Taking advantage of digital power conversion solutions

Taking advantage of digital power conversion solutions

A digital feedback loop is often the best choice

BY STEVE BAKOTA
Texas Instruments
Dallas, TX
http://www.ti.com

Many articles have been written and papers presented that argue for and against the value of a digital PWM controller in a power supply. The most common questions from power supply design engineers are simply, “Why should I consider using a digital PWM controller for loop control? What are the benefits and tradeoffs?”

Power supply engineers have been using analog PWM controllers for switching power supply designs since the SG1524 was introduced in 1976. As the need for monitoring, management, and communication grew, designers started using microcontrollers to perform these functions. Integrating these two technologies was the first step power controller IC suppliers delivered to the marketplace. For any new technology to be considered of value for a design, three fundamentals must be considered: performance, time, and cost. This principle also holds true when considering the transition to digital power controllers.

Meeting designers needs

To be embraced as mainstream technology some fundamental criteria need to be met: (1) the power supply performance, at a minimum, must be comparable to analog solutions while delivering value-added features; (2) design tools must make task easier; and (3) the solution must be cost competitive for the performance it enables. First, the digital controller-based power supply must achieve the performance of existing analog technology. Designers need to see improved reliability, power density, transient response (closed loop bandwidth), stability over line/load/temperature, and efficiency. Additionally, the devices must offer useful features that are either not possible or practical with analog controllers.

The initial objective digital controllers had to achieve was to deliver a solidly performing PWM controller, and products on the market today demonstrate loop response and stability comparable with existing technology.

The next challenge was to deliver on the promise of enhanced features. One good example is the ability to have a controller with a wider operating “sweet spot” than analog technology. By sweet spot, I’m referring to not having performance compromises (transient response, efficiency, etc.). Techniques such as dynamic filter response and improved phase management available on products such as the recently announced UCD9240 deliver on this promise (see Fig. 1 ).

Fig. 1. Digital controllers such as the UCD9240 can significantly reduce performance compromises.

Another benefit of a digital controller that has continued to evolve from the initial concept is improved flexibility. With an architecture that allows for flexibility, it’s possible to make configuration adjustments on the fly as well as change some features with minimal re-engineering. An example of this is highlighted in the emerging need for better power supply monitoring.

Design engineers are just starting to take advantage of enhanced operating monitoring features——and with them come evolving system requirements. Microcontroller-based architectures such as the UCD91xx and UCD92xx allow changes such as what is monitored, tracked, and recorded without releasing a new device. This allows a solution to be tailored to a customer’s needs during development with minimal incremental time/cost/component changes.

Design tools and advanced features

For digital power controller technology to become widely accepted, development tools need to be available that are both intuitive and valuable to both veteran and novice power supply engineers. Today’s power engineers have a thorough understanding of the analog building block of the PWM controller and are able to add external circuits that achieve the desired power design.

With digitally controlled PWMs, this same flexibility needs to be provided, but not at the price of learning how to write C or assembly code. The power designer will simply trade in their soldering iron for a keyboard. Graphical user interface tools are very important collateral for market success of digitally controlled power.

Another area of development is adaptive supply control, which tunes the loop to the characteristics of the load on the supply. Advancements in loop control will lead to improved transient response, improved efficiency over a broader range of loads, and lower cost of external active and passive components. An example of this technology was illustrated in a 1-kW telecom rectifier evaluation concept, developed by TI, that used a single TMS320F28x DSC for both interleaved PFC and a phase-shifted full bridge converter. An interesting part of the design was the development and configuration tools created to optimize its performance.

These tools allowed the designer to measure, model, and implement changes to their loop using a GUI to view/adjust Bode Plots and poles and zeros (not Z transforms). This allows the customization of a supply to meet a design target just by making changes in registers and also provides for tuning the control loop for variations in components.

While these advances in technology are certainly interesting, it’s understood that these solutions must be offered at a competitive cost point for the benefits to be considered in mainstream designs. Products such as the UCD9240 are integrating many features into a single packagefrom offering the ability to regulate/control up to four independent dc/dc converter loops (and up to eight phases) to offering a full range of configuration, monitoring, sequencing and margining capability. The device is valuable to customers who have multiple rails in their product and want flexibility without having to pay a premium.

Fig. 2. A GUI-configurable, four-output, multiphase power system controller such as the UCD9240 integrates many features into a single package.

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