Boost Converter Calculator
Calculate duty cycle, inductor, and output capacitor for a step-up (boost) DC-DC converter.
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How the Boost Converter Calculator Works
A boost converter steps up voltage from a lower input to a higher output using an inductor, switch, diode, and capacitor. The energy stored in the inductor during the ON phase is released to the output during the OFF phase, raising the voltage.
Duty Cycle
D = 1 − Vin / Vout
Converting 12V to 48V gives D = 1 − 12/48 = 0.75 (75%). The switch is ON for 75% of each cycle. Note that high duty cycles (above 0.85) are generally avoided due to efficiency and control loop stability concerns.
Input Current
Iin = Iout × Vout / Vin (assuming 100% efficiency)
The input current is much higher than the output current. For a 12V-to-48V, 0.5A output design, the input draws ~2A. Size the input capacitor and trace width accordingly.
Inductor and Output Capacitor
L = Vin × D / (f × ΔIL) Co = Iout × D / (f × ΔVout)
The output capacitor in a boost converter must handle a large discontinuous current (the diode current). Use low-ESR capacitors and verify the RMS current rating against the datasheet.
Design Example: 12V to 48V, 0.5A, 200kHz
- D = 1 − 12/48 = 0.75
- Iin = 0.5 × 48/12 = 2.0A
- With 20% ripple → ΔIL = 0.4A
- L = 12 × 0.75 / (200e3 × 0.4) = 112.5 μH
- With 100mV ripple → Co = 0.5 × 0.75 / (200e3 × 0.1) = 18.75 μF
Common Mistakes in Boost Converter Design
- Sizing the diode for output current instead of peak current. The boost diode carries the full inductor current pulse, not the averaged output current — under-rating it here is one of the more common ways a first-pass boost design overheats during bring-up.
- Ignoring right-half-plane (RHP) zero in the control loop. Boost (and buck-boost) converters have a RHP zero that limits achievable bandwidth. A control loop tuned like a buck converter will often be unstable or ring badly under load steps — cross over well below the RHP zero frequency.
- Forgetting inrush at startup. At power-up, the inductor and output capacitor look like a near-short through the diode before the controller establishes regulation. Add soft-start (most controller ICs support it) or you will see input fuse blows / voltage droop on bench power supplies with current limiting.
- Using the input capacitor spec from a buck design. Boost converters draw continuous (not pulsed) input current, so input capacitor RMS current requirements are usually lower than a buck of similar power — over-speccing here just wastes board space and cost.
Frequently Asked Questions
Why is the boost converter input current so high?
Power conservation: Pin = Pout (ignoring losses). If Vin is much lower than Vout, the input current must be correspondingly higher. Always size your input components — traces, connectors, capacitors — for the input current, not the output current.
What is the maximum practical duty cycle for a boost converter?
Most boost converter ICs limit the maximum duty cycle to 80–92% to ensure the inductor can fully transfer energy and to maintain control loop stability. Designs requiring D > 0.85 often use a cascade or SEPIC topology instead.