TVS diodes are used in automotive systems for protection against transients. In the 12-V battery protection application circuit shown in Figure 9-2, a bi-directional TVS diode is used to protect from positive and negative transient voltages that occur during normal operation of the car. These transient voltage levels and pulses are specified in ISO 7637-2 and ISO 16750-2 standards.

Two important specifications are breakdown voltage and clamping voltage of the TVS. Breakdown voltage is the voltage at which the TVS diode goes into avalanche, similar to a Zener diode, and is specified at a low current value (typical 1 mA). The breakdown voltage must be higher than worst-case steady state voltages seen in the system. The breakdown voltage of the TVS+ must be higher than 24-V jump start voltage and 35-V suppressed load dump voltage and less than the maximum ratings of LM74703-Q1 and LM74704-Q1 (65 V). The breakdown voltage of TVS- must be beyond the maximum reverse battery voltage –16 V, so that the TVS- is not damaged due to long time exposure to reverse connected battery.

Clamping voltage is the voltage the TVS diode clamps in high-current pulse situations. This voltage is much higher than the breakdown voltage. TVS diodes are designed to clamp transient pulses and must not interfere with steady-state operation. In the case of an ISO 7637-2 pulse 1, the input voltage goes up to –150 V with a generator impedance of 10 Ω. This behavior translates to 15 A flowing through the TVS- and the voltage across the TVS is close to the clamping voltage.

Figure 9-2 Typical 12-V Battery Protection With Single Bidirectional TVS

The next criterion is that the absolute maximum rating of anode-to-cathode reverse voltage of the LM74703-Q1 and LM74704-Q1 (–75 V) and the maximum V_DS rating MOSFET must not be exceeded. In the design example, 60-V rated MOSFET is selected and the maximum limit on the cathode to anode voltage is 60 V.

In case of an ISO 7637-2 pulse 1, the anode of LM74703-Q1, LM74704-Q1 is pulled down by the ISO pulse and clamped by TVS-. The MOSFET is turned off quickly to prevent reverse current from discharging the bulk output capacitors. When the MOSFET turns off, the cathode to anode voltage seen is equal to (TVS Clamping voltage + Output capacitor voltage). If the maximum voltage on output capacitor is 16 V (maximum battery voltage), then the clamping voltage of the TVS- must not exceed (60 V – 16) V = –44 V.

The SMBJ33CA TVS diode can be used for 12-V battery protection applications. The breakdown voltage of 36.7 V meets the jump start, load dump requirements on the positive side and 16-V reverse battery connection on the negative side. During an ISO 7637-2 pulse 1 test, the SMBJ33CA clamps at –42 V with 15 A of peak surge current as shown in Figure 9-5 and meets the clamping voltage ≤ 44 V.

The SMBJ series of TVS diodes are rated up to 600-W peak pulse power levels. This rating is sufficient for ISO 7637-2 pulses and suppressed load dump (ISO-16750-2 pulse B).