The Buck Converter

Don't burn the difference — refuse to take it

The linear regulator wastes power because its transistor stands half-open, dropping voltage while current flows — the exact situation Lesson 3.2 warned about. The switching insight: a transistor that is only ever fully on (no voltage across it) or fully off (no current through it) dissipates almost nothing in either state. So: switch hard, tens of thousands of times per second, and let PWM set the ratio.

The buck converter

Chop 12 V with PWM at, say, 42% duty and the switch node averages 5 V — but as a violent square wave. Now the reunion tour: an inductor (2.4) resists the current changes and a capacitor (2.3) resists the voltage changes — together a low-pass filter (5.3) whose cutoff sits far below the switching frequency. The chop is filtered away; the average sails through:

(One more familiar face: when the switch turns off, the inductor’s current must keep flowing — a diode gives it the path, precisely your flyback diode from 2.4. In modern converters a second transistor plays that role.)

Efficiency lands at 85–96%, nearly independent of how big the voltage step is. That is why every phone charger, laptop brick, LED driver, car ECU and server farm runs on switching conversion — the electricity saved worldwide by this one circuit is measured in power stations.

The engineer's trades

• Ripple vs size: bigger L and C smooth better but cost space and money. ΔI ∝ 1/(L·f_sw).
• Frequency: switching faster shrinks L and C — but each switching edge wastes a crumb of energy, so efficiency dips. Modern designs run 0.5–2 MHz.
• Noise: all that chopping radiates. Switchers need careful layout, and sensitive analog stages often get a quiet linear regulator fed from a switcher — both lessons in one supply.