M5 · Flyback voltages and the primary clamp

Goal

• Work out the voltages a flyback’s switch and output diode block, and check the switch’s in the simulator.
• Add leakage inductance and see why a clamp is needed at all.
• Size an RCD or a TVS clamp that leaves margin to the switch’s rating, and state what it costs in dissipation and in the output voltage it allows without a load.

Before you start

• Read the flyback converter, leakage inductance and snubbers and clamps.
• The turns ratio is 1:$n$ with $n$ the secondary turns over the primary turns, so the primary sees the output divided by $n$.

Steps

1. The ideal flyback’s voltages

The flyback example in CCM has $V_g$ = 48 V, $n$ = 0.25 and $D$ = 0.4. Work out its output voltage, the voltage its switch blocks while off (switch voltage while off) and the reverse voltage of its output diode while the switch is on (diode reverse voltage). Enter the two voltages below.

2. The same in the simulator

Open the example in the simulator and compare its switch voltage with yours in the table under the charts:

▶ Open in the simulator: Flyback
Preset from the example flyback-ccm

The simulator’s transformer has no leakage inductance, so the switch voltage has no spike at turn-off: it steps straight to the input voltage plus the reflected voltage.

3. The reflected voltage

With leakage, the leakage inductance still carries the peak primary current when the switch opens, and it drives the drain above the input voltage plus the reflected voltage until a clamp limits it [Kollman 2024]. The clamp examples run the same kind of flyback at a higher input voltage. Work out their reflected voltage (reflected voltage): any clamp must hold more than that.

4. The RCD clamp

Open the clamp check with the RCD example:

▶ Open in the clamp check: Example flyback with an RCD primary clamp
Preset from the example clamp-rcd

Enter the switch voltage it gives below. It leaves little margin to the switch’s rating of 150 V. Change the resistor until the margin is at least a tenth of the rating, and write down the clamp loss at that value. The clamp trades its voltage against its loss [Kollman 2024].

5. The TVS clamp and the open load

Open the TVS example at the same operating point:

▶ Open in the clamp check: Example flyback with a TVS primary clamp
Preset from the example clamp-tvs

Note its clamp voltage, its loss, and the output voltage it allows without a load (output ceiling set by the clamp). Enter that ceiling below, and compare it with the RCD clamp’s behaviour without a load, which the tool warns about.

Acceptance criteria

• The switch voltage of the CCM flyback example while the switch is off, without leakage.
  Your value (V)Check
  Show the example's value

  80 V (from flyback-ccm: Flyback example in CCM, ideal diode)

• The reverse voltage of its output diode while the switch is on.
  Your value (V)Check
  Show the example's value

  20 V (from flyback-ccm: Flyback example in CCM, ideal diode)

• The reflected voltage $V_\mathrm{OR}$ of the clamp examples.
  Your value (V)Check
  Show the example's value

  50 V (from clamp-rcd: Example flyback with an RCD primary clamp)

• The switch voltage with the RCD example's clamp.
  Your value (V)Check
  Show the example's value

  147 V (from clamp-rcd: Example flyback with an RCD primary clamp)

• An RCD resistor that leaves at least a tenth of the rating as margin, and the clamp loss it costs.
• The output voltage the TVS clamp allows without a load.
  Your value (V)Check
  Show the example's value

  16 V (from clamp-tvs: Example flyback with a TVS primary clamp)

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Gotchas

• The reflected voltage includes the diode drop: the primary carries the output voltage plus the drop, divided by $n$.
• A clamp voltage close to the reflected voltage looks safe for the switch, but the leakage current then takes long to reset and the clamp absorbs much more than the leakage energy [Kollman 2024]. The tool warns when the reset would take a whole period.
• Check the switch at the highest input voltage and the highest peak current together, even if they never occur at once: it is conservative.
• An RCD clamp holds its voltage only while the output takes the power. Without a load its resistor must dissipate everything, and the voltages rise far above the values computed here (the clamp check’s output voltage without a load).

Go deeper

• Technical article Snubbing the flyback converter (SSZTCV6) — R. Kollman, Texas Instruments · retrieved 2026-09-24
  How a flyback's leakage inductance drives the switch above the reflected voltage, and how the clamp voltage trades against the clamp's loss.
• Book chapter Converter Circuits (Ch. 6) — R. W. Erickson, D. Maksimović · retrieved 2026-09-23
  How the basic topologies relate, and transformer-isolated versions including the flyback and forward converters.

Quiz

1. A flyback with $n$ = 0.5 (1:0.5) delivers 12 V through a diode with a 0.5 V drop. What is its reflected voltage?

   1. 6.25 V
   2. 24 V
   3. 25 V
   4. 12.5 V
   Answer and explanation

   Correct answer: C The primary carries the output voltage plus the diode drop, divided by $n$: 12.5 V over 0.5 is 25 V.

2. The same flyback runs from 100 V. Without leakage, what does its switch block while it is off?

   1. 100 V
   2. 125 V
   3. 75 V
   4. 112 V
   Answer and explanation

   Correct answer: B The input voltage plus the reflected voltage: 100 V plus 25 V.

3. And its output diode, while the switch is on?

   1. 62 V
   2. 50 V
   3. 12 V
   4. 112 V
   Answer and explanation

   Correct answer: A The secondary carries $n$ times the input, 50 V, in series with the 12 V output: 62 V.

4. An RCD clamp's resistor is made larger. What happens?

   1. The clamp voltage rises and the clamp dissipates less
   2. The clamp voltage falls and the clamp dissipates less
   3. The clamp voltage rises and the clamp dissipates more
   4. Nothing: the capacitor sets the clamp voltage
   Answer and explanation

   Correct answer: A The clamp voltage settles where the resistor dissipates what the clamp absorbs. A larger resistor needs a higher voltage for that, and a clamp voltage further above the reflected voltage absorbs less.

5. Why must the clamp voltage be above the reflected voltage?

   1. Otherwise the switch would never turn off
   2. Otherwise the clamp conducts whenever the output diode does and takes the energy meant for the output
   3. Otherwise the output diode would break down
   4. It need not be: any clamp voltage works
   Answer and explanation

   Correct answer: B While the output diode conducts, the primary sits at the reflected voltage. A clamp below it would conduct in parallel with the transformer's output path.

6. A flyback with an RCD clamp loses its load. What happens to the clamp voltage?

   1. It stays where it was
   2. It falls to the reflected voltage
   3. It rises far above its design value, because the resistor must now dissipate the whole input power
   4. It falls to zero
   Answer and explanation

   Correct answer: C Without a load, all the energy the converter takes ends in the clamp. The RCD clamp's voltage rises until its resistor dissipates it, and the output and the switch voltage rise with it.