ProcirelBuck Converters: Stepping Down Efficiently
The buck converter reduces voltage by switching. Duty cycle sets the output, an inductor smooths, a diode (or synchronous FET) catches. Vout = D × Vin and 90 %+ efficiencies follow from physics that wastes almost nothing.
> At a glance: 8 minute guide · part 2 of 10 in the power supplies and batteries complete guide track · includes a worked example and a quick-reference table.
## The operating cycle
When the switch closes, current ramps through the inductor into load and capacitor. When it opens, inductor current keeps flowing through the diode. Averaged by the LC filter, the output is a clean DC set by duty cycle under feedback control.
| Quantity | Formula/Rule | Design target |
| — | — | — |
| Output voltage | Vout = D × Vin | Feedback sets it |
| Inductor | L = (Vin−Vout)·D ÷ (ΔI·f) | ΔI 20–40 % of Iout |
| Output cap | Sets ΔV ripple | Low ESR class |
| Efficiency | 85–95 % typical | Better at higher Vout |
| Controller choice | Synchronous for >1 A | Less loss, more pins |
## Choosing L and C
What this means at the bench. Ripple current (typically 20–40 % of load) sets the inductor; ripple voltage sets the capacitor. Too small an inductor saturates and spikes current; too small a capacitor makes the control loop ornery. Follow the controller datasheet formulas they exist for a reason.
## Modules versus design
For most projects, a pre-built buck module ($1–3) outperforms a hand-laid first attempt. Design your own to integrate, learn, or meet EMI formally and copy the reference layout literally.
## How to apply this in your build
Work through the sequence below each step assumes the previous one passed. For numbers that need calculating, the linked tools at the end of this guide do the arithmetic instantly.
1. Define Vin range, Vout and load current
2. Size inductor and capacitors from datasheet equations
3. Copy the reference layout switching nodes stay short
4. Verify ripple and thermal under real load
### Worked example
Converting 12 V to 5 V at 2 A. A linear part dissipates 14 W (impossible); a buck at 92 % dissipates ~0. 9 W a small PCB heats area, no sink needed. Run the numbers yourself with the Battery Life Calculator and the result should agree to within rounding.
> Practical note from the bench. Every buck we lay out follows the same rule: switching loop smaller than a thumbnail. The oscilloscope rewards geometry every time.
## Field mistakes we see again and again
– Undersized inductors saturating audibly
– Long switching-node traces radiating EMI
– Substituting electrolytics where low-ESR ceramics/polymer are specified
## Key takeaways
– The operating cycle the foundation of this guide; revisit it if any measurement here surprises you.
– Choosing L and C the foundation of this guide; revisit it if any measurement here surprises you.
– Modules versus design the foundation of this guide; revisit it if any measurement here surprises you.
## Prerequisites and preparation
Before starting: define vin range, vout and load current and size inductor and capacitors from datasheet equations. Keep the [Battery Life Calculator](/tools/battery-life) and [LM317 Regulator](/tools/lm317-regulator) open every number in the worked example is reproducible. Total time including the bench steps: about 6–8 minutes.
## Who benefits most
Hobbyists meeting this topic for the first time, students who want the version with real numbers instead of abstract symbols. Returning engineers refreshing a corner of the craft. The mistake list alone justifies the visit every entry in it was learned the expensive way.
### Quick reference card
| Aspect | Where to find it in this guide |
| — | — |
| Core theory | The operating cycle |
| Application steps | How to apply this in your build |
| Worked numbers | Worked example |
| Failure modes | Field mistakes we see again and again |
## How this fits the power supplies and batteries complete guide track
This guide is one stop in the structured learning path. Start from the [power supplies and batteries complete guide](/tutorial/power-batteries-complete-guide) pillar page for the full map, or continue with [boost converter basics](/tutorial/boost-converter-basics) and [linear versus switching](/tutorial/linear-vs-switching-regulators). For the arithmetic, open the [Battery Life Calculator](/tools/battery-life) or [LM317 Regulator](/tools/lm317-regulator).
## Frequently asked questions
Can a buck boost voltage?
No bucks only step down; boost or buck-boost topologies cover the other directions.
Why does my module whine?
Usually Burst Mode at light load or a marginal inductor load it or pick a different part.
Is there a calculator for this?
Yes the [Battery Life Calculator](/tools/battery-life) and [LM317 Regulator](/tools/lm317-regulator) tools run the formulas from this guide instantly, client-side, with no signup.
## Keep going with this track
– The complete power & batteries guide: [Power & Batteries complete guide](/tutorial/power-batteries-complete-guide)
– Read next: [bms design tutorial: battery management systems explained](/tutorial/bms-design-tutorial)
– Also in this track: [solar mppt explained: maximum power point tracking](/tutorial/solar-mppt-explained)
– Continue with: [linear vs switching regulators: choosing correctly](/tutorial/linear-vs-switching-regulators)
– Calculate as you go: [runtime estimator](/tools/battery-life) · [regulator designer](/tools/lm317-regulator) · [capacitor code tool](/tools/capacitor-code)
– Bookmark this page against the day a measurement surprises you. Most readers return to the table and the mistake list first, and that is the correct order.
## Working method notes
Keep a lab notebook entry for every build in this track. The measured values, the deviations from the guide and the reason for each. Six months from now, those notes are worth more than any tutorial. They describe your bench and your components rather than a general case.
When a result here disagrees with your expectation, write down both numbers before changing anything. The gap between predicted and measured is where the real engineering lives. It is usually a tolerance, a parasitic or an assumption that was never checked.
## Formulas and checks from this guide
Verification checklist for this track. Verify regulation under load, not just open-circuit, measure inrush where it matters. Treat every lithium cell as energetic chemistry that has earned its protection chain.
Bookmark this page against your next build in the track. The checklist above is the same one used across 18 guides in this series.
## What the bench taught us
No. Oversizing hammers the diodes with inrush and buys ripple you no longer need once regulation follows.
Any lithium pack of 2S or more, without exception.
## A parting thought for builders
A note on protection chains, non-negotiable in this track: cell, then BMS, then charger, then fuse. Each link assumes the previous one exists.
Working through The operating cycleand Choosing L and C with that habit in mind takes minutes, and it is the difference between reading about this topic and owning it.