Redundant power supplies the key to hot swap

Redundant power supplies the key to hot swap
Ever since hot swapping was first used in telecom switching equipment some 40 years ago, the technique has continued to evolve with the times. Hot swapping, or the ability to remove and insert printed circuit boards, power supplies, subassemblies and other types of modules into electronic systems without powering down the system, is an imperative for those indispensable, high-availability systems such as telecom switches, bank servers, cell phone base stations and others. These applications and many others are subject to constant use. As a result, they must never shut down, even when they are being reconfigured or updated with a new printed circuit board or module.

Any failure of a printed circuit board or power supply should not bring down a high-availability system. That is, the system must continue to function even if performance is diminished. Redundant power supplies, are among the essential capabilities for achieving this objective. If one supply were to fail, the system must automatically switch to a second supply in order to continue operating.

Diode OR-ing technique is commonly used to accomplish this automatic switch-over. OR-ing involves placing a diode in series with each power supply output and tying either the cathodes or anodes (depending on whether the power supply is positive or negative) together at the load. This architecture ensures that the system has at least one backup supply and that neither supply appears as a load to the other. Unfortunately, OR-ing has its drawbacks, including significant power losses caused by the inefficiencies of the circuitry and the absence of any protection against inrush current or overcurrent when modules in the system are hot swapped. Recently, hot swapping controller devices have begun replacing OR-ing techniques because these controllers are much more power efficient and they offer undervoltage and overvoltage limits, a current limit, a pre-set current slew rate, circuit breaking and a fault timer to prevent nuisance trips.

In addition to being able to hot swap among two or more power supplies, additional levels of hot swapping are now built into high-availability systems. For example, some high-availability systems have mirrored circuit boards so that if one fails, the other can continue running. In some cases, these redundant system components do not stand idle, waiting for a failure to happen before they are pressed into service. The system's operating load is often shared among the redundant modules during the course of normal operations. Then, if one module fails, the load is shifted to the remaining, functioning modules. Performance may diminish but the system continues to operate until the failed module can be hot swapped out and replaced by a new, fully functional module.

Evolution

When hot swapping was first deployed four decades ago in the telecom industry, the typical practice was to distribute a negative high voltage of -48V because engineers felt that they had to guard against the potential for ionic corrosion that is associated with positive power when it is exposed to the elements. The hot swapping practices of that time eventually were codified into industry accepted standards.

Negative distribution of high voltage power persists to this day in applications such as optical networking and base stations in the cellular telephony infrastructure. In fact, the early hot swapping standards that used negative high voltages have contributed to recently developed standards that are transitioning to positive high voltage power. These new standards include Power over Ethernet (PoE), the PCI standard and others. The PoE standard uses +48V power, while PCI dictates +12V. In addition, several different types of networking equipment distribute power at voltage levels anywhere from +24V to +42V power.

Various hot swapping techniques like redundant power supplies with a diode OR-ing architecture or a hot swapping controller, the mirroring of system modules, load sharing, and others are transitioning to the mainstream world of office electronics where positive high voltage power is the norm.

In fact, the demand for hot swapping in systems with positive high voltage power is growing steadily, as evidenced by its use in storage area networks (SANs), high-availability servers based on the PCI bus and other types of buses, networking equipment with PoE, and Internet switching systems.

For most of these applications, the type of power distributed within the system at the level of the ICs within the system is dictated by the presence of microprocessors, ASICs, complex programmable logic and other kinds of advanced ICs. These devices require power with a low voltage and high current. Unfortunately, low voltage, high current power is difficult to distribute effectively over the small conductor traces of contemporary circuit boards, motherboards and backplanes. As a result, high voltage, low current power must be distributed within the system because it can be bused on smaller conductors with higher resistance. To accommodate the low voltage, high current needs of advanced ICs, various point-of-source power conversion techniques, such as DC/DC conversion, voltage bucking and fly backs, are implemented to convert the power to the voltage and current levels needed by the devices on the board.

Mid-plane hot swapping

Over the last several years, a new method of power distribution has emerged and it has revived many of the layering concepts that were originally developed for hot swapping in telecom switching systems and optical networking equipment. Known as mid-plane power distribution, this concept will continue to increase in popularity for the foreseeable future because it has several economic benefits and it blends well with the layered architecture of high-availability, hot swapping systems.

Mid-plane power distribution takes positive or negative power at a high voltage of around 40V or more and makes use of a layered power architecture to distribute power at the voltage and current level needed throughout the system. At each layer in a mid-plane power distribution architecture, hot swapping techniques can ensure a high degree of redundancy within the system and high availability of the entire system.

In a mid-plane architecture, the 40V supply is distributed via inexpensive 12V supplies at a secondary level within the system. This level could be comprised of a single board or multiple boards. At this layer, the 12V supplies do not have to be particularly accurate because the third layer, or the point-of-source level, utilizes various voltage conversion techniques like sequencing, DC/DC conversion, bucking and fly backs, which further refine the accuracy of the power to the requirements of the ICs at the device level in the system. On complex boards, for example, five or six, or even as many as seven or eight power supplies may be needed to provide the various voltages required by the processors, microcontrollers and programmable logic devices on the board.

This type of mid-plane architecture has distinct advantages because a high voltage power source can be fanned out through inexpensive power supplies at the various layers in the architecture. Then, at each of these layers, hot swapping techniques also can be deployed to ensure a very high level of system availability. As electronic computing and communications systems become more and more prevalent, society becomes more dependent upon them. When a pervasive system fails, it is not merely a question of convenience. Rather, the failure of an electronic system that has become integral to a societys functioning can have adverse effects on the society.

For example, it wasn't too long ago that the only way to withdraw money from a bank was to go to a bank building, wait in line and talk to a person on the other side of a counter. Now, because of networked electronic systems, society has become dependent upon readily available cash on practically every street corner. If this network of electronic tellers were to fail, consumers would certainly be inconvenienced greatly, but, more importantly, the economy as a whole would slow down because the velocity of money moving through the economy would slow down.

In the future, electronic systems will only continue to infiltrate culture at every stratum. Ultimately, these systems are woven into the very fabric of a society. As such, they must be as reliable as possible and that means making them high-availability systems with hot swapping capabilities. Hot swapping will continue to be a critical means of ensuring the availability of important electronic systems.

Steve Hemmah is Hot Swap/Power-over-Ethernet Product Marketing Manager at Texas Instruments, Dallas