Figure 7A: Clamped voltage and current of SM5S24A in ISO 7637-2 test
In Figure 7B the clamped voltage and current of load dump TVS fail in the ISO-7637-2 test of 87V Vs, 13.5V V batt., 0.5 Ohm Ri and 400ms pulse width. The clamping voltage drops to near zero, and the current passed through the device is increased to the maximum allowed by the line impedance.
Figure 7B: Clamped voltage and current of load dump TVS failures in ISO7637-2 test.
Maximum clamping capability of Vishay load dump TVS of ISO-7637-2 pulse 5 test condition with 13.5V Vbatt and 400ms pulse width is as Figure 7C.
Figure 7C: Maximum clamping capability of Vishay load dump TVS in ISO7637-2 test.
Two Groups of Load Dump TVS
There are two kinds of load dump TVS for the primary protection of automotive electronics: EPI PAR TVS and Non-EPI PAR TVS. Both product groups have similar operating breakdown characteristics in reverse bias mode. The difference is that EPI-PAR TVSs have low forward voltage drop (VF) characteristics in forward mode, and non-EPI PAR TVSs have relatively high VF under the same conditions. This characteristic is important to the reverse voltage supplied to the power line. Most CMOS ICs and LSIs have very poor reverse voltage capabilities.
The gates of MOSFETs are also weak in reverse voltage, at - 1 V or lower. In the reversed power input mode, the voltage of the power line is the same as the voltage of the TVS VF. This reverse bias mode causes electronic circuit failure. The low forward voltage drop of EPI PAR TVSs is a good solution to this problem. Another method to protect circuits from reversed power input is utilizing a polarity protection rectifier into the power line, as shown in Figure 8. A polarity protection rectifier should have sufficient forward current ratings, and forward surge and reverse voltage capabilities.