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Flyback Transformer Calculator

Work out flyback transformer instantly with clear inputs, formula shown and shareable results.

About Flyback Transformer Calculator

In a flyback the output voltage plus diode drop is reflected onto the primary multiplied by the turns ratio. The switch must withstand the input voltage plus that reflected voltage plus leakage spike, and the turns ratio is therefore a direct trade between switch voltage rating and rectifier rating.

Formula

Flyback voltages

Vreflected = (Vout + Vd) x N; switch stress = Vin + Vreflected + spike; D = Vr / (Vin + Vr)

Worked example

With the default values below, Flyback Transformer Calculator returns Reflected voltage on the primary of 254.00. Change any input above and every figure here is recalculated in your browser.

Inputs used

Input voltage
325 V
Output voltage
12 V
Primary to secondary turns ratio
20 :1
Maximum duty cycle
45 %

Results

Reflected voltage on the primary
254.00 V
Switch voltage stress
694.80 V
Secondary peak reverse voltage
28.25 V
Duty cycle required
43.87% %

Step by step

  1. 1Values used
  2. 2Flyback voltages
  3. 3Reflected voltage on the primary= 254.00 V
  4. 4Switch voltage stress= 694.80 V
  5. 5Secondary peak reverse voltage= 28.25 V
  6. 6Duty cycle required= 43.87 %

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
  • Electrical results are planning estimates only. Verify against the applicable wiring rules (IEC 60364, BS 7671, NEC or local code) and have all installation work designed and checked by a qualified electrician.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Electrical results are planning estimates only. Verify against the applicable wiring rules (IEC 60364, BS 7671, NEC or local code) and have all installation work designed and checked by a qualified electrician.

Frequently Asked Questions

Why not use a very high turns ratio?

It raises the reflected voltage and so the switch stress. A lower ratio instead raises the secondary diode's reverse voltage, so the choice balances the two.

What sets the leakage spike allowance?

Transformer leakage inductance and the snubber design. A 20-30% margin on top of the calculated stress is normal practice.

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