Secondary Protection of the Automotive Power Line
The primary target of protection circuits in automotive systems is high surge voltages, but the clamped voltage is still high. Secondary protection is especially important in 24- V powertrains, such as found in trucks and vans. The main reason for this is the maximum input voltages for most regulators and dc-to-dc converter ICs for automotive applications are 45 V to 60 V. For this kind of application, using secondary protection, as shown in Figure 9, is recommended.
Figure 9: Secondary protection circuit
Adding resistor R onto the power line reduces the transient current, allowing smaller power-rating TVSs as the secondary protection. Current requirements for microprocessor and logic circuits in electronic units are 150 mA to 300 mA, and the minimum output voltage of a 12-V battery is 7.2 V at - 18 °C, or 14.4 V for a 24-V battery under the same conditions. In a 24-V battery under the above conditions, the supply voltage at a 300-mA load is 8.4 V at R = 20 Ω, and 11.4 V at R = 10 Ω at a minimum voltage of 14.4 V (24-V battery voltage in - 18 °C).
VL = (Vmin ⁄ ( Vmin ⁄ IL)) × (( Vmin ⁄ IL) – R)
VL: Voltage to load
Vmin: Minimum input voltage
IL: Load current
R: Resistor value
Power rating of R = I2R
This supply voltage is higher than the minimum input voltages for most voltage regulators and DC/DC converter ICs.
About the author:
Soo Man (Sweetman) Kim is Senior Application Manager for Vishay Intertechnology, Inc.