Boost Converters: Raising Voltage From Less

The boost converter makes 12 V from a 3. 7 V cell by storing energy in an inductor while the switch conducts, then releasing it in series with the input when the switch opens. Flyback spikes become useful.
> At a glance: 7 minute guide · part 3 of 10 in the power supplies and batteries complete guide track · includes a worked example and a quick-reference table.

## The cycle

Here is the working theory in one pass. Switch on: current ramps in the inductor, diode blocks, load runs from the capacitor. Switch off: inductor voltage adds to the input, current forced through the diode tops the capacitor higher than the source ever was. Vout = Vin ÷ (1 − D).

| Quantity | Formula | Note |
| — | — | — |
| Output voltage | Vin ÷ (1 − D) | D < 1 always | | Input current | Pout ÷ (η·Vin) | Exceeds output current | | Duty at high ratio | → 1 | Stress and losses rise | | Inductor | Sized at worst-case Vin | Lowest battery voltage | | Efficiency | 80–90 % typical | Falls at extremes | ## Practical limits Boost ratios beyond ~6:1 get awkward duty cycles crowd, switch stress rises, and transient response suffers. Extreme ratios belong to transformer-based topologies. Input current always exceeds output current (Pout ÷ η ÷ Vin). A 5 V/1 A output from 3. 7 V draws ~1. 5 A from the cell. ## Battery projects Single-cell Li-ion to 5 V (USB) and to 9–12 V are the classic duties. Watch inductor saturation at low-battery voltage where current peaks highest. Protect the battery with a proper low-voltage cutoff. ## 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 minimum input voltage (battery empty) and output 2. Size magnetics for worst-case input current 3. Verify switch and diode ratings at maximum battery 4. Test transient response and cutoff behaviour ### Worked example Boosting a single 18650 (3. 0–4. 2 V) to 5 V/1 A: at 3. 0 V the input draws ~1. 9 A the inductor and switch must be sized for that corner, not the fresh-battery case. Run the numbers yourself with the Battery Life Calculator and the result should agree to within rounding.

> Practical note from the bench. Battery corners rule boost design: everything is sized at the empty cell, where current is highest and patience lowest.

## Pitfalls that cost real hardware

– Sizing components at nominal rather than empty-battery voltage
– Forgetting the output is live through the inductor-diode path even when “off” (load switches fix this)
– Light-load oscillations mistaken for faults

## Key takeaways

– The cycle the foundation of this guide; revisit it if any measurement here surprises you.
– Practical limits the foundation of this guide; revisit it if any measurement here surprises you.
– Battery projects the foundation of this guide; revisit it if any measurement here surprises you.

## Who this guide is for

Beginners get a single focused topic instead of a whole textbook chapter. It works as an early stop in the power supplies and batteries complete guide path. Intermediate readers use it as a reference the table, the worked example and the mistake list answer the questions that come up mid-build. If you teach, the structure (theory, application, example, failure modes) maps cleanly onto a lab session.

## What you need before starting

Nothing exotic: the parts or tools named in the guide, a multimeter. The Battery Life Calculator open in a tab. Define minimum input voltage (battery empty) and output before you begin the guide assumes it and keep the quick-reference table above within sight while you work through the steps.

### Quick reference card

| Aspect | Where to find it in this guide |
| — | — |
| Core theory | The cycle |
| Application steps | How to apply this in your build |
| Worked numbers | Worked example |
| Failure modes | Pitfalls that cost real hardware |

## 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 [buck topology](/tutorial/buck-converter-basics) and [battery chemistries](/tutorial/lithium-ion-vs-lipo). For the arithmetic, open the [Battery Life Calculator](/tools/battery-life).

## Frequently asked questions

Why does efficiency drop at light load?
Fixed switching and quiescent losses dominate when output power shrinks some parts enter power-save modes.

Boost then buck why do that?
Wide-input systems (solar, battery) use both to hold a stable intermediate rail.

Is there a calculator for this?
Yes the [Battery Life Calculator](/tools/battery-life) tool runs the formulas from this guide instantly, client-side, with no signup.

## Continue the learning path

– 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.

## Measurement discipline

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.

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.
## 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.

## Hard-won notes

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.