A transient-voltage-suppression (TVS) diode is an electronic component used to protect sensitive automotive electronics from voltage spikes induced on connected wires. There are two types of TVS diodes: one offers bidirectional clamping, while the other is unidirectional.
Figure 1 shows an application circuit with two unidirectional TVS diodes. TVS+ clamps for positive pulses, as seen in load dump, and TVS- clamps for negative pulses, as seen in International Organization for Standardization (ISO) specifications.
Figure 1: Protection scheme for positive and negative clamping
There are two important specifications to be aware of: breakdown voltage and clamping voltage (see Table 1). Breakdown voltage is the voltage at which the TVS diode goes into avalanche. It’s similar to a Zener diode and is specified at a low-current-value of 1mA typical. Clamping voltage is the voltage that the TVS diode clamps to in high-current-pulse situations.
Table 1: Example of an electrical specification for a unidirectional TVS diode
In the case of ISO 7637-2 pulse 1 (Figure 2), the voltages go to -150V, with a generator impedance of 10Ω. This translates to 15A flowing through the TVS-, making the voltage across the TVS close to its clamping voltage.
Figure 2: ISO-7637-2 pulse 1
A rule of thumb with TVS diode voltage selection is that the breakdown voltage should be higher than the system’s worst-case steady-state voltages. TVS diodes are meant to clamp pulses and are not meant for steady-state voltages.
Select the value of TVS+ such that the breakdown voltage of the TVS is higher than 24V, which is a commonly used battery voltage for jump-starting. The LM74610-Q1 does not have a positive voltage limit, so the selection of the voltage rating of TVS+ is determined by the maximum voltage tolerated by the downstream electronics. If the downstream parts can withstand at least 37V (suppressed load dump), then there is no need to use TVS+. In this case you can replace the TVS+ with a diode, as shown in Figure 3. I recommend a 1A diode with a 30A surge-current rating and at least a 40V reverse-voltage rating. If you would like a positive clamping voltage, I recommend the Vishay SMBJ24A/SMBJ26A for TVS+, as shown in Figure 1.
Figure 3: Protection scheme for negative clamping
Select the value of TVS- to meet two criteria. First, the breakdown voltage of the TVS should be higher than the maximum reverse battery voltage, which is typically 15V. Second, take care not to exceed the absolute maximum rating for the reverse voltage of the LM74610-Q1 (-45V).
In case of reverse-voltage pulses such as in ISO specifications, the LM74610-Q1 turns the N-channel field-effect transistor (NFET) off. When the NFET turns off, the negative voltage seen by the LM74610-Q1 is the clamping voltage of TVS- plus the output capacitor voltage.
For example, if the maximum voltage on the output capacitors is 16V, then the clamping voltage of the TVS- should not exceed 45V - 16V = 29V.
You can use the SMBJ14A/SMBJ15A/SMBJ16A TVS diodes for TVS-. The breakdown voltage of the SMBJ14A is 15.6V; for the SMBJ16A it is 17.8V. This meets your first criterion. The clamping voltage of the SMBJ14A is 23.2V; for the SMBJ16A it is 26V. This meets your second criterion.
I do not recommend bidirectional TVS diodes due to their symmetrical clamping specifications. The SMBJ26CA has a breakdown voltage of 31.9V and a clamping voltage of 42.1V. The breakdown voltage meets the criteria for being higher than 24V. And while the clamping voltage is 42.1V, a high clamping voltage is not an issue for positive pulses. For a negative ISO pulse, however, the absolute maximum of the LM74610-Q1 can be violated. Voltage across anode to cathode in this case is -(42.1V + 16V) = -58.1V, which violates the absolute maximum rating of -45V.
Figure 4: Negative clamping action for an ISO7637 pulse 1 with the SMBJ26CA TVS
Looking at Figure 4, you can see that a SMBJ26CA TVS diode – which has a breakdown voltage specification of 31.9V (max) and a clamping voltage of 42.1V (max) – actually clamps at the breakdown voltage specification. The actual clamping voltage can be dependent on impedances in the test circuit, however, so I advise considering the worst-case value (which is the clamping voltage). This is why I don’t recommend bidirectional TVS diodes with the LM74610-Q1.
Figure 5: Voltage levels for unsuppressed and suppressed load dump
The peak-power-pulse dissipation (PPPM) is the key specification for TVS diodes. You can calculate it in the actual application by multiplying the clamping voltage by the peak impulse current.
In the case of ISO7637-2 pulse 1, the peak impulse current is 15A (-150V with 10Ω internal resistance). When using the SMBJ14A TVS, the clamping voltage is 23.2V. Thus, PPPM is 348W.
In the case of suppressed load dumps with the SMBJ24A TVS, the clamping voltage is 38.9V and the peak impulse current is (40)/4Ω = 10A. Thus, PPPM is 389W.
As far as power levels for TVS diodes, the “B” in SMBJ stands for 600W peak-power levels. This is sufficient for ISO 7637-2 pulses and a suppressed load dump case (ISO-16750-2 pulse B). Unsuppressed load dumps (ISO-16750-2 pulse A) may require higher-power TVS diodes such as the SMCJ (1500W peak-power level) (see Figure 5).
In conclusion, to insure that your design will meet the ISO specification, the bidirectional clamping TVS diode is best-suited for reverse polarity in automotive applications.
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