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.
- Define minimum input voltage (battery empty) and output
- Size magnetics for worst-case input current
- Verify switch and diode ratings at maximum battery
- 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 complete guide for the full map, or continue with buck topology and battery chemistries. For the arithmetic, open the Battery Life Calculator.
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 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
- Read next: bms design tutorial: battery management systems explained
- Also in this track: solar mppt explained: maximum power point tracking
- Continue with: linear vs switching regulators: choosing correctly
- Calculate as you go: runtime estimator · regulator designer · capacitor code tool
- 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.
Extended Application Notes
This section expands the practical application of boost converters: raising voltage from less beyond the worked example, into the situations builders actually meet. Component substitution: when the exact specified part is unavailable, the substitution logic follows the governing parameter of this design, not the nominal value, and the verification step after any substitution is to re-measure the one quantity this guide identified as critical. Batch variation: components vary, and the design margins recommended in the sections above absorb that variation; if a second build behaves differently, the difference itself is diagnostic and points to the tolerance that dominated. Environmental limits: temperature, supply variation and ageing each push a real circuit away from its bench behaviour, and the recommended practice is to test the extremes deliberately rather than discover them in the field. These notes exist because the bench taught them, repeatedly, and each one was once a real troubleshooting session that ended in understanding.
Failure Analysis in Depth
The mistakes section above lists the traps; this section explains why each trap exists and how to recognize it early. 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. Each of these failures has a signature that appears in measurement before it appears in smoke: a reading that drifts, a waveform that differs from the prediction, a temperature that climbs faster than the calculation. The discipline this guide teaches is to measure at the first sign, not at the last, and the sections above give the specific instrument and setting for each check. Failure analysis is not pessimism; it is the fastest curriculum in electronics, because a fault understood once is a fault prevented forever.
Pre-Build Checklist
Before powering any build of this design, run the list: every component value verified against the specification above, the critical measurement points identified and accessible, the instrument modes and ranges chosen in advance, the expected values written down beside the bench, and the power source current-limited for first application. The checklist takes two minutes and replaces the most expensive class of beginner error, which is not ignorance but confidence outrunning verification. Builders who adopt the checklist across the guides in this track report first-apply success rates that feel like cheating, but it is not cheating, it is engineering.
What Comes Next
Having worked through this guide, the natural next steps are the adjacent guides in the track index above, each of which assumes exactly the vocabulary this page built. The calculators linked in the tools section verify every number in seconds, and the complete guide at the head of this track maps the entire curriculum. Read once, build once, measure always: that is the method this site teaches and the method every section above followed before publication.
Theory in Practice, Extended
The theory section of boost converters: raising voltage from less deserves one more pass with the bench in mind, because knowing a relationship and applying it under constraint are different skills. In application, the relationship is never isolated: it interacts with tolerances, with temperature, with the behaviour of adjacent stages, and with the measurement itself. The extended practice is to take the governing formula from the sections above and stress it, deliberately. Push the input to the edge of its specified range and watch the output follow the prediction, then push past it and watch the prediction break, because the edge of the specification is exactly where the formula stops being the whole story. That boundary, found on the bench rather than in the datasheet, is the real knowledge this guide offers beyond the mathematics.
Component Sourcing and Substitution Notes
Real builds meet real supply chains, and this section addresses the practical reality. The specified components in this guide were chosen for the reasons stated in the design sections, but equivalent parts from reputable manufacturers almost always serve, provided the governing parameters match, not merely the nominal ones. The substitution checklist: match the parameter this guide identified as critical, verify the package and pinout against the physical part before layout, check the datasheet revision for silent changes, and re-run the verification measurement after installation. Avoid unbranded surplus and marketplace components for anything this guide treats as safety-relevant; the failure mode of a counterfeit is not degradation, it is unpredictability, and unpredictability defeats every other design decision in the chain.
Instrumentation for This Design
Every measurement recommended in this guide maps to a specific instrument configuration, and this section consolidates them. Voltage checks: DC range selected before probing, leads verified against a known source, meter burden considered when the node is high impedance. Current checks: circuit broken at the defined point, meter inserted with the correct range and fuse status confirmed first. Waveform checks: probe compensated against the reference before any amplitude claim, ground lead kept short, bandwidth sufficient for the edge rather than the repetition rate. The instrumentation discipline matters more than the instrument class, and a modest instrument used correctly outperforms an expensive one used casually, a claim this site demonstrates throughout its measurement guides.
Documentation Template for This Build
Close the loop the way professional builds do: record the design values from this guide, the as-built values including every substitution, the measured results beside the predicted ones, and the deviation notes that explain every gap. The template is short, a single page, and it converts a successful build into a reference that survives component changes, firmware updates and the passage of months. Every guide on this site was built and documented exactly this way before publication, and the discipline is offered here as part of the curriculum rather than an afterthought. A build that is documented is twice built, once in copper and once in confidence.
The Three Formulas That Size a Boost Converter
Sizing a boost converter is three lines of arithmetic plus derating. Duty cycle: D = 1 - (Vin × η) / Vout, where η is efficiency (start with 0.85); boosting 3.7V to 12V at 85% gives D = 1 - (3.7 × 0.85)/12 = 0.74, meaning the switch is on 74% of the time and your inductor stores energy for the other 26%. Inductor current: the average input current is Pout / (Vin × η), so 12V at 500mA output from 3.7V input is 6W / (3.7 × 0.85) = 1.9A average, and the inductor must carry this plus half the ripple without saturating; a 4.7µH inductor at this power runs roughly ±0.6A of ripple, so specify a 3A-plus saturation rating. Switch and diode: both see Vout plus spikes, so rate both at 1.5× Vout minimum, and prefer a Schottky diode or a synchronous FET because the diode's forward drop is paid on every switching cycle. When a boost converter misbehaves, check the inductor's saturation rating first: it is the component where saving money shows up as mysterious output droop under load.
Last updated 23 August 2026