SLYT818 November   2021 LM74701-Q1 , LM74721-Q1

 

  1. 1 Introduction
  2. 2 Automotive reverse-battery protection with ideal diode controllers
  3. 3 TVS-less automotive reverse-polarity protection using ideal diode controllers
  4. 4 TVS-less ideal diode: mode of operation and working principle
  5. 5 External MOSFET
  6. 6 Output Capacitor (COUT)
  7. 7 TVS-less ideal diode controller EMC performance
  8. 8 Conclusion
  9. 9 Related Websites
  10. 10Important Notice

2 Automotive reverse-battery protection with ideal diode controllers

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.

Figure 2-1 ISO 7637-2 Pulse 1 characteristics.

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.

  • Size, cost and efficiency over conventional power diode and P-Channel MOSFET solutions benefit at higher current levels
  • Robust system level EMC performance
  • 2 x Input TVS diodes required for transient voltage suppression
    • D1: Mainly to block forward current from D2 in normal operation
    • D2: Mainly to suppress negative transient ISO7637-2 Pulse 1
  • Area of Ideal Diode Controller + N-Channel MOSFET: 24 mm2
  • Area of 2 x input TVS diodes (D1, D2): 35 mm2
  • The TVS occupies ~ 70% of total system solution of ideal Diode
  • The TVS adds additional system cost and BoM count
  • Removing the TVS improves the system reliability due to removal of a shunt component on the VBATT terminal

Figure 2-2 Typical ideal diode application circuit and PCB area comparison.