio, the output voltage divided by the input voltage, the red band of the linear bar grows as the ratio gets smaller, while the switching bar stays almost all gold regardless of the ratio. Next to the slider you can read what voltage the input is converted to.
The linear approach burns the difference as heat
A linear regulator uses a 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 difference (Vin - Vout), and the power (Vin - Vout)·I turns entirely into heat. The useful power going to the output is Vout·I, so the efficiency is η = Vout / Vin, the conversion ratio itself. Stepping 12 V down to 5 V loses more than half as heat, and stepping it down to 3.3 V loses nearly 3/4. The larger the step-down, the hotter the device and the greater the waste.
An ideal switch has no loss: chop, then average
A switch has only two states. When it is on, the voltage across it is 0, so V·I = 0; when it is off, the current is 0, so again V·I = 0. Either way, an ideal switch uses almost no power. So instead of dropping the voltage across a variable resistor, the switch is turned on and off rapidly to chop the input into small pieces. If the fraction of each period during which the switch is on is called the duty ratio D, the average of the chopped voltage is D·Vin. When an LC filter made of an inductor and a capacitor smooths and averages these pieces, the output becomes Vout = D·Vin. Even while the switch is off, the inductor current keeps flowing through a diode or a second switch, so a rectifying device does its part here too. Changing only the duty ratio sets the output voltage, and nowhere along the way is power burned.
Efficiency does not care about the ratio
The efficiency of the linear method was the conversion ratio itself. So making 1.2 V from 12 V drops the efficiency to 10 percent. Switching is different. Its losses come only from the switch and inductor not being ideal, not from the input-output difference, so even a large conversion ratio barely shakes the efficiency, which often exceeds 90 percent. With a very large step-down or a very light load, though, the share of losses from switching on and off grows and the efficiency drops a little. That is why almost every power supply, from phone chargers to laptop adapters to data center power, uses switching. The price of fast switching is switching noise and the need for inductors and capacitors, which makes the circuit more complex than a linear one, but the larger the step-down, the more overwhelming the advantage of switching.
Back to the first screen
When the conversion ratio was small, red heat filled most of the linear bar, and as you raised the ratio toward 1, gold useful power filled the bar. That is because linear efficiency is the conversion ratio itself, η = Vout / Vin. The switching bar stayed almost all gold throughout. An ideal switch, with 0 voltage when on and 0 current when off, burns no power, and the LC filter averages the chopped input to make Vout = D·Vin. Going one step beyond rectification with the diode, a one-way valve, you now stand at the threshold of power electronics, which converts voltage efficiently just by switching devices on and off.
What comes next
Here the switch was treated as ideal. The last unit opens up that switch. The power MOSFET is that switch, and a real switch loses power in its small on-resistance while it is on, and in the brief overlap of voltage and current at each turn-on and turn-off. These conduction and switching losses set the ceiling on efficiency, and they are why designers seek faster devices with lower resistance.