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; maggie.nadjmi@power-one.com; www.power-one.com.