Analog Core Insight | Redundant Power Supply Topologies for Automotive Applications Using Ideal Diode Controllers

introduction

Redundant power supplies use multiple power supply units to provide the required power to the load. They help improve system reliability and availability and ensure system safety in the event of a failure in one of the power supply units. In automotive systems, redundant power supplies are particularly important for safety-critical applications such as autonomous driving, where power outages can have serious consequences.

ORing and priority power multiplexing are two common techniques for implementing redundant power supplies in automotive systems. In ORing, the system selects the highest voltage supply from multiple inputs, while power multiplexing allows the system to switch between different power supplies based on priority or other criteria. Designers have traditionally used Schottky diodes and/or P-channel field-effect transistors for redundant circuits in power supplies.

Ideal diode controllers are integrated circuits (ICs) that can control external metal oxide semiconductor field effect transistors (MOSFETs) to emulate the behavior of an ideal diode. They offer several advantages over traditional diodes, such as lower power dissipation, higher current capability, reverse polarity protection, reverse current blocking, and load dump protection. Ideal diode controllers can also provide inrush current limiting as well as overvoltage and overcurrent protection.

In this article, we will discuss the concepts and benefits of using ideal diode controllers for ORing and power multiplexing, the different types and architectures of ORing and power multiplexing circuits, and the challenges and solutions for implementing ORing and power multiplexing using ideal diode controllers in automotive systems.

ORing and Power Multiplexing

Both ORing and power multiplexing techniques use ideal diodes to connect multiple input power supplies to a single output load, but they differ in how they select and switch between different input sources . Figure 1 shows a typical use case for power ORing and priority multiplexing.

The ORing circuit helps the system select the best available power source from multiple inputs based on the highest input voltage. The ideal diode acts as a switch, turning on when the input voltage is higher than the output voltage and turning off when the input voltage is lower than the output voltage. In this way, the ORing circuit ensures that the input source with the highest voltage is connected to the output and prevents reverse current and transconductance between the input sources. If the two input supplies are almost equal, the load can be powered by both supplies at the same time without any circulating current between the two supplies. Therefore, reverse current blocking is the main feature required to implement an ORing circuit.

Power multiplexing circuits allow the system to switch between different power supplies based on criteria such as power priority or input voltage availability and magnitude, regardless of voltage magnitude. In this configuration, the control circuit needs to switch the power path between each power supply and the load, specifically controlled by its own priority logic or external signals (such as microcontroller general-purpose input/output pins). The power multiplexing circuit ensures that only one input source is connected to the output at any one time and prevents reverse current and cross-conductance between input sources. Therefore, in this configuration, the circuit needs to have reverse current blocking and load path switching control functions to enable the priority power supply to power the load.

Typical application circuit of power ORing

ORing circuits are widely used in automotive subsystems such as infotainment, body control modules, advanced driver assistance systems, and lighting modules; they provide redundancy and reliability when the power supply fails or is disconnected. Figure 2 shows different ORing topologies using an ideal diode controller IC and an external N-channel MOSFET.

An effective ORing solution needs to operate extremely fast to limit the duration and magnitude of the reverse current when one of the power supplies fails. An ideal diode controller in an ORing configuration constantly senses the voltage difference between the anode and cathode pins, which are the voltage levels at the power supplies (V _IN1 , V _IN2 ) and the common load (V _{OUT ) points, respectively. As soon as V IN} – V _OUT drops below a specified reverse threshold (usually a few millivolts), a fast comparator turns the gate driver off within milliseconds via a fast pull-down resistor. Texas Instruments Ideal diode controllers feature a fast reverse current sensing comparator and a linear gate regulation scheme to ensure zero DC reverse current in the event of input power loss.

Figure 2: Typical ORing topology using an ideal diode controller

A few subsystems require the load to be disconnected from the power supply to achieve low quiescent current or to protect the system from fault conditions. Topology 2 in Figure 2 shows a typical dual-supply input ORing application circuit with universal load disconnect control using Texas Instruments LM7480-Q1 and LM7470-Q1 devices. FETs Q1 and Q2 are driven by the LM7470-Q1 and LM7480-Q1, respectively, to provide the ORing function, while the Q3 FET driven by the LM7480-Q1 isolates the load from the power supply. When V _IN1 is greater than V _IN2 , the independent control of the FETs by the LM7480-Q1 allows Q2 to block reverse current while Q3 remains on, connecting V _IN1 to V _OUT . Topology 3 in Figure 2 shows a typical application circuit for ORing with load disconnect for each voltage rail, allowing the system designer to specify different load disconnect criteria for each voltage rail.

Figure 3 and Figure 4 show the power ORing switching performance between two power rails when V _IN1 = 12V and V _{IN2 = 15V.}

Figure 3: Power switching from V _IN1 to V _IN2

Priority Power Multiplexer Configuration

The priority power multiplexer automatically switches the main power source to the auxiliary (AUX) or secondary power source when the main power voltage drops below a specified threshold. The main power source is always the preferred source to power the load if it is available and within acceptable limits. For example, if an upstream smart fuse in the power distribution unit trips on the main power source of a subsystem, the priority power multiplexer circuit automatically connects the AUX power source to the output and disconnects the main power source from the output to avoid any interruption in the operation of the subsystem. If the upstream smart fuse resets and the main power voltage rises above an acceptable threshold, the priority power multiplexer circuit automatically connects the main power source back to the output and disconnects the AUX power source.

The power multiplexer circuit requires a controller such as the LM74800-Q1 or LM74900-Q1 to control two back-to-back MOSFETs on each power rail. When both the main power supply and the AUX power supply are present and within acceptable ranges, and the main power supply is powering the load, the AUX path controller must block reverse current when the main power supply voltage is higher than the AUX power supply voltage. Similarly, the AUX path controller must block forward current when the main power supply voltage is lower than the AUX supply. This ensures that the main power supply with the highest priority powers the load, while the AUX power supply is isolated from the main power supply and the load.

The LM74900-Q1 ideal diode controller drives and controls external back-to-back N-channel MOSFETs to emulate an ideal diode rectifier with power path switch control and overcurrent and overvoltage protection. Figure 5 is a schematic of a priority power multiplexer using two LM74900-Q1 devices in a common drain topology . The overvoltage pin of the LM74900-Q1 in the VAUX path is configured so that if VPR _IM is disconnected for any reason, the _VAUX supply is immediately connected to the load and ensures continuous power to the load.

Figure 5: Typical priority power multiplexer application circuit using the LM74900-Q1

The purpose of the power multiplexer circuit is to switch the load to be powered by VAUX while keeping the output voltage low when VPRIM is cut off or out of acceptable range. To keep the output voltage low during the transition, the load switch FET (Q4) ( driven by the LM74900-Q1 in the V _{AUX path) must turn on very quickly when the power path to V PRIM} is off (by turning off Q2). However, the HGATE pin is designed to provide only 55μA gate current for slow startup to provide inrush current limiting, which is too low to quickly turn HGATE high. A small circuit consisting of a resistor (RCP), a transistor (Q5), and a diode (D2) can increase the HGATE source current. The gate source current can also be increased by connecting the emitter of Q5 to the gate of Q4, because Q5 allows the charge pump capacitor to directly pull HGATE high. Alternatively, you can adjust the Q4 gate source current by changing the resistance value of R _CP . D2 provides a path around Q5 to turn off Q4.

Figure 6 shows the waveforms captured when V _PRIM is disconnected and the load is quickly switched to the V _AUX rail. The HGATE of the AUX rail is turned on within 20µs to reduce the output voltage drop.

_{Figure 6: V PRIM} to V _AUX switching in a power multiplexer application

Conclusion

Ideal diode controllers with advanced functions support different architectures of ORing and power multiplexing circuits. Ideal diode controllers have features and benefits such as reverse polarity protection, reverse current blocking, load dump protection, active rectification, overvoltage protection, and inrush current limiting, thus achieving comprehensive input power path protection and helping to ensure system reliability and safety.