A TVS judged by its breakdown voltage
Symptom
Section titled “Symptom”A flyback’s switch fails, or its drain voltage at turn-off peaks above the value the primary clamp was chosen for, although the TVS’s breakdown voltage left margin to the switch’s rating.
A TVS begins to conduct at its breakdown voltage, but while it carries a current it holds its clamping voltage: the breakdown voltage plus its dynamic resistance times the current [STMicroelectronics 2022] (clamping voltage). The data sheet’s clamping voltage is a maximum at a standard pulse’s peak current; the dynamic resistance depends on the pulse’s duration, and the breakdown voltage rises with temperature [STMicroelectronics 2022]. In a flyback the clamp takes the leakage inductance’s current at turn-off, and the switch sees the input voltage plus the clamp’s voltage (switch voltage while the clamp conducts).
In the clamp tool’s example, a TVS that breaks down at 60 V and clamps at 90 V at its 10 A pulse has a dynamic resistance of 3 Ω. At the flyback’s 2 A it holds 66 V, and the switch sees 138 V against its 150 V rating.
How to confirm
Section titled “How to confirm”Capture the drain voltage at turn-off, with a probe rated for it and a short ground connection (probes). Where the primary’s ground is not the oscilloscope’s earth, as in an off-line converter, use a differential probe rated for the voltage: a probe’s ground lead is tied to the oscilloscope’s earth (gate drive). While the clamp conducts, the drain sits at the input voltage plus the clamp’s voltage. Compare that plateau with the clamping voltage at the actual peak current, and its peak with the switch’s rating.
Choose the TVS by its clamping voltage at the actual peak current and at its highest temperature, with margin to the switch’s rating; the clamp check computes it from the data sheet’s breakdown and clamping voltages. Keep the clamp’s voltage well above the reflected output voltage as well: a clamp voltage close to it takes much more than the leakage energy [Kollman 2024].
References
Section titled “References”- [STMicroelectronics 2022]: the clamping voltage at the application’s current from the breakdown voltage and the dynamic resistance, and what the data sheet’s clamping voltage means.
- [Kollman 2024]: a clamp voltage close to the reflected voltage wastes power.
- Snubbers and clamps and the clamp check tool.
The numbers in the examples, presets and exercises on this site are synthetic: round teaching values, not data from any real design.