Diodes AN1197: Power ORing Application Using Ideal Diode Controllers

Introduction

Redundant power supply configurations, known as "Power ORing," are employed to ensure continuous power delivery to a load, thereby enhancing system reliability, availability, and safety. This is particularly critical in automotive applications, such as autonomous driving, where power interruptions can lead to severe consequences.

ORing circuits automatically select the power source with the highest voltage. Power multiplexing enables dynamic switching between sources based on predefined priorities or operational conditions. Traditional implementations have relied on Schottky diodes, P-channel MOSFETs, or hybrid arrangements for source selection and isolation.

Ideal diode controllers, which are integrated circuits designed to control external N-channel MOSFETs, offer a more efficient alternative. These controllers emulate the behavior of ideal diodes, providing benefits such as reduced power losses, higher current handling, reverse polarity protection, and reverse current protection. They also include features like inrush current limiting and load dump protection.

This application note explores the principles and advantages of ORing and power multiplexing using ideal diode controllers. It examines various circuit topologies and addresses key design challenges and mitigation strategies for automotive power systems.

What is Power ORing?

In an ORing configuration, the system automatically selects the input with the highest voltage to supply the load. Ideal diodes function as directional switches, conducting only when the input voltage exceeds the output voltage and blocking current when the input voltage falls below the output. This mechanism prevents reverse current flow and cross-conduction between sources. When input voltages are closely matched, both supplies may share the load without circulating current, making reverse current blocking a key requirement for effective ORing.

Figure 1 illustrates a typical power ORing application using the AP74700Q ideal diode controller from Diodes Incorporated. The circuit shows two input power rails, VIN1 and VIN2, each connected through an N-channel MOSFET (Q1 and Q2 respectively) and input capacitors (CIN) to a common output (VOUT). Each MOSFET is controlled by a Diodes AP74700Q ideal diode controller. The controllers have ANODE, GATE, CATHODE, EN, C_CP, VCP, and GND pins. TVS diodes are included for transient voltage suppression.

Figures 2 and 3 show power supply ORing switch-over performance between two power supply rails (VIN1 = 12V, VIN2 = 15V). These oscilloscope traces illustrate the transition from VIN1 to VIN2 and vice-versa, displaying signals like VIN1, VOUT, VGATE1, and IIN1 over time.

Power ORing Applications

ORing circuits are widely used in automotive subsystems such as infotainment units, body control modules, advanced driver-assistance systems (ADAS), and lighting systems. They enhance power redundancy and system reliability in the event of a power supply failure or disconnection.

For an effective solution, the system must respond rapidly to changes in input conditions to minimize reverse current during a supply fault. Ideal diode controllers continuously monitor the voltage differential between their anode and cathode terminals, corresponding to the input voltage (VIN1, VIN2) and the shared output (VOUT). When the input voltage drops below the output voltage by a small threshold (typically a few millivolts), a high-speed comparator disables the gate drive via a fast pulldown mechanism, typically within microseconds. In addition to fast reverse-current detection, Diodes' ideal diode controllers incorporate linear gate regulation to ensure smooth operation.

Circuit Diagrams and Block Diagrams

Figure 1: Typical Power ORing Application

A typical Power ORing application circuit shows two input power rails, VIN1 and VIN2, each connected through an N-channel MOSFET (Q1 and Q2 respectively) and input capacitors (CIN) to a common output (VOUT). Each MOSFET is controlled by a Diodes AP74700Q ideal diode controller. The controllers have ANODE, GATE, CATHODE, EN, C_CP, VCP, and GND pins. TVS diodes are included for transient voltage suppression.

Figure 4: AP74700Q Reverse Current Block Diagram

The AP74700Q Reverse Current Block Diagram illustrates the internal architecture of the controller. Key functional blocks include 'Full Conduction Mode' (active when the voltage difference across the device, VAC, is greater than 50mV), 'VAC Regulation' (typically maintaining a 20mV drop), a 'Gate Driver' to control the external MOSFET, and a 'Reverse Current Block' (active when VAC is less than -10mV). It shows connections for ANODE, GATE, CATHODE, VEN, VCP, VCP_UV, and GND.

Figure 5: ORing with a Common Load Disconnect

This diagram shows a dual-input ORing configuration with common load disconnect capability. VIN1 is connected via Q1 (controlled by AP74700Q) and VIN2 via Q2 (controlled by AP74800Q) to VOUT. Q3, controlled by the AP74800Q's ON/OFF pins, provides common load disconnect. The AP74800Q monitors VBAT_MON and uses resistors R1, R2, R3 for configuration. It features DGATE, VCP, VIN, CATHODE, HGATE, OUT, VSNS, SW, EN/UVLO, ON OFF, OVLO, and GND pins.

Figure 6: ORing with Load Disconnect Functionality

This diagram presents an alternative approach where load disconnect functionality is applied individually to each input rail. Two AP74800Q controllers are used. The first AP74800Q controls Q1 and Q3 for VIN1, and the second AP74800Q controls Q2 and Q4 for VIN2. Each controller manages its respective MOSFETs and load disconnect function, allowing designers to define separate disconnect criteria for each power source.

How to Select Appropriate Power MOSFETs

Ideal diode controllers, such as ORing MOSFET controllers, require one or two back-to-back N-channel MOSFETs to function effectively. Selecting the appropriate MOSFET is critical to ensure system robustness and reliable operation under all conditions. The following criteria should guide your selection:

1. Voltage Rating: Choose an N-channel MOSFET with a drain-to-source breakdown voltage (VDS) that exceeds the system bus voltage. For instance, in a 48V system, a MOSFET rated for at least 60V is recommended to provide adequate margin.
2. Current Handling Capability: Ensure the MOSFET can support the maximum expected load current. In redundant power systems where one supply may fail, the remaining supply must be able to handle the full load. For example, if the system load is 40A, the selected MOSFET must be rated to carry more than 40A continuously.
3. On-Resistance (RDS(ON)) Considerations: The MOSFET's RDS(ON) directly impacts power dissipation and thermal management. Select a device with a low enough RDS(ON) to:
   • Maintain voltage regulation with sufficient margins.
   • Minimize power losses and heat generation under worst-case conditions.
   • Reduce or eliminate the need for extensive heat sinking or forced airflow.
   This selection may require iterative optimization based on thermal constraints, efficiency goals, and cost trade-offs. In high-power applications, investing in a lower RDS(ON) FET can reduce overall system costs by minimizing cooling requirements. Further considerations on RDS(ON) selection are discussed in the section, "Maximizing Efficiency with FETs in Redundant Power Systems."
4. Gate Drive Compatibility: Ensure the MOSFET performs well with the ideal diode controller's gate drive voltage. For example, the AP74700Q controller provides a gate drive of 15V above the source. Always evaluate the MOSFET's characteristics at this gate-source voltage (VGS(ON) >= 15V).
5. Switching Speed: Since ORing applications involve slow switching transitions, fast switching characteristics and a low gate charge are not critical. Focus instead on conduction performance and thermal behavior.

Maximizing Efficiency with FETs in Redundant Power Systems

To fully realize the advantages of using MOSFETs over diodes in redundant power distribution systems, the design objective should be to minimize power dissipation under worst-case conditions, achieving levels significantly lower than what diodes can offer.

In a typical diode-based system, each diode may dissipate approximately 10W during normal operation. If one diode fails, the remaining diode must conduct the full load (e.g., 40A), resulting in a forward voltage drop of 0.6V and a worst-case power dissipation of 24W.

By contrast, in a MOSFET-based system, the worst-case scenario also involves one device carrying the full load. However, by selecting a MOSFET with a low RDS(ON), power losses can be substantially reduced. For example, a 60V, 100A-rated MOSFET with an RDS(ON) of 4.3mΩ at 125°C can be sourced for under $1 in 1k-unit quantities. Under a 40A load, the power dissipation would be calculated as:

P = I² × RDS_ON = 40² × 0.0043 = 6.88W

This is significantly lower than the diode-based alternative. Under normal conditions, each MOSFET would carry only 20A, resulting in even lower conduction losses due to reduced RDS(ON) at lower temperatures.

To select an appropriate FET:

• Use distributor search tools to filter by key parameters such as breakdown voltage, current rating, and RDS(ON).
• Evaluate cost-performance trade-offs, as lower RDS(ON) devices may reduce or eliminate the need for heat sinks or forced airflow, potentially lowering overall system costs.
• Confirm that the voltage drop across the FET under a full load remains within acceptable limits. For instance, with 40A through a 4.3mΩ device, the voltage drop is:

V = I × RDS_ON = 40 × 0.0043 = 0.172V

This results in a voltage deviation of only 0.3% from a 48V rail, well within typical regulation tolerances.

Finally, use this power dissipation data to inform thermal design decisions, considering the cumulative heat load from all system components and any planned cooling strategies.

Important Notice

Diodes Incorporated (Diodes) and its subsidiaries make no warranty of any kind, express or implied, with regards to any information contained in this document. The information provided is for informational purposes only and is intended for skilled and technically trained engineering customers and users.

Diodes assumes no liability for any application-related information, support, assistance, or feedback provided. Customers and users are responsible for selecting appropriate Diodes products, evaluating their suitability, and ensuring their applications comply with applicable legal and regulatory requirements, as well as safety and functional-safety standards.

Products described herein may be covered by patents and trademarks. Diodes does not convey any license under its intellectual property rights or the rights of any third parties.

Diodes' products are provided subject to Diodes' Standard Terms and Conditions of Sale, available at https://www.diodes.com/about/company/terms-and-conditions/terms-and-conditions-of-sales/. This document does not alter or expand applicable warranties.

Diodes' products and technology may not be used for or incorporated into any products or systems whose manufacture, use, or sale is prohibited under any applicable laws and regulations. Customers and users will be solely responsible for any damages, losses, or penalties arising from noncompliance.

While efforts have been made to ensure accuracy, this document may contain technical inaccuracies, omissions, or typographical errors. Diodes reserves the right to make changes without further notice. The English version of this document is the final and determinative format.

Any unauthorized copying, modification, distribution, transmission, display, or other use of this document is prohibited. Diodes assumes no responsibility for any losses incurred by customers or users arising from such unauthorized use.

This Notice may be periodically updated with the most recent version available at https://www.diodes.com/about/company/terms-and-conditions/important-notice.

The Diodes logo is a registered trademark of Diodes Incorporated. All other trademarks are the property of their respective owners. © 2025 Diodes Incorporated. All Rights Reserved.