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.