In the field of electronics, power supply design has often been done "just-in-time". While "just-in-time" can be the ideal in manufacturing, it often leads to inefficiencies in total system design. Fortunately, many designers are realizing that just-in-time power supply design is increasingly untenable. That's because the simple power supply is not so simple anymore.

It is certainly understandable that as the complexity of electronic systems has increased, the complexity of analog power supplies has increased as well. We only fool ourselves if we cling to the belief that a design team can largely ignore the power requirements of a system until most of the digital design has been completed. When designers have this attitude, the system usually ends up with a compromise power supply solution instead of the most effective solution or the design of the power supply can involve more work than it should.

Exacerbating the problem is the scarcity of power supply designers -- some of the hardest analog engineers to find these days. Typically, power system designers acquire their skills through several years of on-the-job learning rather than classroom work.

Reshaping power supplies

Over the last 10 years many forces in the industry have reshaped the venerable power supply. Gone are the days when we just rectified 120 V from ac to dc, filtered it and stepped it down to plus or minus 15 V. Back then it mattered little if we wasted as much power as the system used. Now, power supply design must contend with portable applications powered by limited-life batteries, a "green" movement intent on conserving energy wherever and whenever possible, higher-speed, lower-voltage processors with high current requirements, mixed voltage mode systems and other industry trends that are forcing designers to examine a system's power supply needs earlier in the development cycle rather than later.

Digital and analog co-existence

Although most designers in our industry are digital designers, electronic systems are still made up of both digital and analog paths. These digital and analog paths must be kept separate so that digital signals won't couple across to the analog path. If designers had their preference, a simple, off-the-shelf power supply solution would always be available to meet their needs.

In recent years the processing speeds and capabilities of TI's digital signal processors (DSPs), as well as DSPs and microprocessors from other companies, have increased dramatically. At the same time, chip geometries have decreased and low-power CMOS processes are being used more and more. These developments place a great deal of pressure on power supply design because the system's electrical load has become much more dynamic. As voltages have decreased from 5 V to 3.3 V, or 2.5 V (and soon 1.8 V), current requirements have remained high and many processors generate high load transients. For example, a processor might only draw a few amperes (A) during normal operations, but to execute certain functions like a write-to-memory the current requirement could jump to several amps with even higher load transients. How the power supply will handle such dynamic electrical loads is just as critical to consider early in the design cycle as any of the fundamental digital design issues.

Meeting tighter tolerances

Decreasing operating voltages have required greater precision on the part of power supplies. Heavy loads and extreme load transients make it much more difficult to maintain a 3.3-V supply within a plus or minus five-percent tolerance than it is to maintain a 15-V supply within five percent. More complex and sophisticated switch-mode power supplies are increasingly being used to stay within the prescribed tolerances of low-voltage supplies.

Portability brings complexity

Portable applications like laptop or notebook computers and wireless communications devices have raised the complexity of power supply design to a level never thought of 10 years ago. At the same time, the demands of portable applications have heightened the industry's awareness of the need for sophisticated power supplies.

Complex questions concerning partitioned power distribution, power supervision and battery management must be addressed early in the development cycle by designers of portable applications or the system will not be able to compete in the marketplace. Indeed, many laptop or notebook computers succeed and grab market share based on the fact that their power supplies are able to squeeze more life from finite battery capacities.

Many of the recent innovations in power supply design have come about because of the increasing demands of portable applications. Soon, many of these innovations will find their way into other types of non-portable applications as well.

The growing popularity of portable applications has not been lost on integrated circuit manufacturers. Indeed, some of the premier vendors of analog ICs including TI have turned their attention to power technology. Broad-based vendors who supply digital and analog technology, and who, because of their broad understanding of the many different issues at the level of system design, will be able to continually innovate new power supply solutions.

Portable power

In a portable application like a notebook computer, even power distribution -- the most basic level of a power supply subsystem -- is more complex than you might expect. For a notebook computer, the power supply designer must develop a partitioned power distribution network so that segments of the system can be shut down or put in a sleep mode when they are not operating. This conserves power and extends the life of the battery. Only with great difficulty can this type of a power distribution network be overlaid on top of the notebook's system design once it's been completed. A sophisticated power distribution network should be developed concurrently with the system's overall design.

The next level in a notebook computer's power supply system involves power supervision. Here, the power supply must be intelligent enough to monitor the voltage being provided to the notebook computer. If the supply voltage drops below a certain tolerance level, the power supervisor can send an early warning to the processor for notification of the user or the supervisor can switch to a back-up power source. If the voltage continues to drop, the power supervisor can send a reset signal to the processor to shut down the system.

The third level of sophistication in a power supply system is battery management. At this level, the power supply system is able to monitor the load current to determine how quickly the battery is being drained. It can also switch the battery from a discharging mode to a charging mode if a power source is introduced and the battery needs to be recharged.

Mixed voltage modes

Another complicating factor that is moving power supply design from the back end to the front end of the system development process is the increasing occurrence of mixed voltage modes in a single system. At one time, supplying one voltage to a system was all that was required. Today, we commonly encounter systems with a 2.5-V processor core, 3.3-V I/Os and, undoubtedly, 5-V power too. Many times two separate power supplies will be required to support the system's I/O. In addition, the analog signal path might require higher voltages, like 12 V, 15 V or even 40 V for a backlit display screen on a portable computer.

Designing in the voltage steps, down and up, after the system design has been completed adds to the complexity of the task. With a little foresight and planning, the number of headaches involved with designing a power supply for a mixed-voltage mode system can be reduced.

The squeeze continues

Conditions in the industry will only make it more imperative in the future that power supply design be conducted as an integral part of system design. Processing speeds will continue to increase, operating voltages will decline, current requirements will remain high and system power budgets will drop. In addition, several other developments are on the horizon and these will continue to put the squeeze on power supply design.

Although great strides have been made with new types of batteries for mobile systems, the power capacity of the most advanced batteries is not keeping pace with users' voracious appetite for portable applications and extended battery life. On the other hand, portable system manufacturers are always looking for an advantage in the marketplace and a claim of longer battery life is something consumers can easily understand. As a result, sophisticated switched power supplies are being relied upon to reduce system power usage and extend the battery's life.

A trend toward distributed power supplies is also requiring greater sophistication in power supply design. In many applications today, power is brought through a voltage regulator and dropped to the required voltage. The power is then bused throughout the system. Designers have now realized that busing power at a low voltage may not be the most effective power distribution system. Some systems now distribute power throughout the system at a high voltage, like 12 V, 24 V or even 48 V. This high voltage power is then passed through local voltage regulators or other distributed power supply blocks at those local points in the system where power is needed. Such a power supply system is a far cry from the days when a simple off-the-shelf voltage regulator was all the power supply most systems needed.

Know thy power budget!

In the years ahead semiconductor companies like TI will draw on their expertise in end-equipment segments of the industry like wireless communications and networking to develop more sophisticated and more capable power supplies. It's incumbent upon designers to know the system's power budget at the beginning of a development project. Only then can the system's power supply be developed concurrently with the system itself.