POWER SUPPLY Explaining the importance of power rails

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Power rails are “invisible heroes” of semiconductor chips and PCBs that integrate power electronics and digital systems. Contrary to the name, power rails are not the type of power supply found on railway tracks. They ensure the continuous functioning of electronic devices and computers. The article describes power rails and their significance in the industry.

A power rail is a type of conductor that carries a constant power supply throughout the circuit. Just a few mm or cm wide, power rails are needed to deliver power across all components, whether active or passive, on a PCB, power management IC, processor, or system. Power rails are also known as voltage rails or DC power rails.

Power rails are physically present in PCBs and semiconductor chips like power management integrated circuits, microprocessors, and microcontrollers. Sometimes, power rails can be invisible but measured through copper traces, wires, power planes, via arrays, rings, grids, meshes, metal layers, and bus bars. They are also available in schematic and other digital files of computer-aided design (CAD) and electronic design automation software (EDA).

Types of power rails

Engineers are habitual to using 5V, 3.3V, and 12V supply voltage. Power rail is Vcc in bipolar transistors and Vdd in field-effect transistors. These values are some examples of power rails as follows:

• 5 V power rail: Logic circuits

• 3.3 V power rail: Modern digital electronic systems

• 12 V power rail: Power amplifiers and audio amplifiers

• 1.2 V, 1.8 V, and 2.5 V power rails: Low-power ICs, microcontrollers, and microprocessors used in computer CPUs.

BASIC KNOWLEDGE

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Can power rails be negative?

Yes, power rails can be positive, negative, or zero. Negative power rails such as -5 V and -12 V are used in op-amps and analog electronics. Negative power rails are called Vee in bipolar transistors and Vss in field-effect transistors. The 0 V power rail is the reference point called the ground rail. A single system can use multiple types of power rails, whether positive, negative, or ground.

Power rail features

Just like arteries and veins supply oxygen-rich blood to our body parts, power rails supply energy (voltage) to keep every component alive in a system, board, or chip. Power rails are heavily used in digital applications. Some of the power rail features are listed below.

Power rails do not carry information

Power rails supply power to each component in a system, PCB, or chip. It is critical to differentiate between power rails and signal lines. A signal line carries information, but a power rail just supplies power to neighboring components. The main function of power rails is voltage supply— they do not have to do anything with information transfer.

Multi-power rails

Each system can use multiple power rails based on device count and application. Power management integrated circuits (PMICs) implement advanced power functions dependent upon power rails. Designers can modify the power rail structure in PCBs and processors, offering a combination of high and low voltages. Different components on a chip can access a 5 V or a 3.3 V power rail alike.

Power rails are a part of the power tree

A power tree is a hierarchical power supply network present in each device, PCB, or IC. It showcases how power is distributed from the main source to the connected devices and the load. The power rail is the main input source at the top of the power tree. Each branch is divided into new rails. Hybrid systems with PCBs, power ICs, processors, and analog devices implement multi-tree power rails.

Power rails are only DC

In the first stage, power rails convert the supply AC to DC. The entire power rail operation runs on DC-DC conversion. At each branch, the converter or a regulator performs power-related operations to define a new power rail. Power AC rails also exist, but are rarely used. They can be applicable in systems that rely only on AC for operation. Power rails convert DC to AC for components that need AC power.

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Power rail collapse leads to system failure

Transients, surges, and faulty conditions can facilitate power rail failure. A power rail can also fail if it is overloaded or cannot supply enough power to all the connected components. Once a power rail collapses, the entire system can be turned off. Effective output stage design and power rail supervision methods, such as reset circuits, advanced failure detection, and battery backup, can combat power rail collapse.

Processors have shifted to low-power rails

Earlier, large-sized microprocessors would use 48 V, 24 V, or 12 V power rails. Ongoing miniaturization trends, low power usage, and the race to achieve lower losses gave rise to lower voltage usage. Modern microprocessors have shifted from 2.5 V and 3.3 power rails to 1.2 V, 1.5 V, and even as low as 1 V or 0.8 V power rails.

Power rails should exhibit low impedance

Ripples, EMI, noise, crosstalk, signal reflections, ground bounce, and various other undesirable factors can interfere with power rails. When a single power rail feed experiences a voltage drop, it can disrupt the operation of all connected components. Ideally, power rails should exhibit zero impedance. Practically, it must have the lowest impedance on the board or in the system.

References

• https://www.sciencedirect.com/topics/engineering/power-rail

• https://resources.altium.com/p/why-power-rails-need-be-low-impedance

• https://www.intel.com/content/www/us/en/docs/programmable/683564/current/power-rails.html

• https://www.powerelectronictips.com/dc-power-rails-part-1-basics-and-designations-faq/

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