Redundant power supplies use more than one power-supply unit to provide the necessary power for a load. They help increase a system’s reliability and availability, and ensure system safety in case one of the power-supply units fails. Redundant power supplies are especially important in automotive systems for safety-critical applications, such as automated driving, where a loss of power could result in serious consequences.

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

Ideal diode controllers are integrated circuits (ICs) that can control external metal-oxide semiconductor field-effect transistors (MOSFETs) to emulate the behavior of ideal diodes. They offer several advantages over conventional 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 and overvoltage and overcurrent protection.

In this article, we will discuss the concept and benefits of ORing and power multiplexing using ideal diode controllers, 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.

Both ORing and power multiplexing techniques use ideal diodes to connect multiple input power sources 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 supply ORing and priority multiplexing.

An ORing circuit facilitates system selection of the best available power source from multiple inputs, based on the highest input voltage. The ideal diodes act as switches that turn on when the input voltage is higher than the output voltage, and turn off when the input voltage is lower than the output voltage. This way, the ORing circuit ensures that the input source with the highest voltage is connected to the output, and prevents reverse current flow and cross conduction between the input sources. If the input power supplies are almost equal, it is possible that both power supplies share the load without any circulating current between them. Thus, reverse current blocking is the primary feature required for realizing an ORing circuit.

A power multiplexing circuit allows the system to switch between different power sources irrespective of the voltage magnitude, based on criteria such as source priority or input voltage availability and magnitude. In this configuration, the control circuit needs to switch power paths between each power supply and load on and off, controlled by its own priority logic or an external signal, such as a microcontroller general-purpose input/output pin. The power multiplexing circuit ensures that only one input source is connected to the output at any point in time, and prevents reverse current flow and cross conduction between the input sources. The circuit in this configuration is therefore required to have both reverse current blocking and load path on and off control features to enable the prioritized power supply to serve the load.

ORing circuits are popular in automotive subsystems such as infotainment, body control modules, advanced driver assistance systems and lighting modules; they provide redundancy and reliability in case of a power-supply failure or disconnection. Figure 2 shows different ORing topologies using ideal diode controller ICs combined with external N-channel MOSFETs.

An effective ORing solution needs to be extremely fast in order to limit the duration and amount of reverse current in case one of the supplies fails. The ideal diode controllers in an ORing configuration constantly sense the voltage difference between the anode and cathode pins, which are the voltage levels at the power sources (V_IN1, V_IN2) and the common-load (V_OUT) point, respectively. A fast comparator shuts down the gate drive through a fast pulldown – within microseconds as soon as V_IN – V_OUT falls below a designated reverse threshold, typically a few millivolts. Along with a fast reverse-current detection comparator, TI ideal diode controllers have a linear gate regulation scheme that ensures zero DC reverse current in the event of an input supply loss.

Few subsystems require disconnecting the load from the power supplies to achieve low quiescent current or to protect the system from fault conditions. Topology No. 2 in Figure 2 shows a typical application circuit for a dual-supply input ORing with a common load disconnect control using TI’s LM7480-Q1 and LM7470-Q1 devices. FET Q1 and Q2, driven by the LM7470-Q1 and LM7480-Q1, respectively, provide ORing functionality, whereas the Q3 FET driven by the LM7480-Q1 can isolate the load from power supplies. When V_IN1 is greater than V_IN2, the independent control of FETs by the LM7480-Q1 allows Q2 to block reverse current, while Q3 remains on, connecting V_IN1 to V_OUT.

Topology No. 3 in Figure 2 shows a typical application circuit for ORing with load disconnect functionality for individual rails, thus allowing system designers to assign different load disconnect criteria for each rail.

Figure 3 and Figure 4 shows power-supply ORing switchover performance between two power-supply rails where V_IN1 = 12V and V_IN2 = 15V.