Figure 4: Typical vehicle power bus
What is Load Dump?
The worst instances of surge voltage are generated when the battery is disconnected when the engine is in operation, and the alternator is supplying current to the power line of the vehicle. This condition is known as “load dump,” and most vehicle manufacturers and industry associations specify a maximum voltage, line impedance, and time duration for this load dump status, as shown in Figure 5.
Figure 5: Output voltage of alternator in load dump condition
Two well-known tests simulate this condition: the U.S.'s ISO-7637-2 Pulse 5 and Japan's JASO A-1 for 14-V powertrains and JASO D-1 for 27-V powertrains. In this section we review the application of TVS for load dump in 14-V powertrains.
Specification and Results of Load Dump Tests
The U.S.'s ISO-7637-2 Pulse 5 and Japan's JASO A-1 tests for 14-V powertrains are simulated in Table 1.
Table 1: Major load dump test conditions for 14-V powertrains. For full resolution, click here .
Figure 6: For ISO-7637-2 test conditions, the standard condition is a VS range of 65 V to 87 V, and Ri (line impedance) range of 0.5 Ω to 4 Ω.
Some vehicle manufacturers apply different conditions for the load dump test based on ISO-7637-2 Pulse 5. The peak clamped current of the load dump TVS will be estimated by the following equation:
Calculation for peak clamping current
IPP = ( Vin – VC) ⁄ Ri
IPP: Peak clamping current
Vin: Input voltage
VC: Clamping voltage
Ri: Line impedance
The current and voltage waveforms of Vishay's SM5S24A in the ISO-7637-2 test of 87V Vs, 13.5V V batt., 0.75 Ohm Ri and 400ms pulse