Why we need ‘green’ POL dc/dc converter designs

Why we need ‘green’ POL dc/dc converter designs

Cutting the current energy consumption of appliances by 15% is more cost effective than building new power facilities

BY TONY ARMSTRONG
Linear Technology
Milpitas, CA
http://www.linear.com

The concept of “green environment protection” has been in the news a great deal lately. As a result, most industrialized nations generally accept that they need to conserve energy. They realize that as populations increase, so do the demands for energy to power new homes with heating/cooling systems, lighting, and electrical appliances.

It costs a great deal of money not only to build new power-generating facilities, but also to deliver this power to the users once it is generated. It is more cost effective to cut the current energy consumption of most electrical appliances by 15% to 20% than to build new power facilities.

As a result of the high costs associated with the building of new power-generating facilities, many countries have adopted a so-called “green policy” whereby they encourage manufacturers to incorporate energy-saving techniques into their end products. This has led many power management product suppliers to make a lot of progress in improving their products’ power conversion efficiency and power consumption when placed in a standby mode.

Background

Most embedded systems are powered via a 48-V backplane. This voltage is normally stepped-down to a lower intermediate bus voltage of 24, 12, or 5 V to allow power to the racks of boards within the system. However, most of the subcircuits or ICs on these boards are required to operate at voltages ranging from sub-1 to 3.3 V, at currents ranging from tens of milliamps to tens of amps.

As a result, point-of-load (POL) dc/dc converters are necessary to step down from the 24-, 12-, or 5-V voltage rails to the desired voltage and current level required by the subcircuits or ICs.

Even manufacturers of battery-powered portable products are under ever-increasing pressure to pack more features into an already constrained form factor while simultaneously gaining longer battery runtimes. As an example, most PMPs have the functionality of both a video player and a MP3 player. As a result, the internal electronics requires multiple low-voltage output rails with varying power levels. The primary reason for this is obvious; the majority of the digital large-scale integrated (LSI) ICs have operating voltage of 1.2 V or less. At the same time, memory and I/O voltage requirements can vary between 2.2 and 3.3 V. Thus, it is becoming impractical to use multiple single-POL dc/dc converters directly from the Li-ion battery, and so system designers are adopting a more integrated approach.

Saving energy with PMICs

For a power-management IC to be used as part of an energy-saving dc/dc converter, it must have two main attributes. First, it must have very high efficiency of conversion over a wide range of load currents. And second, it must have low standby and shutdown quiescent current.

Embedded energy demands

For many embedded systems, the growing demands for increased current at ever-decreasing voltages continue to drive power-supply development. Much of the progress in this area can be traced to gains made in power conversion technology, particularly improvements in power ICs and power semiconductors.

In general, these components contribute to enhancing power supply performance by permitting increased switching frequencies with minimal impact on power-conversion efficiency. This is made possible by reducing switching and on-state losses while allowing for the efficient removal of heat. However, the migration to lower output voltages places more pressure on these factors, which in turn creates significant design challenges.

Mulitphase considerations

Multiphase operation is a general term for conversion topologies where a single input is processed by two or more converters, where the converters are run synchronously with each other but in different, locked phases. This approach reduces the input ripple current, the output ripple voltage, and the overall RFI signature while allowing high-current single outputs, or multiple lower current outputs with fully regulated output voltages. It also allows smaller external components to be used, which for a monolithic device increases output current capability, as multiple, smaller MOSFETs can easily be fabricated “on-chip.” This also has the added benefit of improved thermal management.

Multiphase topologies can be configured as step-down (buck), step-up (boost), and even forward, although generally buck is the more prevalent application. Conversion efficiencies of up to 95% from 12 V into 1.xV out (for example, 1.0 V, 1.2 V, 1.5 V, and even 0.9 V) are commonplace today. Further, by incorporating a pulse-skipping pulse-width modulation (PWM) technique, high-efficiency operation over multiple decades of load current can easily be obtained. This also has the added benefit of being able to obtain low quiescent current when delivering low levels of current to the load. A quiescent current in the range of tens of microamperes are the norm.

The scenario described for embedded systems is not so different for handheld battery-powered devices, with the possible exception that many of these portable applications have strict limitations on component height. This can be a challenge for a power converter, since the inductor and filter capacitors are usually among the tallest components. Nevertheless, a multiphase architecture is ideal for these applications even down to component heights of only 1.5 mm.

Many of the monolithic multiphase converters from the various analog IC suppliers can deliver more than 10-W output power in a small size, with higher efficiency, lower profile, and lower output ripple than is achievable with a comparable single-phase converter.

Mulitphase considerations

Consider, as an example, a monolithic, synchronous, high-switching-frequency (up to 2 MHz per phase), four-phase power IC architecture. An example of such a product is the LTC3425 (see Fig. 1) . This would allow the use of small, low-cost inductors rather than a single large bulky inductor, and require much less output filter capacitance than an equivalent single-phase circuit because the effective output ripple frequency is up to 8 MHz. In addition, all the power MOSFETs required are fabricated on-chip. This is ideal for space-constrained boards and portable devices that demand the use of low-profile components.

Fig. 1. The LTC3425 boosting from two NiCd/MH cells to 3.3 V. This design can supply over 2 A of load current with efficiencies up to 94% while switching at 1 MHz per phase (4-MHz output ripple frequency).

Furthermore, designing a converter using a multiphase approach is no different than designing a traditional single-phase converter. All the power switches are internal, so the four-phase operation is transparent. The current limit and switching frequency for all four phases could easily be programmed by a single resistor, as in single-phase designs. Similarly, setting the output voltage and compensating the loop would be no different than other familiar dc/dc converter designs.

The synchronous four-phase architecture of this type of POL converter achieves high efficiency over a wide range of loads, while enabling the use of low-profile components. Finally, with a four-to-one reduction in output ripple current, it is possible to achieve very low output voltage ripple using small, lower-cost ceramic capacitors.

Designers of POL dc/dc converters for almost any kind of system face many challenges due to the multiple constraints of limited space and cooling within a given enclosure, as well as the need for the correct power supply tracking for improved system reliability. Despite having to navigate through these myriad constraints, many of the recently introduced regulators from the various analog IC manufacturers provide simple, compact, efficient, and feature-rich solutions. ■