Designing circuit protection for autonomous driving systems
Ensure robust and reliable circuits for driver and passenger safety
Widespread adoption of autonomous vehicles cannot be achieved unless their electronic circuits are reliable and robust to electrical shocks. Electronics engineers can reduce the risk of critical circuit failures by providing protection from electrostatic discharges (ESD), overcurrents, transient surges and reverse polarity for three mission-critical autonomous vehicle subsystems — the camera, radar and advanced driving assistance systems (ADAS).
Camera subsystem protection
Multiple cameras work together, providing situational awareness for the driver while also converting visual light through a CCD/CMOS image sensor into signals that are sent to the communication/control circuits (Figure 2).
Protect the camera power-supply subsystem from high energy transients, overcurrent and reverse polarity. For overcurrent protection, select either a surface-mountable ceramic fuse with single-shot usage or a polymer-based positive temperature coefficient (PPTC) resettable fuse. The PPTCs do not need replacement if an overcurrent condition occurs. In response to the heat generated by an overcurrent, the PPTC exponentially increases in resistance. Once the overcurrent condition is removed, the PPTC recovers to a low resistance, and the circuit resets.
Power supply circuits also require protection from high energy transients inside the vehicle (energizing/de-energizing of motors) and should withstand transients as defined by ISO Standards 7637 and 16750. A transient voltage suppressor (TVS) diode can safely absorb both low- and high-energy transients as specified in the above-referenced standards.
In the event that the voltage polarity to the power supply is reversed, utilize a Schottky diode in series with the fuse to avoid failure. The diode’s low forward voltage drop has a minimal impact on power supply performance while also providing reverse polarity protection.
The controller area network (CAN) transceiver requires ESD protection. TVS diode arrays have high ESD robustness with numerous options (discrete, two-channel arrays) having 30 kV air and 30 kV contact discharge capabilities. These devices help meet the ISO 10605 standard for ESD in vehicles. An efficient solution is to protect both the high and low lines with a two-channel diode array (Littelfuse AQ24CANA, Figure 3).
The Ethernet transceiver also needs ESD protection. Diode arrays and polymer ESD suppressors can provide the necessary protection for the high-speed differential data lines. These diode arrays can provide up to ±30 kV ESD protection and protect a differential line pair in a single package for PCB space savings. In systems where the capacitance must be the absolute lowest, consider polymer ESD suppressors with 0.04 pF (Littelfuse AXGD, Figure 4). Since the capacitance is this low, it also will not impede 1 Gbit Ethernet transmission rates.
Protect the image sensor and CCD/CMOS imaging module with a bidirectional, low-capacitance protection component (Figure 5). This TVS diode array can withstand ESD strikes of up to ±30 kV, has capacitance around 0.35 pF, as well as extremely low leakage current (typically less than 10 nA). Available in an ultra-small SOD882 package, it provides exceptional space efficiency.
Ensure a reliable visible-light detection system by using the recommended components as close as possible to the four camera subsystem’s inputs. This will keep extraneous energy from damaging critical circuits.
Radar subsystem protection
The radar subsystem provides the input for the important forward and side pedestrian detection and collision avoidance functions (see Figure 6). The circuit has two DC power supplies. The power supply that powers the analog radar transmitter is a low-noise supply. A conventional power supply powers the logic/communication circuits.
The radar subsystem power supplies, like the camera subsystem power supplies, require overcurrent, transient surge, reverse polarity and ESD protection.
One set of circuit protection devices can protect both supplies from overcurrent and reverse polarity. Use either a surface-mounted fuse or a resettable PPTC. Provide reverse polarity protection for the power supplies and all the radar subsystem circuits by utilizing a low forward voltage Schottky diode in series with the input line to both supplies. Remember to provide each power supply with surge protection at the input.
To adequately protect the power supplies from surges, be sure to choose the TVS diode based on its transient power rating (400 W/600 W for low power transients and 1,500 W to 7,000 W for high power transients).
The waveform generator and analog frontend are part of the radar transmitter and receiver, respectively. They are separated from the transmitter and receiver circuits since the addition of protection components on the transmitter output and receiver input blocks could alter their transmission and reception impedance.
For the camera subsystem’s image sensor, use a bidirectional clamping component, like an ESD diode.
The radar subsystem transmits its information to the vehicle’s central processing subsystem. Use bidirectional diode arrays to provide ESD protection for both the CAN bus I/O lines. Also, use either diode arrays or polymer ESD suppressors for the Ethernet transceiver to minimize signal distortion and not impact the transmission rate.
ADAS communication/control subsystem protection
The control, communications and signal processing subsystem must identify other vehicles in traffic, make fast stops due to an obstruction in the vehicle’s path and needs to have a fail-safe response when a sensor fails. While never-fail firmware is critical, the circuit designer should focus on ensuring that the hardware survives any transient energy strikes. All circuits that supply information to the ADAS control subsystem need robust ESD protection (Figure 7).
The ADAS and control power supply require protection from overcurrent, surges and reverse polarity. Design the power supply fuse within the module, or further upstream in the vehicle’s low voltage junction box. Select a TVS diode by its surge power rating to provide the best possible surge transient protection. A Schottky diode in series with the power supply input line provides reverse voltage polarity protection.
Use ESD protection on each communication link. Design for each port’s unique requirements (Table 1). Select from TVS diode arrays and polymer ESD suppressors with unique characteristics to protect each port without compromising the data rate or high-to-low voltage differential.
Signal lines connecting directly to the DSP circuits require ESD protection. Electronics engineers can use TVS diode arrays or polymer ESD suppressors that provide bidirectional protection for the high and low signal lines.
Ensuring that the ADAS subsystem remains operational is mission-critical. Incorporate ESD protection on all subsystem inputs/outputs, using TVS diodes for surge protection against transients generated by electric and electromechanical devices, installing Schottky diodes for reverse polarity protection, and fuses or PPTCs for overcurrent conditions.
A broad range of components are available to protect automotive circuitry from electrical stresses. AEC-Q qualified components can help accelerate compliance with quality and reliability standard requirements while giving the designer confidence that the board-level components will provide circuit protection to ensure reliable performance and long life.
For more information, see the Automotive Electronics Applications Guide, courtesy of Littelfuse.