[Design View / Design Solution]
Customize Power Supplies Freely With A Digital Feedback Loop
Digital signal controllers plus power-supply-friendly on-chip peripherals are the building blocks for an easy and cost-effective method of digital power conversion.
Tighter power regulations and safety issues are demanding efficient and intelligent power supplies that can be monitored externally and manufactured cost-effectively, with minimal hardware changes.
Power-supply engineering advances have shown that digital control of the power-conversion feedback loop enables designers to create more accurate and reliable power supplies with increased power density, at lower costs and with faster time-to-market. These digital power supplies are easily customizable at any time during production because changes can be made in software, rather than hardware.
Using analog feedback circuitry still makes sense in power supplies with less than 100 W in dc-dc and less than 250 W in ac-dc ratings. However, in high-featured, elevated-rating power supplies, digital control of the power- conversion feedback loop becomes critical, since it overcomes most of the limitations sometimes imposed by a fixed analog approach.
For example, a capacitive load may significantly affect a power supply’s stability. Analog feedback systems can be designed to handle a capacitive load, but major changes in the capacitance of the load could exceed the phase and gain margin of the design. The advantage of a digital feedback system is the ability to change compensation on the fly, which allows the feedback to compensate for a wider variation in load characteristics in real time.
THE SHIFT TO DIGITAL Until recently, digital feedback systems only saw limited use, due to their perceived complexity, the cost of the DSP required, and the limited capabilities of the DSP peripherals. However, through education, the perceived complexity is slowly fading, and the digital signal controller’s (DSC) arrival has helped to alleviate the problems associated with cost and peripheral capability.
DSCs combine the look and feel of an MCU with a DSP’s calculation and processing capabilities. CPU design incorporates the math functions typically found in DSPs, while the functionality and flexibility of the peripherals trace their lineage to embedded controllers. The resulting DSC exhibits the math performance of a DSP, while retaining the flexibility and complex, coordinated features of the peripherals. DSCs significantly ease design complexity, without burning CPU performance to achieve it.
In fact, with these features, designs using DSCs are actually much simpler than DSP design because many DSCs come with power-supply-friendly peripherals onboard. Such peripherals include counter-based pulse-width-modulation (PWM) modules, analog comparators, and analogto- digital converter (ADCs). Hence, analog comparatorbased feedback and ADC sampling are enabled. These capabilities, coupled with fast multiplication in a singleclock cycle, allow DSCs to easily handle the high execution rates needed for power-supply control-loop software.
The DSC’s performance capabilities, coupled with the lower switching frequencies of high-power designs, enable even a moderately performing DSC with the appropriate peripherals to easily handle multiple control loops. This means that a single chip not only improves the response characteristics of the supply, but does so for multiple independent outputs simultaneously.
Before starting a power-supply design, designers must make three basic choices:
What will be the design’s topology?
What will be the operating mode?
What will be the control methodology?
The topology is driven primarily by the design’s input-to-output voltage ratio. The operating mode is driven by the topology and the required output current, as well as by the costs associated with the components. Finally, the control methodology is typically driven by the available technology and, to a lesser extent, component cost. Let’s examine each of these choices, with an eye toward how the use of a DSC will affect the decision.
TOPOLOGY As mentioned above, topology is driven primarily by the input-to-output voltage ratio of the design. Designs with a higher input voltage typically use a buck topology, while lower-input-voltage designs usually go with a boost topology. However, another factor that often drives the choice of topology is the availability of a PWM controller with the requisite features, which is compatible with the chosen topology. After all, if a designer can’t generate the appropriate switching signals, a switch-mode power supply (SMPS) isn’t possible.
This is where the DSC steps in. Because a DSC’s peripherals are programmable, it’s possible to generate a single PWM output, two or more phases of PWM output, half-bridge drive outputs, or even a full H-bridge drive output. In fact, due to the programmability of the DSC’s peripherals, a given topology needn’t remain static.
It’s well within the capabilities of a DSC to switch from a single phase, to two phases, and then to three phases, all while maintaining the appropriate phase shifts between phases. Some DSCs even go so far as to include deadtime control between bridge outputs for the purpose of preventing shootthrough currents in synchronously switched designs (Fig. 1).
Please refresh the page if you have trouble reading this text.
Search Electronic Design
Email Newsletter
Sponsored By:
The Find Power Products monthly newsletter brings you the most important new developments within the world of power design. The newsletter includes exerpts from industry leader Sam Davis's exclusive blog, as well as overviews of the latest new products.
Enter Email to Subscribe
Web Seminar
Sponsored By:
Title: Exploring How Good GUIs Drive Adoption in the Digital Power Management Space
Reprints
