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
| Field | What to enter |
|---|---|
| Input voltage V_in (minimum) | Units: V, mV, kV, µV, MV |
| Output voltage V_out | Units: V, mV, kV, µV, MV |
| Output rectifier drop V_d | Units: V, mV, kV, µV, MV |
| Turns ratio N_p/N_s | A number |
| Output current I_out | Units: A, mA, µA, kA |
| Switching frequency f_s | Units: 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 D |
|---|---|
| 75 | 0.5238 |
| 112.5 | 0.4231 |
| 150 | 0.3548 |
| 187.5 | 0.3056 |
| 225 | 0.2683 |
| 300 | 0.2157 |
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)
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.
Frequently asked questions
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.