Buck-Boost Converter Calculator
Calculate buck-boost duty cycle, inductor value, capacitor value, ripple current and component stress for DC-DC step-up / step-down converter designs.
โก Design Rule: A buck-boost converter is useful when input voltage can be above or below the required output. Use this with the Boost Converter Calculator, Buck Converter Inductor Calculator, MOSFET Power Loss Calculator and Regulator Heat Sink Calculator.
๐ Buck-Boost Converter: Input can be lower or higher than regulated output
Topology / Polarity
Input Voltage Vin
Output Voltage Magnitude
Output Current
Efficiency
Switching Frequency
Inductor Ripple
Allowed Output Ripple
This is a first-pass continuous-conduction-mode estimate. Final values must be checked with the controller datasheet, current limit, thermal design and PCB layout.
Presets:
Topology / Polarity
Input Voltage Vin
Duty Cycle
Output Current
Efficiency
Inductor Optional
Switching Frequency
Use this mode to estimate ideal output voltage from duty cycle. High duty cycle increases input/inductor current quickly.
Duty presets:
Minimum Vin
Nominal Vin
Maximum Vin
Output Voltage
Output Current
Efficiency
This checks duty cycle and input current across a variable source such as battery, solar panel or automotive supply.
Input ranges:
๐ Formula Reference
Ideal Duty Cycle
D = |Vout| รท (Vin + |Vout|)
Ideal Output Voltage
|Vout| = Vin ร D รท (1 - D)
Inductor Estimate
L = Vin ร D รท (ฮIL ร fs)
Output Capacitor
C โ Iout ร D รท (fs ร ฮVout)
๐ Quick Reference
Duty Guide
Low duty<40%
Normal40โ75%
High stress>80%
Ripple Starting Points
Low ripple20%
Typical30%
Small inductor40%
Component Stress
Switch voltageVin + Vout
Diode voltageVin + Vout
Inductor currentinput current
๐ Design Notes
Classic buck-boost is invertingThe simple single-switch buck-boost converter produces negative output polarity. Use SEPIC or 4-switch buck-boost if you need positive output.
Input current can be highWhen stepping up voltage or operating at high duty, the input and inductor current can be much higher than output current.
Check current limitMost failed designs are limited by switch current limit, inductor saturation or thermal rise, not by the formula itself.
Use this as a starting pointFinal capacitor ESR, diode recovery, MOSFET losses, snubber needs and compensation must be checked in the IC datasheet.
What is a Buck-Boost Converter Calculator?
A buck-boost converter calculator estimates the duty cycle, inductor value, capacitor value, input current and switching stress for DC-DC converters that can step voltage up or down.
How to calculate buck-boost converter duty cycle
For an ideal inverting buck-boost converter, duty cycle is D = |Vout| / (Vin + |Vout|). The same magnitude relationship is also useful as a first estimate for SEPIC/Cuk style converters.
When should you use a buck-boost converter?
Use a buck-boost topology when the input source may be below, equal to or above the required output voltage, such as battery-powered devices, automotive supplies, solar circuits and portable electronics.
โ Frequently Asked Questions
Use D = |Vout| / (Vin + |Vout|) for the ideal inverting buck-boost converter. Example: 12V to 24V gives D = 24 / (12 + 24) = 66.7%.
Yes. It is used when input voltage can be higher or lower than the regulated output. The exact operating method depends on the topology and controller.
The classic single-inductor buck-boost topology inverts polarity, so the output is negative with respect to input ground. Use non-inverting buck-boost, SEPIC or 4-switch buck-boost when positive output is required.
A boost converter only steps voltage up. A buck-boost converter can produce an output magnitude that is lower or higher than the input voltage.
Start with L = Vin ร D / (ฮIL ร fs). Then choose an inductor with saturation current above peak current and low enough resistance for efficiency.
High duty cycle increases inductor current, MOSFET current, diode stress and losses. The converter may hit current limit, overheat or fail to regulate.
SEPIC is a non-inverting buck-boost type topology. It can step up or step down while keeping positive output polarity, but it needs more components than a simple inverting buck-boost.
Common causes are input supply current limit, inductor saturation, MOSFET current limit, diode loss, too-small capacitor, poor PCB layout or duty cycle hitting the controller maximum.
For the classic topology, MOSFET and diode voltage stress is approximately Vin + |Vout|, plus switching spikes. Choose extra voltage margin and follow the controller datasheet.