Buck Converter Input Current
Iin = Iout × Vout / (Vin × η)
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Formula
Description
A switching converter conserves power (minus losses), so the input power must equal the output power divided by efficiency. For a buck (step-down) converter, the input current is less than the output current because the input voltage is higher. This formula is essential for sizing input capacitors, selecting input fuses, and calculating the load on upstream power stages. The efficiency term accounts for all internal losses including switching, conduction, gate drive, and quiescent current.
Variables
- Iin — Average input current (A)
- Iout — Output load current (A)
- Vout — Output voltage (V)
- Vin — Input voltage (V)
- η — Converter efficiency (0 to 1)
Practical Notes
Input current is pulsed in a buck converter (flowing only during the switch-on time), so RMS input current is higher than the average. Input capacitors must handle this ripple current. For accurate input fuse sizing, use the average input current at minimum input voltage and maximum load for worst case.
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