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.