Buck-Boost Converter Design Calculator
Calculates duty cycle, minimum inductance, current ripple, peak current, and output capacitance for Buck, Boost, and Buck-Boost switching converter topologies.
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How to Use
- Select the converter topology: Buck (step-down), Boost (step-up), or Buck-Boost.
- Enter input voltage Vin (V), output voltage Vout (V), maximum output current Iout (A), switching frequency f (kHz), and output ripple ΔVout (mV).
- Click "Calculate" (or simply edit any input field to trigger automatic recalculation).
- The results show: duty cycle D, minimum inductance Lmin, recommended inductance (Lmin × 1.2 margin), inductor current ripple ΔIL, peak inductor current, and minimum output capacitance Cout.
- A formula box below lists the four formulas (duty cycle, inductance, current ripple, output capacitance) used for the selected topology, for verification.
- Click "Load Sample Data" to restore the default Buck example: Vin=12V, Vout=5V, Iout=2A, f=100kHz, ΔVout=50mV.
Features
- Supports parameter calculation for three basic DC-DC switching converter topologies: Buck step-down (D=Vout/Vin), Boost step-up (D=1-Vin/Vout), and Buck-Boost (D=Vout/(Vin+Vout)).
- Calculates the minimum required inductance Lmin for each topology using the inductor ripple-current-factor method (a fixed 30% of output current as the ripple factor), and provides a recommended inductance with a 20% safety margin.
- Computes the inductor current ripple ΔIL and the peak inductor current for each topology (Buck: Iout+ΔIL/2; Boost/Buck-Boost: account for the duty-cycle-adjusted average current Iout/(1-D)).
- Given the allowed output ripple voltage ΔVout, calculates the minimum output capacitance Cout required (Cout=ΔIL/(8×f×ΔVout)).
- Displays the exact formulas used (duty cycle, inductance, current ripple, output capacitance) for each topology, so engineers can verify hand calculations.
Use Cases
Inductor/capacitor selection for a Buck step-down supply
For common step-down designs like 5V-to-3.3V or 12V-to-5V, determine the minimum inductor and output capacitor specs based on target ripple and current.
Parameter estimation for a Boost circuit (e.g., battery step-up supply)
For applications stepping a Li-ion battery from 3.7V up to 5V/12V, calculate the required minimum inductance and peak inductor current to verify the inductor's saturation current rating.
Buck-Boost supply design when input range spans the output voltage
When the input voltage range (e.g., a 2.5V–4.2V battery) straddles the target output voltage (e.g., 3.3V), use the Buck-Boost topology to compute the corresponding duty cycle and component parameters.
Comparing switching frequency's impact on inductor/capacitor size
Adjust the switching frequency f to observe how the minimum inductance and output capacitance requirements change, useful for trading off switching losses against component size.
FAQ
The inductor ripple factor is fixed at 30% — can it be customized?
No. The tool internally uses the common engineering default of 30% of output current as the ripple factor (the 0.3×Iout term in the formula) when calculating minimum inductance. This is a widely used rule of thumb. If your design needs a different ripple factor (e.g., 20% or 40%), you'll need to recompute manually using the topology's formula.
What's the difference between the recommended and minimum inductance?
The minimum inductance Lmin is the theoretical lower bound that just satisfies the 30% ripple factor. The recommended inductance applies a 1.2× safety margin on top and rounds up to 0.1μH, giving a more practical reference value (accounting for inductor tolerance, temperature drift, etc.). In practice, choose the nearest standard inductor value (e.g., E12 series) close to the recommended figure.
Why is the peak inductor current much higher than Iout for Boost and Buck-Boost topologies?
In step-up and buck-boost topologies, the average inductor current is not equal to the output current — it's Iout/(1-D), where D is the duty cycle. When duty cycle is large (output voltage much higher than input, or a large step-up/down ratio), (1-D) becomes small, significantly amplifying both the average and peak inductor current. This is inherent to boost-type topologies, and inductors/switches must be sized with sufficient margin for the peak current.
What happens if Vin, Vout, Iout, or f is entered as 0?
The tool validates that all four core inputs (Vin, Vout, Iout, f) must be greater than 0 before calculating. If any one is ≤0, no calculation is triggered and the results section remains hidden, preventing division-by-zero or meaningless negative results.