Digital power supplies for switch mode power supplies
Switched-mode power supplies (SMPS) are the basis for most electronic equipment in today's high-tech world. Every electrical device requires power to operate, and different devices require different forms of power. This results in tremendous architectural diversity, with solutions ranging from ultra-small SMD 1W step-down DC/DC converters to high-power, high-efficiency 50kW rectifiers.
The SMPS industry has come a long way from the early REC-30 mercury vapor thyristor power supplies used in Navy teletype systems in the 1940s to today's ultra-high efficiency, high power density telecom rectifiers. With the promise of new technologies such as GaN and SiC devices, the SMPS market is expected to reach over $6.3 billion by 2025, according to the Report Linker assessment. This advancement is the result of continued technological developments that have touched every component of design. However, there is one common thread behind this ubiquitous transformation: the digital revolution.
In this article, we will discuss some of the important aspects surrounding the matter. We first outline the historical conditions that led to this shift and then clarify what is meant by the commonly used term “digital power” and some related concepts.
Figure 1. SMPS historical timeline.
A brief history of SMPS
There is no clear consensus in the SMPS community on what is meant by the term “digital power supply.” This can be attributed to a variety of factors, including digital technology that introduced the first PWM controller ICs as early as the 1970s, the varying degrees of digitization that have occurred over the past few decades, and of course the creative nature of marketing. In fact, the switching action itself - which gives the entire technology its unique name - is essentially a digital phenomenon. Despite this confusion, as we will see in this section, the technological evolution of modules is historically clear.
Although the SMPS revolution has been known since the early 1930s, it did not truly occur until the development of two key technologies in the late 1970s: power MOSFETs and PWM controller ICs.
Previously, SMPS were controlled by analog devices; the controller board was designed with operational amplifiers, comparators, transistors and passive components to perform all the functions required to supervise and control the module. As designs evolve, larger and more complex functionality is required, and the challenge of solving such complex logic with discrete components often results in performance limitations and high parts count.
Many companies saw this opportunity and began producing dedicated analog ICs for PWM control. However, it was not until 1976 that Bob Mammano integrated all functional blocks into one chip (SG1524) and invented the first PWM controller IC, marking the beginning of the digital power era (see Figure 2).
Figure 2. Block diagram of the SG1524. Image courtesy of Tautec Electronics.
Once Mammano hit the scene with its SG1524, a variety of PWM controllers began to appear, offering more and more features and improved functionality. In the mid-1980s, the increasing diversity of power ICs expanded beyond the realm of PWM control and provided solutions for other tasks such as monitoring and fault management.
At the same time, new control strategies (such as current mode control) are being researched with promising results, posing another new set of requirements for power ICs. The market continued to expand into the 1990s, and the huge growth in consumer electronics and data processing equipment gave rise to environmental concerns, plans to reduce power consumption, and a strong need to improve SMPS conversion efficiency (e.g., US Energy Star and EU Code of Conduct) .
Increasingly stringent requirements for SMPS impact every aspect of the design, requiring more complex functionality such as data logging and external communications. This complexity coincides with the increasing popularity and falling prices of MPU and MCU chips, which entered the market in the mid-1970s with devices such as TI's TMS1000 and Intel's I8080 and proliferated rapidly (see Figure 3).
Figure 3. Texas Instruments' TMS1000 and Intel's 18080. Image courtesy of Texas Instruments and Intel.
The events leading to the adoption of these technologies in SMPS are not well documented, as this is a clear design trend toward eliminating an increasing number of discrete components for non-critical monitoring and control functions. However, by the 1980s, the motor drive industry began using MPUs and MCUs with great success for a variety of purposes, including feedback control.
However, SMPS designers (a typically conservative group) only use this experience to perform non-critical tasks. Later, in the 1990s, a step was finally taken with the popularity of digital signal processor-based controllers (DSCs). This is the birth of fully digital control SMPS.
Digital power supply and software controlled SMPS
All in all, we can say that digital power is not an all-or-nothing term when it comes to SMPS. SMPS are complex systems containing various functional blocks that have become increasingly digitized as technology has evolved. In this transformation, deep digital technology penetration has been achieved through the introduction of DSCs and their respective firmware to perform all monitoring and control tasks required to perform power conversion.
Well, we can define digital power as power generated by SMPS where all control related functions are performed by digital technology. However, this definition does not distinguish between power supplies whose control functions are implemented by non-programmable digital controller ICs and power supplies that rely on MPUs, MCUs, DSCs, FPGAs, or combinations thereof. This distinction is becoming increasingly important as many SMPS applications and new technologies such as SiC and GaN require the latter.
Similar to what happens in other fields, many possibilities arise when choosing a software-oriented approach to power design. From introducing new IP protection mechanisms through the use of code encryption to reducing reliability issues caused by component aging, the benefits are numerous and can significantly improve performance and reduce the cost and duration of development cycles.
However, these advantages come at a high cost, as changing the design paradigm to introduce a completely new discipline (i.e., software engineering) at the module level presents its own challenges, to say the least.
Some of these challenges have to do with developing the right skills. Now, the team needs digital designers, digital controls and firmware developers. Others involve employing the right tools and processes to execute the right design. Indeed, in order to move in this direction without creating significant risks, a number of factors must be considered.
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