SLYT818 November 2021 LM74701-Q1 , LM74721-Q1
An automotive battery connects to multiple loads, including electronic control units (ECUs), relays and motors. Several system-level events, such as turning inductive loads on or off, can create voltage transients along battery supply lines. All reverse-polarity protection circuits must protect downstream electronic loads against these system level transient events.
Ideal diode reverse-battery protection typically comprises of an ideal diode controller, N-channel metal-oxide semiconductor field-effect transistor (MOSFET) and an input-side transient voltage suppression (TVS) diode to clamp transient events. This TVS diode consumes as much as 70% of the solution footprint, however, which is a challenge when designing dense ECU designs such as automotive driver assistance system (ADAS) cameras, USB hubs and display ECUs. Removing the input-side TVS saves system cost, reduces solution size and improves system reliability.
This article describes a TVS-less reverse-battery protection system design using an ideal diode controller, analyzing the system architecture for protection and electromagnetic compliance (EMC) in accordance with International Organization for Standardization (ISO) 7637-2 and 16750-2, and original equipment manufacturer (OEM) standards such as VW8000 (LV124) from German auto manufacturers.
An ideal diode controller driving an N-channel MOSFET is a low-loss reverse-polarity protection solution that replaces traditional solutions based on power diodes and P-channel MOSFETs. Apart from providing protection against input polarity reversal, an input protection solution should also safeguard downstream electronic circuits from various system-level transient events. Automotive standards such as ISO 7637-2, VW 80000 (LV124) and ISO 16750-2 define such system-level transient events.
A typical application circuit comprises an ideal diode controller driving an N-channel MOSFET and an input-side TVS diode used to suppress various automotive EMC transients. The main purpose of the input-side TVS diode is to protect against automotive high-energy negative transient generated from the disconnection of supply from inductive loads and described by ISO7637-2 Pulse 1 transient event. As shown in Figure 2-1, voltage transients occur when current through inductive load is interrupted. As per ISO7637-2 standard, this transient event typically lasts for 2 ms (td) with amplitude (US) ranging from –75 V to –150 V. The total duration between two consecutive pulses is 200 ms (t2). There are other low energy, short-duration transient events defined by the ISO 7637-2 standard such as Pulse 2A, 3A, 3B caused by sudden switching and current interruption in the inductance of wiring harness. The input and output capacitors used in ideal diode protection circuit filters these short duration transients and do not impact overall system performance.
Most vehicles have a centralized load-dump clamp that, in the case of 12-V battery-powered vehicles, clamps the maximum transient voltage during a load-dump event to 35 V. But electronic circuits need protection from negative transients that occur when turning off of an inductive load. An input-side TVS diode clamps these transients within the safer limits so that electronic circuits can continue to operate without any damage.
Figure 2-2 shows a typical printed circuit board (PCB) for an ideal diode reverse-polarity protection solution and the contribution of the input TVS diode toward the total solution size. For space-constrained ECU designs such as ADAS cameras, Radar and LIDAR ECUs, and USB hubs, eliminating the input-side TVS diode and simultaneously ensuring robust system-level EMC performance has many advantages. Eliminating the input-side TVS diode also improves overall reliability, because there is no longer a need for a shunt component between the battery and ground line.
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TI’s LM74701-Q1 and LM74721-Q1 ideal diode controller integrates a unique feature to achieve a TVS-less input reverse-polarity protection solution. The device operates an external MOSFET as an active clamp element to dissipate the energy of negative transient event described by ISO 7637-2 Pulse 1. During a ISO 7637-2 Pulse 1 transient event, where there is no output voltage holdup requirement, these devices regulate the voltage across the external FET to a pre-defined threshold by monitoring the voltage drop across the external MOSFET’s drain-to-source (VDS) pins and enabling the gate drive. The reverse current flows from the output capacitor back to the input source, and transient pulse energy dissipates across the MOSFET.
Figure 3-1 shows the ISO 7637-2 Pulse 1 operation differences between a standard ideal diode and a TVS-less ideal diode solution.
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During VDS clamp operation, there is current conduction from the output capacitors back to the input source. This operation is acceptable during an ISO 7637-2 Pulse 1 transient event, where the output voltage holdup requirement is not mandatory given the long brownout duration of 200 ms. However, reverse-current protection is a critical feature needed in system tests, where the supply interruptions are for short durations such as an input micro short (LV124-E10) as well as during AC superimposed testing (LV124-E06). The VDS clamp operation should not engage during these tests, ensuring reverse-current protection.
Figure 4-1 is a flow chart that can help you decide the VDS clamp threshold to ensure EMC performance across various test conditions.
For a TVS-less ideal diode controller, when the input voltage is lower than the output voltage but has not reached the VDS clamp threshold, the ideal diode controller remains in reverse-current blocking mode and the external FET remains off, in order to ensure that the output voltage does not drop much from its nominal value. This mode of operation is preferable in system-level EMC tests with output-voltage holdup requirements such as input micro-short interruptions (LV124-E10, ISO 16750-2). When the voltage difference between the output and input exceeds the VDS clamp threshold, such as in the case of an ISO 7637-2 Pulse 1 transient event, the ideal diode controller enables the gate drive and the external FET operates in the saturation region, acting as a current source. The output capacitor provides reverse energy back to the input source and the output voltage drops from its nominal value. Figure 4-2 shows the equivalent circuit of ideal diode controller TVS-less operation during an ISO 7637-2 Pulse 1 transient event.
Section 5 and Section 6 discusses external component selection considerations.