Digital or analog power supply?

In recent years, the use of microprocessors to control switching power supplies has continued to develop. There is still much debate about the advantages of digital power supplies over analog power supplies, with two camps arguing back and forth.

In reality, each approach has its own advantages and disadvantages. But designers must ultimately make a choice between using an analog or digital solution, and making an informed decision requires understanding the advantages and disadvantages of each.

Analog switching power supplies have been used for decades. The design is well understood, and there are many excellent textbooks, simulation kits, application notes, and seminars. There are also a large number of low-cost integrated circuits available from a variety of vendors that package many functions, from integrated gate drivers and switches to current sensing and protection. In short, the idea that digital power supplies will make analog power supplies redundant is far-fetched in any way.

Digital control has some features that are not available in the analog world, which enables switch-mode power supply design to achieve functions that have not been possible until now. As with other aspects of engineering, these benefits come at a cost, and the choice of using digital solutions must be based on whether these advantages outweigh the problems they bring.

A frequently cited advantage of digital control is that it allows the removal of some passive components in the controller, thereby eliminating component tolerance and aging issues. In addition, this advantage is more valuable and far-reaching in some applications. For example, in some multi-loop designs, the use of digital processors can concentrate control functions in one device, thereby implementing functions such as power rail sequencing, margin setting, load sharing, phase compensation, and software-implemented fault prediction.

Modular power supply designers have even more advantages. Think of a power supply manufacturer with many different power stages. With a digital control solution, a single processor can work with individual custom software to meet the needs of each power stage. When mass-produced, the resulting economies of scale can be enormous.

Before using digital power, there are of course some issues that must be considered. The PCB board space of the digital controller must include the MCU, crystal clock, protection/filtering and ADC pin buffer. In addition, there are some limitations on PWM accuracy and ADC dynamic range. Nevertheless, some of the latest digital power (please visit http://www.ti.com/piccolo ) dedicated MCU products can still solve most of these problems.

Another concern for analog engineers is the time and effort required to master these digital design techniques. In many cases, concepts such as Z-transforms and sampling theory have not been touched upon since graduation day! Fortunately, a number of online tools are now available for free ( http://www.biricha.com/ ) which automatically convert an analog design to its discrete time equivalent based on a set of user frequency domain specifications. Biricha also offers a multi-day course in digital power supply design, tailored for engineers who wish to master digital techniques or simply want to brush up on knowledge that has been neglected for a long time. In addition, Bodo Power will be publishing a series of technical articles over the coming months aimed at providing assistance to analog engineers in digital PSU design.

In summary, digital power is not a "magic bullet" that makes analog power redundant. Both analog and digital power will exist and will coexist harmoniously. If additional digital control functions are not required, then analog power is the obvious choice. Conversely, in complex systems with many tightly regulated power stages, requiring monitoring, data communications, and flexible control loops, digital solutions can play a more appropriate role. Fortunately, the introduction of microprocessors that optimize digital power control and the emergence of some new tools and training allow designers to choose the best solution for the application at hand.