# Flyback Converter Calculator

URL: https://speedycalculator.com/calculators/physics/flyback-converter/
Last updated: 2026-10-30

> The flyback converter calculator designs a flyback transformer operating at the boundary of continuous conduction: duty cycle, secondary and primary peak currents and inductances from the input and output voltages, turns ratio, output current and switching frequency.

## Inputs

- Input voltage V_in (minimum) (V, mV, kV, µV, MV)
- Output voltage V_out (V, mV, kV, µV, MV)
- Output rectifier drop V_d (V, mV, kV, µV, MV)
- Turns ratio N_p/N_s
- Output current I_out (A, mA, µA, kA)
- Switching frequency f_s (kHz, MHz, Hz)

## Results

- Duty cycle D
- Secondary peak current
- Secondary inductance
- Primary peak current
- Primary inductance
- Reflected voltage on the switch

## Formula

```
D = N·(V_out + V_d) / (V_in + N·(V_out + V_d)),  I_pk,s = 2·I_out/(1 − D),  L_s = (V_out + V_d)(1 − D)/(f_s·I_pk,s)
L_p = N²·L_s,  I_pk,p = I_pk,s / N  (N = N_p/N_s)
```

## Explanation

Flyback converters are the workhorse of small isolated power supplies – phone chargers, standby supplies and LED drivers. A coupled inductor stores energy while the switch is on and releases it to the secondary when it turns off.

### How to use the Flyback Converter Calculator

- **Input voltage V_in (minimum)**
- **Output voltage V_out**
- **Output rectifier drop V_d**
- **Turns ratio N_p/N_s**
- **Output current I_out**
- **Switching frequency f_s**

Results update as you type, every field has its own unit menu, and everything is calculated in your browser.

### How it works

Enter the minimum input voltage, output voltage, rectifier diode drop, turns ratio N_p/N_s, output current and switching frequency.

### Inputs explained

FieldWhat to enterInput voltage V_in (minimum)Units: V, mV, kV, µV, MVOutput voltage V_outUnits: V, mV, kV, µV, MVOutput rectifier drop V_dUnits: V, mV, kV, µV, MVTurns ratio N_p/N_sA numberOutput current I_outUnits: A, mA, µA, kASwitching frequency f_sUnits: kHz, MHz, Hz

### What each result means

- **Duty cycle D** = [Turns ratio N_p÷N_s]×([Output voltage V_out] + [Output rectifier drop V_d])÷([Input voltage V_in (minimum)] + [Turns ratio N_p÷N_s]×([Output voltage V_out] + [Output rectifier drop V_d]))
- **Secondary peak current** = 2×[Output current I_out]÷(1 - [Duty cycle D])
- **Secondary inductance** = ([Output voltage V_out] + [Output rectifier drop V_d])×(1 - [Duty cycle D])÷([Switching frequency f_s]×2×[Output current I_out]÷(1 - [Duty cycle D]))
- **Primary peak current** = 2×[Output current I_out]÷(1 - [Duty cycle D])÷[Turns ratio N_p÷N_s]
- **Primary inductance** = [Turns ratio N_p÷N_s]^2×([Output voltage V_out] + [Output rectifier drop V_d])×(1 - [Duty cycle D])÷([Switching frequency f_s]×2×[Output current I_out]÷(1 - [Duty cycle D]))
- **Reflected voltage on the switch** = [Input voltage V_in (minimum)] + [Turns ratio N_p÷N_s]×([Output voltage V_out] + [Output rectifier drop V_d])

### Example values

How the main result changes when **Input voltage V_in** changes and everything else stays at the example values above:

Input voltage V_in (minimum) (V)Duty cycle D750.5238112.50.42311500.3548187.50.30562250.26833000.2157

## Worked example

Inputs — Input voltage V_in (minimum): 150 V; Output voltage V_out: 5 V; Output rectifier drop V_d: 0.5 V; Turns ratio N_p/N_s: 15; Output current I_out: 2 A; Switching frequency f_s: 100 kHz.

Results — **Duty cycle D = 0.3548**; Secondary peak current = 6.2 A; Secondary inductance = 5.7232 µH; Primary peak current = 0.41333 A; Primary inductance = 1,287.72112 µH; Reflected voltage on the switch = 232.5 V.

## Key terms

- **Coupled inductor**: The flyback "transformer", which stores energy in its gap.

## FAQ

**Why is it called flyback?**
Energy is delivered when the switch turns off, like the flyback of old TV deflection circuits.

**Does the transformer provide isolation?**
Yes – the primary and secondary are galvanically isolated.

**Why design at the boundary of conduction?**
It gives a simple design and low turn-on losses.

**What about leakage inductance?**
It causes voltage spikes; snubbers or clamps are needed.


## Sources

- [Texas Instruments – Power supply design application notes](https://www.ti.com/power-management/overview.html)
- [OpenStax – University Physics Volume 2](https://openstax.org/details/books/university-physics-volume-2)