Your 15 V Zener and TVS diodes are avalanche diodes
Abstract
Zener diodes are very common components provided from 1.8V up to 200V by suppliers. Avalanche diodes are specific diodes between 200V and 1000V characterized in avalanche clamping voltage mode. The connection in series of avalanche diodes allows designing high voltage diodes. For most components avalanche is often understood as a catastrophic phenomenon of failure of the component. The goal of this presentation is to explain the Zener effect and the avalanche effect, the area of voltage where Zener clamping voltage may occur and the impact on the i(V) curves. In fact, most Zener diode or Transient Voltage Suppressor (TVS) work in avalanche mode.