Power & Energy
How to choose a buck converter's inductor and capacitor
A buck converter's output filter is a coil and a capacitor, and they're chosen in that order. The inductor sets how much the current wobbles each cycle and must keep it flowing the whole cycle. The capacitor absorbs that wobble and sets how much the output voltage ripples. Pick the coil first, then the capacitor for the ripple you can accept.
Step 1: the inductor
While the switch is on, the coil has Vin − Vout across it and its current climbs. The swing each cycle is the ripple current:
D is the duty cycle and f the switching frequency. A common target is ΔI of 20 to 40 % of the load current.
Step 2: the capacitor
The capacitor soaks up the ripple current. Its voltage wobbles by about:
Worked example
12 V in, half duty, 1 kHz, a 0.1 H coil and a 60 Ω load. The output sits near 5.75 V (the diode takes a little), so the load draws about 96 mA.
- Ripple current: (12 − 5.75) × 0.5 ÷ (1000 × 0.1) ≈ 31 mA, about a third of the load. The coil's current climbs and falls between about 81 and 111 mA.
- With 100 µF: ΔV ≈ 0.031 ÷ (8 × 1000 × 0.0001) ≈ 0.04 V. The output holds within 5.73 to 5.77 V.
- With 10 µF: ten times the ripple, about 0.4 V, 5.56 to 5.94 V.
Live experiment
Smooth the buck's output
A 12 V buck switching at 1 kHz into a 60 Ω load. With the 0.1 H coil, pick the capacitor: 1 µF swings 4.96 to 6.54 V, 10 µF 5.56 to 5.94 V, 100 µF 5.73 to 5.77 V. Then swap in a 10 mH coil and watch its current run dry and the output drift up.
“Ripple” is lesson 4 of 6 in Buck Bay, Power & Energy World 2. Free with an account. Lessons unlock in order, so you’ll do the 10 before it first.
Too small a coil: running dry
Swap the coil for 10 mH, a tenth of the inductance. The ripple current would be ten times bigger, about 310 mA, but the coil's current can't go below zero, and it averages only 96 mA. So it falls to zero every cycle and stays there for part of it: discontinuous conduction. The output no longer sits at D × Vin, and creeps up towards 7 V. No capacitor fixes that.
The boundary is where ΔI equals twice the load current:
Here that's about 16 mH, so 10 mH runs dry and 0.1 H is comfortable.
Faster switching shrinks everything
Double f and the ripple current halves, and the ripple voltage, with f in its formula twice, falls to a quarter. That's why real converters switch at hundreds of kilohertz. Take 12 V to 5 V at 1 A and 500 kHz with a 22 µH coil: ΔI = 7 × 0.42 ÷ (500,000 × 0.000022) ≈ 0.27 A, about 27 % of the load. A 22 µF capacitor then ripples by about 3 mV. At these speeds the capacitor's ESR often matters more: 0.27 A through 0.05 Ω adds 13 mV, which is why such supplies use ceramic capacitors.
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.
“Ripple” is lesson 4 of 6 in Buck Bay, Power & Energy World 2. Free with an account. Lessons unlock in order, so you’ll do the 10 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: Runs dry every cycle, and the output wanders up to nearly 7 V. The current wobble is ten times bigger than with 0.1 H, so it hits zero each cycle. With the current stopped, the output isn't held at half the input any more.
- Why not “Shorts the battery”? The switch and diode still take turns, so there's no direct path across the battery. The current just falls to zero each cycle.
- Why not “Smooths better”? A smaller coil lets its current swing more, ten times as much here, so it smooths worse.
- Why not “Makes no difference”? The coil's current ripple is inversely proportional to its inductance, so a tenth the inductance gives ten times the swing.
Show the answer
Answer: Is smooth but sits too high, near 6.7 V. The capacitor smooths it, but the coil still runs dry every cycle, and the average climbs above half the input.
- Why not “Is perfect”? A big capacitor fixes the ripple, but with the coil running dry the average voltage drifts high.
- Why not “Swings more”? More capacitance always smooths more. The trouble left over is the voltage level, not the swing.
- Why not “Drops to 3 V”? When the coil runs dry the output rises above the duty-cycle value, not below it.
Show the answer
Answer: Sits near 5.7 V but still ripples ±0.2 V. The coil fixes the average; the small capacitor still lets it wobble.
- Why not “Drops out”? The coil and switch still deliver the full duty-cycle voltage, so the output holds near 5.7 V.
- Why not “Runs dry”? A bigger coil keeps its current flowing all cycle. It's the small coil that runs dry.
- Why not “Is perfect”? The coil fixes the level, but 10 µF still lets the voltage swing about 0.4 V from top to bottom.
“Ripple” is lesson 4 of 6 in Buck Bay, Power & Energy World 2. Free with an account. Lessons unlock in order, so you’ll do the 10 before it first.
Questions people ask
How much ripple current should a buck converter's inductor have?
A common starting point is a ripple current of 20 to 40 % of the full load current. Less needs a bigger, slower inductor; more means more output ripple and higher peak currents in the switch and coil.
What is discontinuous conduction mode?
When the coil's current falls all the way to zero before the end of each cycle. It happens with a small coil or a light load. The output then no longer sits at duty cycle times input, and rises above it unless a feedback loop corrects the duty.
Does the capacitor's ESR matter?
Often more than its capacitance. The ripple current flowing through the capacitor's equivalent series resistance adds ΔI × ESR of ripple. That's why switching supplies use low-ESR ceramic or polymer capacitors on their outputs.
Related topics
- How does a buck converter work?Switch, coil, diode and capacitor: the step-down converter that halves a voltage with almost no heat.
- How does an inductor work?A coil that fights changes in its current, the high-voltage spike it makes, and how to tame it.
- Linear regulator vs buck converterBrakes or gears: a linear regulator burns the spare volts, a buck trades them for amps. When each wins.
- What is a decoupling capacitor?Why every chip has a capacitor beside its power pins, how big, and why it must be close.