The power rating of a TVS is its surge-absorbing capability under specific test or application conditions. The industrial-standard test condition of 10 μs/1000 μs pulse form (Bellcore 1089 spec.) is shown in Figure 1. This test condition differs from the TVS ESD test condition of 8 μs/20 μs pulse form, as shown in Figure 2.
Figure 1: Test waveform of TVS
Figure 2: Test waveform of TVS ESD
Breakdown Voltage (VBR)
The breakdown voltage is the voltage at which the device goes into avalanche breakdown, and is measured at a specified current on the datasheet.
Maximum Breakdown Voltage (VC: Clamping Voltage)
The clamping voltage appears across the TVS at the specified peak pulse current rating. The breakdown voltage of a TVS is measured at a very low current, such as 1 mA or 10 mA, which is different from the actual avalanche voltage in application conditions. Thus, semiconductor manufactures specify the typical or maximum breakdown voltage in large current.
Stand-Off Voltage (VWM): Working Stand-Off Reverse Voltage
The stand-off voltage indicates the maximum voltage of the TVS when not in breakdown, and is an important parameter of protection devices in circuits that do not operate under normal conditions. In automotive applications, some regulation of the automotive electronics is provided by “jump-start protection.” This condition supplies 24 VDC in 10 minutes to 12-V type electronics, and 36 VDC in 10 minutes to 24-V type electronics without damage or malfunction of the circuit. Thus, the stand-off voltage is one of the key parameters in TVS for automotive electronics.
Figure 3: Parameters of voltage and current
Primary Protection of the Automotive Power Line (Load Dump)
Automotive electronics, such as electronic control units, sensors, and entertainment systems, are connected to one power line. The power sources for these electronics are the battery and alternator, both of which have unstable output voltages that are subject to temperature, operating status, and other conditions. Additionally, ESD,