Electronic Design

DSP Corrects Switch-Mode Power Factors

With the advent of high-speed, low-cost DSPs, the interest in using digital control in switch-mode power supplies (SMPS) has risen significantly. This has prompted researchers at Texas A&M University's Power Electronics and Power Quality Laboratory, College Station, to explore the digital control of power-factor correction (PFC) in SMPS. Traditionally, analog control has dominated this sector.

To implement DSP in this application, the researchers developed a fuzzy-logic control algorithm. The algorithm shapes the utility input current to a sinusoidal waveform. This enables ac-dc converters to comply with standards like IEC 1000-3 and IEEE-519 requirements for input-current harmonics and conducted EMI.

The proposed fuzzy-logic control algorithm is implemented on Texas Instruments' TMS320LF2407, which offers 16-bit fixed-point arithmetic and a number of control-optimized peripherals on-chip. Plus, it provides 32 kbytes of flash and 128 bytes of RAM to accommodate corresponding DSP code in the on-chip memory, with the ability to reprogram for future updates.

In essence, the C2407 DSP provides the digital control for the PFC boost converter. By sensing the utility voltage, output dc voltage, and inductor current, the DSP generates the necessary gating signal for the MOSFET to minimize the input total harmonic distortion (see the figure). In fact, to curb harmonics, the shape of the input current is made identical to the shape of the input voltage. The power-factor value realized is very high as well. Measurements taken by re-searcher Sangsun Kim show power factors on the order of 0.99.

According to project supervisor Prasad Enjeti, DSP control has many advantages over the analog approach. Programmability, adaptability, fewer parts, less susceptibility to environmental variations, and lower switching frequencies are among them, he asserts. "Input voltage distortions are prevalent in environments using three-phase, four-wire systems. The DSP controller can be programmed not to respond to these distortions," he notes.

In contrast, he adds, analog design is inflexible, requires more parts, and is susceptible to aging and environmental variations. Inflexibility means performance cannot be optimized for various utility distortions. Unlike analog, a DSP can do more than simple correction. As clock speeds continue to get better, one DSP controller has the processing power to do other tasks in addition to PFC. For instance, Enjeti explains, the same DSP can monitor the power-supply voltage and communicate with the central control processor.

Some key factors that motivated the researchers to adopt TI's C2407 include its single development platform, sufficient on-chip memory and optimized control peripherals, a fast on-chip analog-to-digital converter, software development tools, an evaluation module, and low cost. Recently, TI released several lower-cost members in this family. The researchers say the technology is ready and the software code generated is available for moving this concept to the commercial world.

For more information, point your browser to www.enjeti.tamu.edu.

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