A flyback leakage inductance spike is not the ordinary reflected voltage that appears when a switch opens. It is a separate, faster and much higher excursion caused by energy stored in the flux that never coupled to the secondary. Because no rectifier on the output can absorb that energy, the switching device must be protected by a clamp or snubber deliberately designed for it.
Two different voltages appear when the switch opens
When the switching device in a flyback topology turns off, the drain or collector voltage rises to the input voltage plus the reflected output voltage, which is the secondary voltage scaled by the turns ratio. That part is intentional, predictable and necessary for the converter to work. Sitting on top of it is a much sharper spike. During conduction, some of the flux produced by the primary links only the primary and never reaches the secondary, and the energy associated with that leakage flux has nowhere to go when the switch opens. The secondary winding and its rectifier cannot receive it because they were never coupled to it. So that energy drives current into whatever capacitance is available, principally the switch’s own output capacitance, and the resulting voltage rise can far exceed the reflected voltage.
Why this is not the same as a relay coil kickback
The familiar case of switching an inductive load, where a diode across the coil catches the reverse voltage, involves a single winding whose entire stored energy can be recirculated. A coupled magnetic component is different because its energy divides. The portion in the mutual flux transfers to the secondary and does useful work; the portion in the leakage flux does not, and adding a diode across the secondary does nothing for it. This is why a flyback transformer needs a clamp on the primary side even though the output rectifier is already handling the main energy transfer. It is also why leakage inductance is treated as a headline parameter for such transformers, quoted with the secondary shorted, rather than as an incidental imperfection.
What the spike does if you ignore it
The immediate risk is exceeding the switch’s breakdown voltage. Modern devices may survive a limited amount of avalanche energy, and some are explicitly rated for it, but relying on repetitive avalanche as a design strategy consumes device life and is rarely what the manufacturer intended. Even below breakdown the spike causes trouble. The fast edge rings with circuit parasitics, radiating and conducting interference that shows up as electromagnetic compatibility failures late in a project when they are expensive to fix. It stresses the transformer insulation. It couples into control circuitry and causes erratic switching. And because the peak depends on load current, a design that looks safe at moderate load can exceed the device rating during a load step, at startup, or into a short circuit.
Clamps and snubbers decide where the energy goes
Every practical solution routes leakage energy somewhere tolerable. A resistor-capacitor-diode clamp catches the spike on a capacitor and burns the energy in a resistor, limiting the peak at the cost of continuous dissipation that must be budgeted thermally. A transient voltage suppressor or Zener-based clamp holds the voltage to a defined level and is more predictable, but it also dissipates and must be rated for the repetitive energy rather than a one-off surge. Active clamp topologies recycle the energy back into the input instead of wasting it, improving efficiency at the cost of an extra switch and its drive. None of these eliminates leakage inductance; they manage its consequences. Reducing the leakage itself, through winding technique, interleaving and core choice, is the complementary approach and is where the transformer specification earns its keep.
Measuring before sizing, and doing it safely
Sizing a clamp from theory alone is unreliable because leakage inductance varies between transformer samples and the parasitic capacitances are hard to predict. Measure the switch node voltage with a probe that has a very short ground connection, since a long ground lead adds its own ringing and will mislead you badly. Capture the peak under the genuinely worst case: maximum load, minimum input voltage, startup, load step and short circuit recovery, at the highest ambient temperature. Then choose a clamp level that leaves margin below the device rating and verify the clamp components’ dissipation with a thermal measurement. Remember that a flyback primary is frequently mains-referenced and not isolated from the supply, so an oscilloscope ground clipped to that node can be dangerous; use an isolation approach appropriate to the equipment and follow safe work practices.
What to check before you act
- Probe the switch node with the shortest possible ground connection.
- Capture the peak at minimum input, maximum load and during faults.
- Budget clamp dissipation thermally, not just electrically.
- Treat a mains-referenced primary as live before connecting any instrument.
Common questions
Can I remove the clamp if my switch is rated well above the reflected voltage?
No. The leakage spike is a separate excursion above the reflected voltage and can be several times larger, and it also creates interference that the clamp helps to control.
Does a bigger snubber capacitor always help?
It lowers the peak but increases the energy circulating each cycle and therefore the resistor dissipation and the efficiency penalty. Size it from measurement rather than by increasing it until the spike disappears.
The practical takeaway
Leakage energy has to be given a destination, and the clamp is that destination. Measure the switch node properly under worst-case conditions, size the clamp for repetitive energy rather than a single event, and reduce leakage inductance in the transformer itself wherever the specification allows it.
References and further reading
- NFPA: electrical safety research and guidance on overheating equipment and fire risk
- CDC NIOSH: electrical safety fundamentals for anyone working on powered equipment
- OSHA: Subpart S, the full set of electrical safety standards for workplaces
Numerical scenarios are illustrative unless identified otherwise. Follow the exact product instructions; component ratings and local installation requirements can differ.
