Power & Energy
Linear regulator vs buck converter
A linear regulator drops the spare voltage across a transistor and turns it into heat, like riding the brakes down a hill. A buck converter switches the supply on and off and smooths the result with a coil, trading volts for amps like a bicycle's gears. Linear is simple, cheap and quiet; a buck is far more efficient when the drop or the current is big.
The linear regulator's heat
A linear regulator passes the load's full current through its pass transistor, and the transistor holds back whatever voltage is spare:
So its efficiency can never beat Vout ÷ Vin. From 12 V to 4.8 V at about 100 mA, the load takes 0.47 W and the transistor burns 0.69 W, more than the load.
It also needs headroom: some voltage across the pass transistor to keep control. Too little and the output drops out; too much and it all becomes heat. The best supply for a linear regulator sits just above its dropout.
Live experiment
Find the regulator's window
A linear pass regulator feeds a 50 Ω load at about 4.8 V while you slide its battery from 4 to 15 V. Every volt above the output is heat in Q1: 0.69 W at 12 V, more than the load's 0.47 W. Below about 6 V it drops out.
“Headroom and heat” is lesson 2 of 6 in Buck Bay, Power & Energy World 2. Free with an account. Lessons unlock in order, so you’ll do the 8 before it first.
The buck converter's trick
A buck converter chops the input with a switch that's always fully on or fully off, so it barely heats. A coil and capacitor smooth the chopped voltage to its average:
where D is the fraction of the time the switch is on. Power in roughly equals power out, so half the volts out means nearly twice the amps out: the input current is smaller than the output current.
A worked comparison
Make 5 V at 1 A from 12 V.
| Linear | Buck at 90% | |
|---|---|---|
| Power to the load | 5 W | 5 W |
| Power from 12 V | 12 W | 5.6 W |
| Heat | 7 W | 0.56 W |
| Input current | 1 A | 0.46 A |
The linear regulator needs a sizeable heatsink; the buck runs barely warm, and the battery lasts more than twice as long.
Which to use
Choose linear when:
- the current is small, so the heat is small anyway
- the input sits just above the output, where an LDO is nearly as efficient
- noise matters: audio, radio receivers and precise sensors
Choose a buck when the drop is large, the current is big, or it runs from a battery. It costs a coil, a little more design care, and some switching ripple. A common compromise uses both: a buck does the big step down efficiently, and a small LDO after it cleans up the ripple.
Free lesson
Now build it yourself
Each lesson comes in three short parts: see the idea, try it on a live board, then build it yourself, with quick checks as you go.
“Headroom and heat” is lesson 2 of 6 in Buck Bay, Power & Energy World 2. Free with an account. Lessons unlock in order, so you’ll do the 8 before it first.
Or start from the very first lesson, no account neededWork it out
Practice questions
Three quick questions from the lessons. Pick an answer to check it.
Show the answer
Answer: 0.7 W. (12 − 4.8) × 0.1 = 0.72 W, as heat.
- Why not “Nothing”? A linear regulator always turns the spare volts times the current into heat.
- Why not “7 W”? 7 W would need 1 A. At 100 mA the regulator burns about 0.7 W.
- Why not “0.07 W”? That's a tenth of the real figure: (12 − 4.8) V × 0.1 A = 0.72 W.
Show the answer
Answer: 6 V. Output ≈ input × on-fraction.
- Why not “3 V”? 3 V would be on for a quarter of the time. Output ≈ input × on-fraction, so half gives 6 V.
- Why not “12 V”? 12 V would mean the switch was on all the time. Half the time gives about half the input.
- Why not “24 V”? A buck converter can only step down. It never makes more than its input.
Show the answer
Answer: A switch, a coil, a diode and a capacitor, arranged differently. Same four parts. Switch first steps down; coil first steps up.
- Why not “Different parts”? They share the same four parts. Only the arrangement differs: switch first steps down, coil first steps up.
- Why not “Transformers”? Neither needs a transformer. A single coil stores and releases the energy.
- Why not “Zeners”? Zeners waste power holding a voltage. Switchers avoid that by switching fully on and off.
“Headroom and heat” is lesson 2 of 6 in Buck Bay, Power & Energy World 2. Free with an account. Lessons unlock in order, so you’ll do the 8 before it first.
Questions people ask
Is a buck converter more efficient than a linear regulator?
Usually, by a lot. A linear regulator's efficiency can't beat V_out ÷ V_in, so 12 V to 5 V is at best about 42%. A buck converter typically reaches 85–95% whatever the ratio. When V_in is only a little above V_out, a good LDO can match it.
When should I use a linear regulator instead of a switcher?
When the current is small, the input is close to the output, or noise matters. Linear regulators are cheap, need no coil, and make no switching noise, which suits audio, radio receivers and precise sensors. For big drops at big currents, use a buck.
How do I work out a linear regulator's heat?
P = (V_in − V_out) × I. From 12 V to 5 V at 1 A, that's 7 W, which needs a real heatsink. A buck converter at 90% would waste about 0.56 W doing the same job.
What is headroom in a regulator?
The extra input voltage a linear regulator needs above its output to keep control, also called dropout. Too little and the output sags; too much and it all becomes heat. The sweet spot is a supply just above the dropout.
Related topics
- How does a linear voltage regulator work?A zener sets the voltage and a pass transistor carries the current: the circuit inside every 7805 and LDO.
- How does a buck converter work?Switch, coil, diode and capacitor: the step-down converter that halves a voltage with almost no heat.
- Electrical power explained: P = V × IP = V × I: how fast energy turns into heat, and why a resistor has a power rating.
- How does a boost converter work?A coil fills from the battery, then kicks into the output: how a switch makes more volts than it started with.