The Ins And Outs Of Power Conversion

By Cheryl Ajluni
Power conversion is a general term that refers to a system or device producing an output that is different than its input. It can assume many forms—everything from an inverter to an isolated power supply, uninterruptable power supply (UPS), or AC/DC converter. Power conversion, like low-power design, is fairly commonplace these days. Nevertheless, recent advances in digital power solutions (e.g., the ability to address high-frequency switch-mode operation) are making it much more palatable for engineering low power, portable applications.

In an attempt to gain a better understanding of power conversion and its relation to low-power engineering, LPE recently posed a few questions to Don Alfano, applications director for power products at Silicon Labs.

LPE: How is power conversion beneficial to enabling low-power design?
Alfano: With all of the attention focused on ‘going green,’ low-power design is politically and economically very important. However, the real reason low-power design can be beneficial is based more in practice than populist opinion. One obvious example lies in battery-operated systems such as iPods, cell phones, GPS systems, and so on. The operating time of these systems is directly related to the system power consumption and battery capacity. Since higher capacity batteries are typically bulkier, it is more desirable to limit operating power than to increase battery capacity.

Another example involves consumer devices that are powered by the AC line. Devices such as flat-panel TVs, desktop PCs and consumer audio devices consume relatively large amounts of power due to their sheer volume (e.g., 1000 watts per TV x 5 million TVs = 5 billion watts of electricity). A mere 10% improvement in operating efficiency can save 500 million watts of power, and how many tons of coal does it take to make that much power? The answer is a lot.

It’s because of this that standby power requirements (e.g., power consumed when the TV is “off”) for these devices have come under scrutiny by certification agencies such as EnergyStar. Consider that the average TV consumes 10 watts of power when “off.” Applying the same 5 million TV sets, the power consumed would be 50 million watts. Reducing standby power to the new standard of 1 watt results in a savings of 45 million watts of power. The bottom line: low-power design extends the operating time of battery-operated devices, while reducing battery weight. Low-power design also results in greater operating economy when implemented in AC line-powered devices.

Are there any obstacles or challenges that engineers need to be aware of in applications requiring power-conversion technology, such as ac-dc or dc-dc?
Both linear and switch-mode power conversion systems have been around for decades, and their control technologies are well-developed. That being said, one common obstacle is the tradeoff between the emission of electromagnetic interference (EMI) and efficiency. In any power system design, the engineer strives for the highest efficiency. However, certain portions of the circuit can act as parasitic “radio transmitters,” causing EMI that can be induced into other devices. A classic example is the noise that appeared on a television screen when an old-time vacuum cleaner was turned on. Federal Communications Commission (FCC) regulations now prohibit such emissions, forcing the power system designer to add parts into his design that ‘snub’ (eliminate) these emissions. Such circuits dissipate power and degrade efficiency.

Another classic tradeoff is efficiency vs. physical size. Switch-mode power systems become smaller when they are designed to operate at high frequencies. However, these high frequencies cause lower operating efficiency. Reducing the operating frequency results in higher efficiency but requires larger, bulkier transformers, which adds size, cost and weight.

Have there been any recent advances in power-conversion technologies or techniques?
Yes, digital power control is catching on after a slow start. Digital power effectively uses digital computer technology to control power conversion instead of traditional analog control circuits. Digital power control can enable more sophisticated power-conversion algorithms that can increase efficiency and system responsiveness while (ultimately) lowering cost. Another trend is the use of analog resonant power-conversion control circuits, which can boost efficiency and decrease system cost. These are becoming popular in consumer electronics like flat panel TVs.

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Alfano’s comments serve to underscore an important point—that digital power solutions and digital power control are highly relevant topics these days (See figure 1). One reason is that by minimizing high-speed circuit components and leveraging smaller process geometries, digital power solutions have simply become more practical for portable devices. Additionally, since digital power solutions integrate power control and power management, they are much more flexible than analog solutions and able to address higher levels of complexity, including the different operational modes of portable applications.

Figure 1. One example of a digital power solution comes from Microchip. It offers two device families for digital power applications: the dsPIC30F and dsPIC33F (pictured here) SMPS and digital-power conversion families. These devices include peripherals specifically designed for power conversion. Peripherals such as a high-speed PWM, ADC and analog comparators can be tied together using an internal configurable control fabric that enables them to interact directly with one another, resulting in significant performance gains in digital power applications.

Alfano adds that, “As its name implies, digital power control is based on digital processor technology. As such, the digital power controller can make informed control decisions based on multiple, real-time data it continuously gathers. For example, the digital power controller can measure various key operating parameters within the system such as input voltage and output current, then dynamically adjust or otherwise influence the operation of internal system components so as to maximize system operating efficiency (e.g., minimize power loss). In so doing, the digital power controller has the ability to maximize system efficiency across the power conversion system’s entire operating range.”

In addition to allowing the designer to adapt to different operating parameters, digital power control also has the ability to switch between compensators as a function of operational modes. Phase management, such as phase shedding and adding based on load requirements, is another capability that is possible with digital power control and one that is especially beneficial for applications that utilize multiphase regulators.

With portable applications continuing to weave their way through society, the current focus on low-power design shows no signs of letting up any time soon. Power conversion, digital power solutions and digital power control will therefore continue to be topics of great interest to designers. Such solutions offer flexibility as well as the ability to increase system efficiency—features which are highly prized in today’s portable market.

More information on power conversion, digital power solutions and digital power control is available from an array of companies working in this arena. Among those companies are Freescale, Intel, International Rectifier, Intersil, Microchip, Silicon Labs, STMicroelectronics and Texas Instruments.