How to Design Automotive Transient and Overcurrent Protection Filters

Somewhere in the world today, there are automotive engineers starting to imagine a new car infotainment system that won’t come to fruition for the next five years or more. This is because infotainment system applications have many power requirements and are currently only in the concept stage. As infotainment systems have increasingly complex Electronic functions, they require an increasing number of integrated circuits (ICs) that all share power from the 12V battery.

Somewhere in the world today, there are automotive engineers starting to imagine a new car infotainment system that won’t come to fruition for the next five years or more. This is because infotainment system applications have many power requirements and are currently only in the concept stage. As infotainment systems have increasingly complex electronic functions, they require an increasing number of integrated circuits (ICs) that all share power from the 12V battery.

Power conditioning and protection features need to be incorporated into the design of the power architecture to ensure that the system operates well under various transient events.

In this article, I will describe several typical transients to be aware of and how TI can help meet transient protection needs.

Typical transient

Transients can occur in four common scenarios.

Figure 1 shows the first scenario, a load dump event caused by battery disconnection while the alternator is charging the battery. Load dump will cause the voltage to rise; the centralized clamp circuit of the alternator will see a maximum voltage of 35V.

How to Design Automotive Transient and Overcurrent Protection Filters
Figure 1: Load dump curve for a 12V system

Figure 2 shows the second scenario, that is, when the power supply is disconnected, a large negative voltage peak is generated in the module connected in parallel with the inductive load (such as the International Organization for Standardization). [ISO]7637-2 test pulse 1).

How to Design Automotive Transient and Overcurrent Protection Filters
Figure 2: ISO 7637-2 test pulse

Figure 3 shows the third scenario, where the bulk capacitance of the system causes an inrush current during startup, which induces a larger current as the capacitor charges.

How to Design Automotive Transient and Overcurrent Protection Filters
Figure 3: Inrush current curve during startup (with large capacitive load)

The fourth scenario is when the battery voltage drops. Figure 4 shows a cold start, where the engine is started at a low ambient temperature.

How to Design Automotive Transient and Overcurrent Protection Filters
Figure 4: Typical cold crank waveform

Transient protection

One way to provide transient protection is to use an ideal diode controller. As shown in Figure 5, additional overcurrent protection can be achieved using a current sense amplifier and an ideal diode controller, providing a comprehensive protection solution over filtering and power regulation.

How to Design Automotive Transient and Overcurrent Protection Filters
Figure 5: Block diagram of automotive transient and overcurrent protection filter protection

Load dump protection

The LM74810-Q1 utilizes an overvoltage adjustable feature to provide protection from unwanted load dump events. As shown in Figure 6, the OV pin of the LM74810-Q1 can use a comparator to signal an overvoltage event. This turns off the HGATE voltage used to drive the Q2 metal-oxide-semiconductor field-effect transistor (MOSFET). Downstream components with lower voltage ratings do not have the voltage range required for transients at the input, and can be used by adjusting the resistor divider connected to the OV pin to your desired threshold. The LM74810-Q1 device is rated for a maximum input voltage of 65V and can continue to operate during transient events with a peak voltage of 35V.

How to Design Automotive Transient and Overcurrent Protection Filters
Figure 6: Typical block diagram of LM74800 with overvoltage protection

Negative Voltage Transient Protection

The LM74810-Q1, along with the appropriate MOSFETs and input transient voltage suppressor (TVS), protects the system from high negative voltage transients such as ISO 7637-2 test pulse 1. If the input voltage is negative, the LM74810-Q1 turns off and pulls DGATE low. Then, the body diode of Q1 in Figure 6 provides reverse voltage protection for the system and prevents negative current flow. Once the input voltage returns to nominal, the LM74810-Q1 turns back on and allows the MOSFET to operate normally.

The TVS diode protects the LM74810-Q1 when ISO 7637-2 test pulse 1 (typically 100V or more) produces large negative voltage spikes. The breakdown voltage of the input TVS should be between the 35V load dump and the 65V maximum voltage rating of the LM74810-Q1. In the event of a negative voltage, the TVS diode breakdown voltage should be higher than when the battery is connected in reverse, but also low enough that the negative clamping voltage of the TVS does not exceed the voltage across the Q1 MOSFET.

Inrush current limit

The LM74810-Q1 has an inrush current limit function that controls the amount of current during startup. Capacitors at the output limit the current, ensuring that no current beyond the safe operating range can flow through the component.

As shown in Figure 7, adding a resistor capacitor (RC) to the HGATE pin of the LM74810-Q1 slows down the HGATE voltage ramp during start-up, enabling inrush current limiting.

How to Design Automotive Transient and Overcurrent Protection Filters
Figure 7: Inrush current limiting provided by LM7480x-Q1

How to Design Automotive Transient and Overcurrent Protection Filters

Overcurrent Protection

The INA302-Q1 provides overcurrent detection via two independently adjustable threshold comparator outputs. Connecting the active-low comparator output to the enable pin of the LM74810-Q1 enables the MOSFET to shut down in the event of overcurrent. The ALERT2 comparator provides flexibility to adjust the delay of the output signal, which is useful when there is a small increase in current during normal operation, but the overcurrent protection function does not have to be triggered. You can adjust the delay duration by changing the capacitor value on the DELAY pin of the device, or the current threshold for overcurrent events through the ILIM pin of the INA302-Q1; see R5 in Figure 8.

How to Design Automotive Transient and Overcurrent Protection Filters
Figure 8: Implementing Adjustable Overvoltage and Overcurrent Protection

Low Voltage Transient Protection

Cold crank and warm crank events can cause low voltage transients in the system. In this type of event, the input voltage is lower than the output voltage, resulting in negative current flow. Negative currents are a cause for concern for systems that need to maintain normal operation. Since the output voltage drops in the presence of output capacitance, reverse current blocking is required to ensure that current does not flow back into the battery.

Since the LM74810-Q1 continuously monitors the voltage drop between the A and C pins of the Q1 MOSFET, it provides protection against low voltage transients. As shown in Figures 9 and 10, during normal operation, the voltage across Q1 is positive and current flows into the load. In scenarios where the input voltage is lower than the output voltage and may generate reverse current, the LM74810-Q1 will respond quickly when the voltage across Q1 reaches C4.5mV and turn off the MOSFET to prevent DC reverse current.

How to Design Automotive Transient and Overcurrent Protection Filters

How to Design Automotive Transient and Overcurrent Protection Filters
Figure 10: When the voltage drop across the MOSFET reaches C4.5mV, DGATE driving the Q1 logic gate will pull low, providing reverse current blocking

Flexibility in harsh automotive environments

By providing advanced system protection at the input, designers can increase design flexibility. These protection devices not only do not interfere with the normal operation of the system, but also provide protection in the harsh electrical environment in the car, and more importantly, through more options, they can also drive innovation in the rest of the automotive system.

In addition, a compact two-device protection system similar to those described above can significantly reduce the overall solution size compared to discrete solutions. The smaller solution size provides more space for other parts of the infotainment system, enabling more innovation.

We hope this design not only protects your system, but also helps you quickly upgrade your design.

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