Electronic Design

Tiny Power Supply Delivers A Big WALLOP

Optimized for powering high-speed datacom and telecom applications, a current-delivery system sets a new performance standard for ac-dc power supplies.

The steady advance in silicon technology is altering the landscape for power supplies. With each new generation of CMOS chips, circuit densities and signaling speeds continue to rise, while supply voltages continue to fall. The 5-V standard for logic has given way to 3.3, 2.5, 1.8, and 1.5 V. Furthermore, the Semiconductor Industry Association (SIA) has predicted that voltages as low as 1 V will be in use by 2006.

The combination of falling supply voltages and rising currents has changed power-supply requirements in a fundamental way. Now the focus for much of the power-supply industry is current delivery rather than power delivery. As a result of this shift, current ratings are becoming a more relevant measure of supply performance than power ratings.

The demand for high currents at low voltage is just one of the challenges presented to power-supply designers. With CMOS-based circuits steadily shrinking, the pressure is on power-supply manufacturers to produce smaller supplies that will fit into space-limited applications. To reduce the size of the power-supply package, designers must employ new circuit topologies and components that will boost power-conversion efficiency, which reduces the need for heatsinks and fans while allowing the use of smaller components.

The need for smaller, more-efficient supplies is particularly great in the datacom/telecom industry. In this area, networking equipment designed to move data at 10 Gbits/s is placing stringent demands on the power supply. Devices such as routers, LAN controllers, and servers require high performance. In addition to delivering high currents at low voltages with high efficiency, power supplies also must offer fast transient response, lend themselves to current-sharing operation, and satisfy the new regulations for power-factor correction.

With its introduction of NET1, a multioutput, open-frame ac-dc power supply, Power-One of Camarillo, Calif., has combined advanced circuit and packaging technologies to meet the needs of these current-hungry high-speed datacom/telecom applications.

NET1 exploits a proprietary, self-driven synchronous rectification scheme to generate voltages as low as 1 V at currents up to 100 A with high efficiency. The unit provides a main output with as many as three auxiliary outputs, while operating from a universal (85- to 264-V ac) input. The supply, which measures just 7 by 4.5 by 1.35 in., achieves a current density that's more than twice that of other available ac-dc supplies. Plus, NET1 incorporates some key features to optimize system-level power-supply performance. These include the elimination of minimum-load requirements, single-wire current sharing on all outputs, the reduced demand for forced-air cooling, active power-factor correction, and dual-interconnect technology.

In terms of equivalent performance, there's no comparable power supply on the market, according to the company. But for a comparison on the basis of current capability alone, NET1 may be measured against the company's own SPM2, a 5- by 11- by 3-in. unit that generates 120 A on a single output (Fig. 1). That unit, though, is based on technology that's approximately 10 years old. Another unit, which serves as a reference point, is the more recently introduced 8- by 4.2- by 1.5-in. MPU150. This supply features a 35-A total current capability on its two main outputs.

A High Current Density
To put these numbers in perspective, consider current density. For the SPM2, with a 120-A output at 3.3 V, current density is 0.7 A/in.3 Meanwhile, the MPU150, with main outputs of 30 A at 2.5 V and auxiliary outputs of 2 A at 12 and 10 A at 5 V for a total of 42 A, delivers 0.8 A/in.3 Compare these values with that of the NET1. In one four-output configuration, this supply generates 50 A at 1.8 V, 40 A at 3.3 V, 1 A at 5 V, and 1 A at 12 V, for a total of 92 A. The resulting current density is 2.2 A/in.3 for this NET1 model.

Two of the innovations responsible for the supply's high efficiency are the zero-voltage soft-transition forward converter and the independent self-driven synchronous-rectification circuits. The forward-converter circuit of NET1 is a variation of the popular two-transistor forward-converter architecture (Fig. 2).

The zero-voltage soft-transition variation reduces FET stresses at turn-on and turn-off to improve efficiency and reliability. It does this by reshaping the PWM pulses used in zero-voltage switching of the MOSFETs. Dubbed EDGE (short for efficient dual geometric edge), this technology "softens" or smooths the leading and trailing edges of the PWM pulses, resulting in improved efficiency.

The resulting forward-converter circuit eliminates losses associated with switching and the body capacitance of the MOSFETs. The remaining losses are on-state losses, which are proportional to the on-resistance of the transistors. This forward converter exhibits greater than 90% efficiency. The reduced voltage and thermal stresses provided by this scheme combine with the power supply's low component count to provide a high degree of reliability as calculated per MIL-STD 217.

The independent self-driven synchronous rectifier is a novel topology that can efficiently generate supply voltages as low as 1 V and as high as 48 V. Like other synchronous-rectifier schemes, this architecture lowers output losses and increases current capabilities by replacing Schottky diodes with low-RDS(ON) MOSFET switches.

Yet the synchronous rectifier of NET1 differs from existing self-driven synchronous rectifiers. The latter circuits are designed to operate with an optimum reset voltage across their transformer such that the voltage on the transformer never stays at zero. But the zero-voltage condition is one that commonly occurs in forward-converter circuits. When it does occur, the efficiency of the synchronous rectifier is degraded as it loses gate drive and current begins to flow in the MOSFET's body diode.

Driver Circuit Modified
In the synchronous-rectifier circuit of NET1, the driver circuit is modified to account for zero-voltage conditions. The company has developed two versions of the circuit—one for employment with forward-converter circuits and another for use with full-bridge circuits (Fig. 3).

In the independent self-driven synchronous configuration, the output voltage is self-adjusting, which allows accurate current sharing in redundant power systems. This feature eliminates the need for ORing diodes that lower overall power-system efficiency, while providing current sharing on all outputs. The shared output currents are typically balanced to within 10% tolerances when up to six units are paralleled.

Another advantage of self-driven synchronous rectification is that the outputs can sink as well as source current. As a result, there's no need to preload the supply with external resistors, such as those usually found in redundant power systems. Taken together, all of the improvements introduced in the forward-converter and synchronous-rectifier circuits provide a significant boost in efficiency when compared with traditional designs based on Schottky-diode or synchronous-rectification circuits (Fig. 4). Although efficiency varies with the distribution of loads on the output, power-supply efficiency under full-rated loads is typically 80%.

Such improvements are largely responsible for NET1's compact packaging in the narrow 1U height. With higher efficiency comes reduced heating, which leads to smaller, more densely packed components that require less heatsinking. And with less heat dissipation, the need for forced-air cooling is reduced too. Consequently, the fan necessary to cool NET1 is significantly smaller than the one used to cool the SPM2. The older supply requires a 60-mm fan, while the newer unit needs only a 40-mm fan.

Other design factors also help the NET1 achieve small size. Interleaved PWM reduces peak capacitor currents at the input and reduces the size of input capacitors. The use of a low-leakage laminated foil transformer reduces the package height of the transformer, while low-loss spiral-wound inductors provide low-profile output filtering.

The independent, self-driven synchronous rectifier also delivers very fast transient response. This feature is important in low-voltage high-speed circuits, particularly when a decreasing step-load occurs. For example, a step-load reduction from 25 to 5 A can generate an appreciable voltage swing in the output of a conventional power supply thanks to the existing energy stored in the output filter (Fig. 5). Because it can sink energy from the output filter, the NET1's synchronous rectifier can maintain tighter regulation than a conventional supply, reducing transients by 60% for the cited step-load.

Active Power-Factor Correction
Another feature that adds to the value of the NET1 is built-in active power-factor correction (PFC). Offering a minimum power factor of 0.95, the NET1 meets the EN61000-3-2 requirements for European power-line harmonics. These regulations go into effect January 1, 2001. In addition, PFC allows the power supply to accommodate the square-wave outputs typically produced by uninterruptible power supplies (UPSs) and battery-backup systems.

In designing the NET1, consideration also was given to the assembly of the supply within the application. To this end, the company developed a dual-interconnect technology that allows connection of the main outputs using either Fast-On style connectors or screw fasteners.

Price & Availability
The NET1 is priced at $195 each in quantities of 1000. Evaluation units are available in four to six weeks after receipt of an order.

Power-One, 741 Calle Plano, Camarillo, CA 93012; Contact Maggie Nadjmi at (800) 678-9445, ext. 4230; [email protected]; www.power-one.com.

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