The principle of switching: changing voltage without loss
An ideal switch has zero voltage when on and zero current when off, so it dissipates no power. This property is the starting point of power electronics.
Why switch
The loss in a switch is voltage × current.
| State | Voltage | Current | Loss |
|---|---|---|---|
| Fully on | ≈ 0 | Large | ≈ 0 |
| Fully off | Large | ≈ 0 | ≈ 0 |
| In between (linear) | Medium | Medium | Large |
A linear regulator uses its transistor in the in-between state and gets hot. A switching converter moves only between on and off, sets the average with the duty cycle, and smooths that average with an inductor and a capacitor.
Real losses
A real switch is left with two kinds of loss.
- Conduction loss: Rds(on) × I² while it is on
- Switching loss: the brief instants of turning on and off when voltage and current overlap
Raising the switching frequency makes the inductor and capacitor smaller but increases switching loss. The frequency is chosen on this balance (usually tens of kHz to several MHz).
The role of the inductor
An inductor does not let its current change suddenly, so when the switch turns off the current keeps flowing through a diode or the opposite switch. This "vessel that holds energy for a moment" is what makes it possible to step voltage down (buck) and up (boost).