Calculator KB-14
Buck converter calculator
Choose an inductor and capacitor for the required voltages, current, and frequency.
Result
In brief: Calculate a DC-DC buck converter: inductor value, output capacitance, duty cycle, peak current, and maximum ESR, with an inductor current graph. Example: Inductance: 49.38 µH.
Calculation example
| Input voltage | 24 V |
|---|---|
| Output voltage | 12 V |
| Load current | 3 A |
| Frequency | 150 kHz |
| Current ripple | 30 % |
| Output ripple | 50 mV |
| Expected efficiency | 90 % |
| Inductance | 49.38 µH |
|---|---|
| Duty cycle | 55.6 % |
| Inductor current ripple | 900 mA |
| Peak current | 3.45 A |
| Minimum output capacitance | 15 µF |
| Maximum capacitor ESR | 55.6 mΩ |
| Average input current | 1.667 A |
| Losses | 4 W |
How it is calculated
L = (Ui − Uo) · D / (f · ΔI)
C = ΔI / (8 · f · ΔU)
Higher inductance means less current ripple, but a larger inductor. Ripple is usually set to 20–40% of the load current. Capacitance is calculated from the charge the inductor supplies beyond the load during each cycle, and ESR is often more important than capacitance itself.
What to look for
- The inductor's saturation current should be at least 20–30% higher than the peak current.
- Use the maximum input voltage for the calculation: current ripple is greatest at this voltage.
- The input capacitor has to handle a large ripple current. Check its ripple current rating, not just its capacitance.
Frequently asked questions
How do I choose an inductor for a step-down converter?
L = (Uin − Uout) · D / (f · ΔI), where D = Uout / (Uin · η), and current ripple ΔI is usually set to 20–40% of the load current. For 24→12 V, 3 A, 150 kHz, 30% ripple, and 90% efficiency, the result is about 49 µH.
What current rating should I choose for an inductor?
Choose it for the peak current: Ipeak = Iload + ΔI / 2. At 3 A with 0.9 A ripple, that’s 3.45 A. The inductor’s saturation current should exceed the peak by a margin of 20–30%.
What has the biggest effect on output voltage ripple?
Usually the output capacitor’s ESR, not its capacitance: ΔU ≈ ΔI · ESR. With ΔI of 0.9 A and a 50 mV target, ESR must be no more than 55 mΩ, so low-impedance or ceramic capacitors are used.
