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Flyback converter

Flyback converter: the circuit drawn with the example's values24 V5 Vµ
8.73 : 1, D 67 %, Lp 109 µH for 24 V → 5 V at 2 A

A boost with a transformer: the primary stores energy, the secondary hands it over.

Opens in the app on 1 December, with Circuit Notebook 1.0 See it worked out

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The example, worked out

Given

Lowest input
24 V
Highest input
36 V
Output
5 V
Load current
2 A
Output diode drop
500 mV
Switching frequency
100 kHz
Reflected voltage
48 V
Efficiency expected
85 %
Output ripple allowed
50 mV

Working

  1. the reflected voltage sets the turns ratio
  2. volt-seconds balance on the primary
  3. a triangle of current carries all the power at the boundary
  4. the stresses, before the leakage spike

Results

Turns ratio Np : Ns8.73 : 1
Duty at lowest input66.7 %
Primary inductance109 µH
Primary peak current1.47 A
Secondary peak current12.8 A
Switch voltage (no spike)84 V
Diode reverse voltage9.13 V
Output capacitor267 µF
Input current (average)490 mA
Input power11.8 W

The formulas

  1. n = Np / Ns = Vr / (Vout + Vf)the reflected voltage sets the turns ratio
  2. D = Vr / (Vr + Vin)volt-seconds balance on the primary
  3. Ipk = 2 Pin / (Vin D), Lp = Vin D / (f Ipk)a triangle of current carries the power at the boundary

The usual traps

  • Forgetting the leakage spike: the switch sees Vin + Vr plus 30–50 %, so clamp it.
  • A core without an air gap stores almost no energy.

Questions

How do I calculate the turns ratio and inductance of a flyback converter?

n = Np / Ns = Vr / (Vout + Vf): the reflected voltage sets the turns ratio. D = Vr / (Vr + Vin): volt-seconds balance on the primary. Ipk = 2 Pin / (Vin D), Lp = Vin D / (f Ipk): a triangle of current carries the power at the boundary.

What does it give for typical values?

8.73 : 1, D 67 %, Lp 109 µH for 24 V → 5 V at 2 A. Turns ratio Np : Ns: 8.73 : 1; Duty at lowest input: 66.7 %; Primary inductance: 109 µH.

What are the usual mistakes?

Forgetting the leakage spike: the switch sees Vin + Vr plus 30–50 %, so clamp it. A core without an air gap stores almost no energy.

In the app

Your circuit, drawn live

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The part you can buy

Results rounded to standard values (E12, E24, E96) and power ratings.

The working, shown

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Into your notebook

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Opens in the app on 1 December, with Circuit Notebook 1.0

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