Electronic Design

Power-Supply Designers Are Taking Aim At "Green" Designs

Lower power consumption and higher efficiency levels are constantly being improved in power-supply circuits and components.

Engineers of all types are striving to build better products and processes that will create a "greener" world for us and help our environment. This isn't surprising, considering the huge amount of electronics products that are manufactured every day and the resultant waste produced during their manufacture, end use, and disposal.

Production engineers are contributing to the cause by ensuring that no toxic materials are used in the manufacturing process, that the products are boxed and shipped in only environmentally friendly cases, and that the soldering processes are flux-free and lead-free. But, the area of electronic design is where the green impact is strongly felt. A key element here has been the design of power supplies that are more efficient while consuming less energy.

Just about every piece of electronic equipment and circuit requires a power supply to operate. Therefore, any aspect of a power supply's design that positively impacts the environment can have wide-ranging green ramifications.

Now, such brandings as "Energy Star" or the "Blue Angel" on many new power supplies show that a specific, final product has a reduced power-consumption level. Neither of these standards is very tight, though, calling for greater achievement in less power consumption.

Green Power Modules
One of the first efforts to reduce power-supply consumption came from Philips Semiconductors about two years ago with the introduction of a 100-W multichip green power module (see "Multichip Power Module Targets 'Green' Designs," electronic design, June 8, 1998, p. 41). That product was the first in a "GreenChip" family of "hot side" switched-mode power-supply modules. In fact, Philips has just introduced a second-generation GreenChip module.

Reducing power-supply current consumption while in the standby and sleep modes is one of the most direct approaches to cutting overall energy consumption. These techniques, of course, must be balanced against cost restrictions that can sometimes limit their effectiveness.

To counter this dilemma, Thomson Multimedia came up with what it claims is a very effective solution for switched-mode power supplies under the direction of its Green TV project. The company calls the product its Ecosphere. The name is derived from ECOlogical Series-Parallel Halfbridge Efficient REsonant converter switched-mode power supply. This solution only consumes 100 to 200 mW of power from the ac power line in the standby mode. Moreover, it requires a transformer that's five times smaller than one necessary when using a conventional design (Fig. 1).

"The Ecosphere offers a wide-voltage-regulation range with an output power from 0.5 to 100 W," says Markus Rehm, managing director of IBR Ingenieur-Buero Rehm, an engineering office exploiting and licensing the Ecosphere principle. The current solution was developed for use in the European market and is designed to work with ac input voltages between 180 and 270 V. Essentially, it combines the advantages of both serial and parallel resonant-converter designs.

When the Ecosphere power-supply concept was first conceived, its designers developed several system prerequisites to which its design would comply. For example, in a resonant forward converter (parallel resonant converter), no energy needs to be stored inside the transformer. This happens because energy is transmitted directly to the transformer's secondary side.

By employing a half-bridge circuit in the Ecosphere, the first and third quadrants of the magnetization loop in the ferrite is used. The voltage across the power switches doesn't exceed the supply voltage by more than two diode voltage drops. As a result, the voltage stress is only half of what it would have been if a single-transistor, single-ended converter was implemented. Keep in mind that two transistors and drive circuitry for the low-side and high-side switches are necessary.

The continuous conduction mode of the Ecosphere's design implies that current in the resonant LC circuit is a continuous sinusoidal wave. A disadvantage of the parallel resonant converter is that with low loads, the circulating currents and currents in the transistor aren't smaller than those when driving a high load. In either case, the effective resistance reflected across the parallel capacitor use must be high in order to avoid significant decreases in the Q factor of the circuit.

"If this is true at high loads, it will be even more true with lower loads," Rehm explains. In other words, the current in the reflected load transistor that's across the parallel capacitor at low-load conditions is just a small fraction of the current in the capacitor itself. Therefore, power losses in the transistors don't decrease at low loads. Additionally, the efficiency at such loads is poor.

But in a series resonant converter, the situation is different. Here, to achieve a constant output voltage across the load resistor in series with the resonating LC circuit when the load current decreases, current through the load transistor (which also is the current in the transistors) decreases as well. As a result, efficiency levels remain high during low-load conditions.

"We intended to take advantage of the good low-load efficiency of a series-parallel converter circuit, as well as of a parallel resonant converter circuit's ability to regulate at low- or open-load situations, when we developed the series-parallel resonant converter circuit Ecosphere," Rehm reports.

"Here, the frequency is always above the resonant frequency, or in other words, above the resonance mode. In fact," he continues, "this means that during low-load conditions, the frequency is at its maximum of 800 kHz, while during full-load conditions, it's at its minimum of 300 kHz."

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