Buck Converter Input Current

Iin = Iout × Vout / (Vin × η)

Quick answer

This calculator solves Iin = Iout × Vout / (Vin × η) for Input Current. With Output Current 4.4 A, Output Voltage 12 V, Input Voltage 26.4 V, Efficiency 50 %, the result is 4 A. It solves in 5 directions, so you can enter the values you have and get the one you need.

Calculator

Input Current (A)
—

Formula

Iin = Iout × Vout / (Vin × η)

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.

Symbols and units

Symbol Meaning Default unit Accepted units
Iin Input Current A pA, nA, µA, mA, A, kA
Iout Output Current A pA, nA, µA, mA, A, kA
Vout Output Voltage V pV, nV, µV, mV, V, kV
Vin Input Voltage V pV, nV, µV, mV, V, kV
eta Efficiency — —, %

Step by step

  1. 1. What you know: Output Current 4.4 A, Output Voltage 12 V, Input Voltage 26.4 V, Efficiency 50 %
  2. 2. The relation: Iin = Iout × Vout / (Vin × η)
  3. 3. Substitute: Iout = 4.4 A, Vout = 12 V, Vin = 26.4 V, eta = 50 %
  4. 4. Result: Input Current = 4 A

Related quantities from the same inputs

  • P = V × I = 52.8 W

Worked examples

Minimum range

Given: Output Current 4.4 mA, Output Voltage 0.33 V, Input Voltage 0.726 V, Efficiency 2.5 %

Result: Input Current = 80 mA

Low range

Given: Output Current 44 mA, Output Voltage 3.3 V, Input Voltage 7.26 V, Efficiency 25 %

Result: Input Current = 80 mA

Typical

Given: Output Current 4.4 A, Output Voltage 12 V, Input Voltage 26.4 V, Efficiency 50 %

Result: Input Current = 4 A

High range

Given: Output Current 0.22 kA, Output Voltage 0.23 kV, Input Voltage 0.506 kV, Efficiency 0.8

Result: Input Current = 125 A

Maximum range

Given: Output Current 2.2 kA, Output Voltage 2.3 kV, Input Voltage 5.06 kV, Efficiency 8

Result: Input Current = 125 A

Solving for each variable

Solve for Input Current

Output Current 4.4 mA, Output Voltage 0.33 V, Input Voltage 0.726 V, Efficiency 2.5 % → 80 mA

Solve for Output Current

Input Current 2 mA, Output Voltage 0.33 V, Input Voltage 0.726 V, Efficiency 2.5 % → 110 µA

Solve for Output Voltage

Input Current 2 mA, Output Current 4.4 mA, Input Voltage 0.726 V, Efficiency 2.5 % → 8.25 mV

Solve for Input Voltage

Input Current 2 mA, Output Current 4.4 mA, Output Voltage 0.33 V, Efficiency 2.5 % → 29.04 V

Solve for Efficiency

Input Current 2 mA, Output Current 4.4 mA, Output Voltage 0.33 V, Input Voltage 0.726 V → 1

How Input Current changes with Output Current

Output Current (input) Input Current (result)
0.968 A 0.3999 A
2.42 A 0.9997 A
4.84 A 1.999 A
7.26 A 2.999 A
9.68 A 3.999 A
14.52 A 5.998 A
19.36 A 7.997 A
38.72 A 15.99 A
67.76 A 27.99 A
96.8 A 39.99 A

The same result in other units

Unit Value
µA 4000000
mA 4000
A 4
kA 0.004

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Frequently asked questions

How do you calculate Buck Converter Input Current?

Use Iin = Iout × Vout / (Vin × η). With Output Current 4.4 A, Output Voltage 12 V, Input Voltage 26.4 V, Efficiency 50 %, Input Current works out to 4 A.

What is the formula for Buck Converter Input Current?

The formula is Iin = Iout × Vout / (Vin × η). A switching converter conserves power (minus losses), so the input power must equal the output power divided by efficiency.

How do you find Output Current when you know the other values?

Rearrange for Output Current: with Input Current 2 mA, Output Voltage 0.33 V, Input Voltage 0.726 V, Efficiency 2.5 %, Output Current is 110 µA.

How do you find Output Voltage when you know the other values?

Rearrange for Output Voltage: with Input Current 2 mA, Output Current 4.4 mA, Input Voltage 0.726 V, Efficiency 2.5 %, Output Voltage is 8.25 mV.

What units does Buck Converter Input Current use?

Input Current is shown in µA, mA, A, kA. Values are held in SI base units internally, so you can mix prefixes without converting by hand.

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