DSPs Simplify Digital Control Implementation of SMPS

The greater functionality and new monitoring features of today's power supplies require more sophisticated control and communications. But as complexity and intelligence grow, so does the difficulty of designing power supplies using traditional analog-only techniques. In addition, requirements for lower cost, smaller form factors, and compressed development cycles continue to escalate, pressuring power supply designers as never before.

Fortunately, designers can now leverage the power of digital signal processors (DSPs) to help them tackle these complex challenges. Designed specifically for control applications using 16-bit or 32-bit fixed-point cores, these DSP controllers provide programming flexibility and enable a single-chip solution for control and communications functions of a power supply. Costing as little as $2 per device, DSP controllers are supported by complete libraries of control algorithms and hardware tools that help simplify development for designers who are making the transition from traditional power supply design methods.

Resolving Analog Design Issues

Until now, power supply control has been based largely on analog controllers. Providing some supervisory and monitoring assistance are the 8- and 16-bit microcontrollers. However, they lack the computational performance required to cost-effectively close the voltage and current control loops in a power supply. Since the controllers are analog-based, a typical power supply can employ multiple controllers and use several individual components. Furthermore, because each analog design is unique, control must be redesigned for every new power supply the manufacturer develops.

DSP controllers resolve many of these issues, reducing part counts, board space, and system costs, and enabling the addition of new features for product enhancements. The high performance and integration of these devices give designers the ability to implement closed-loop voltage and current control, as well as supervisory and communication functions — all of which are traditionally designed with analog controllers and 8- or 16-bit microcontrollers.

Today's DSP controllers can produce pulse-width modulated (PWM) switching frequencies at 1 MHz, and at the same time implement wide bandwidth voltage and current control loops. These processors, with up to a 150 MHz CPU clock, allow single chip solutions for power factor correction (PFC) and dc-dc conversion stages, synchronous rectification, digital current sharing, and other critical power supply functions. The high level of performance also allows for the implementation soft start profiles, intelligent fault management and protection, active in-rush control, communication power stage sequencing, and monitoring.

Integration and Flexibility

On-chip peripherals such as analog-to-digital converters (ADCs) and pulse-width modulators (PWMs) save board space and reduce parts counts by up to 40%. The integration of input and output pins (I/O) and communications on the same chip along with control capabilities provides support for the intelligent monitoring and reporting of the system status. Integrated flash memory simplifies software development and permits upgrades that add features and can enhance the algorithm, not only during manufacturing but also in the field. This high level of integration leads to significant part count reduction and, in turn, increases the density of the power supply. To achieve higher efficiencies, power supply manufactures can use the computational power to implement higher complexity topologies such as zero voltage switching (ZVS) and zero current switching (ZCS), which result in higher overall efficiencies. In addition, DSP controllers provide stable, reliable control, even under noisy conditions, enhancing overall reliability of the power supply.

Programming flexibility is one of the key advantages of digital over analog control, contributing significantly to faster development cycles and reduced time to market. Programmability enables manufacturers to use a single control platform in a wide range of power supply products and helps to differentiate the product line with enhanced control and monitoring capabilities. Systems in development can be tuned quickly using software-based calibration. In some cases, essential power supply software is available in source code from DSP controller vendors, saving considerable development effort for system designers. Typically, ready-to-use algorithms include voltage loop/current loop controllers for PFC and dc-dc stages, and features such as input rms voltage measurement, frequency measurement, voltage out-of-range protection, other monitoring and diagnostic functions, primary/secondary sequencing, overcurrent management, and intelligent fault protection. In addition, programmability enables power supply redesigns with more efficient topologies that take advantage of advanced nonlinear digital control techniques for optimum performance over the complete operating range of the power supply.

DSP controller development tools and environments simplify the transition for designers who are accustomed to analog and microcontroller techniques. Flexible, integrated development environments and C compilers provide tools that are familiar to most programmers. Design kits simplify system evaluation and help jump-start development, and additional algorithms and design expertise are available from third parties to help extend system functionality quickly.

Cost Savings

When it comes to the bottom line, DSP-based control is cost-competitive with analog control — especially in power supplies rated at 300W and above. Typical end applications include network servers, telecom switching and routing equipment, high-power rectifiers, wireless base stations, and other types of multichannel equipment. These space-constrained applications can take advantage of the increased features and functionality that are available with a powerful, programmable digital controller. In systems such as these, DSP controllers are being used for functions like inrush control, soft start, PFC, dc-dc conversion, digital current sharing, and all supervision and monitoring. For power supply manufacturers, savings come from a reduced board space, smaller bill of materials, simpler inventory management and manufacturing, and greater reliability in their products. As these savings become evident in higher-rated power supplies, the technology will quickly migrate to more cost-competitive, lower-rated systems, especially since manufacturers can reuse existing control designs for new products.

The savings in space and components that comes from using a DSP controller in a power supply is shown in Fig. 1. Here, a generic switchmode rectifier supply designed using traditional analog controller and microcontroller has been redesigned using a DSP controller. The power stages (filter, rectifier bridge, PFC, dc-dc converter, and auxiliary power supply) stay the same in each design, and analog interface functions remain. However, in the redesign, all of the control and the communication functions originally implemented using analog and microcontroller circuitry have been pulled into a single DSP controller for the primary and secondary sides of the supply.

While the advantages of DSP controllers are apparent, designers with traditional analog and microcontroller expertise may find the idea of DSP-based redesign somewhat daunting. This difficulty is less formidable than it appears — not only because of the strong development support available, but also because analog designers can take the designs they're familiar with and redefine them for their digital implementation.

Essentially, redesigning power supply control with a DSP controller breaks down into two steps: redefining the analog error amplifier compensation digitally and then programming the DSP. Some vendor-supplied algorithms are available for use as reference and starting development on the second step, programming the DSP. The more challenging aspect of redesign lies in redefining the control parameters in the familiar analog or S domain in order to design the controller in the new digital or Z domain. This point is generally true of all converters in power supply control. Therefore, the PFC converter stage offers a good illustration.

PFC Converter Example

An example of a digitally controlled PFC converter is shown in Fig. 2, where an ac-dc boost converter stage has been interfaced to a TMS320C2000^™ DSP controller. The converter is controlled by two feedback loops: an outer loop with a relatively slow response that regulates the average output dc voltage (Vo), and a much faster inner loop that shapes the input current (Iin). Instantaneous feedback from Vo, Iin and input voltage (V_IN) are needed to implement the control algorithm. All three signals are sensed and conditioned, then digitally sampled by the ADC as inputs to the DSP controller. In this example, the entire 16-bit fixed-point range of the DSP controller is used in representing the converted signals as digital values, with the most significant bit representing the polarity of the signal.

In the PFC control implementation, Vo is compared to the desired reference bus voltage (Vref), and the difference is fed into the voltage loop controller (Gvea). The output of Gvea is multiplied by V_IN and 1/Vdc**2 (where Vdc = average V_IN) to calculate the reference current (Iref) for the inner current loop. Iref is then compared to Iin, and the difference is fed into the regulator (Gca), which generates the PWM duty ratio command for the PFC switch.

K_m =V_max/V_min

Implementing this control in software using a DSP controller means it's necessary to redefine the voltage and current feedback gain parameters (the K blocks), then derive and digitize the compensators (the G blocks). For a particular fixed-point representation of the converted signal as an example, the feed-forward voltage sensing gain (Kf) can be defined as 1/V_IN(max), the current sensing gain (Ks) as 1/I_IN(max), and the bus voltage sensing gain (Kd) as 1/Vo(max). Mathematically, the derivation of these definitions is fairly straightforward, though it is too lengthy to discuss in detail here. Interested parties can find it published in an application note from Texas Instruments¹.

Power of Digital

Once the gain parameters have been redefined for digital implementation, the voltage and current compensators can be derived using methods similar to those used in analog designs. The compensators can then be digitized using several techniques — for example, bilinear transformation and pole-zero matching. These transformations generate an equivalent digital controller that can be implemented in software using a DSP controller. Once designers become familiar with this approach, digital control implementation of 3-phase PFC, interleaved PFC, phase shift and resonant bridge topologies, zero-voltage switched (ZVS), and zero current switched (ZCS) converter topologies can be extended. These transformations generate an equivalent digital controller that can be implemented in software using a DSP controller.

As designers become accustomed to working with DSP controllers, they'll realize the inherent power of the digital techniques for power supply control and make increasing use of these and other digitally enabled techniques. For instance, in a PFC circuit, DSP controllers allow changes of operating points in real time. By raising or lowering the dc bus voltage level according to the ac input voltage level, the controller can dynamically achieve an optimum bus voltage level while maintaining a good input power factor — a feature that can't be easily supported with analog design.

Innovations, such as implementation of the power supply control to optimize the performance, continue the trend toward greater efficiency and reliability that's underway in power supply design. However, before power supply manufacturers can take advantage of new functionality, they need to find ways to cut costs on existing systems. DSP controllers offer the means to reduce costs in the short term and open up long-term innovations in power supply control. Since the transition from traditional analog to digital redesign is straightforward and well supported, power supply designers can quickly introduce DSP controllers into their existing products. Then, they can reuse the same control designs, with minor modifications in software, for new products. The performance, integration, ease of design, and flexibility inherent in DSP controllers gives developers what they want: empowerment.

Shamim Choudhury is a senior applications engineer at Texas Instruments in the Digital Power Solutions group and is responsible for providing solutions and support for customers developing digital-based power solutions. Matt Harrison is manger of the Digital Power Solutions group and is responsible for driving TI digital innovation for power supply applications.