AN703

AN703 Vishay Siliconix AN703 www.DataSheet4U.com Designing DC/DC Converters with the Si9110 Switchmode Controller connected through a diode to the power bus to provide emergency back-up. The distributed power approach is employed in telecom systems, large minicomputers, and in other applications where reliability is a primary concern. To illustrate some of the performance capabilities of this BiC/ DMOS switchmode controller IC, a 15-W forward converter design is presented. The converter provides +5-V and ±12-V outputs from a 9- to 36-V input range. This permits the power supply to operate from 12-V or 24-V batteries, or from a 28-V aircraft power source. Before describing the forward converter example, it is instructive to review the operation of each of the Si9110 switchmode controller’s functional blocks. In distributed power systems and battery-powered equipment, the advantages of MOS over bipolar technology for pulsewidth modulation (PWM) controllers are significant. First, by using a BiC/DMOS power IC process, a high-voltage DMOS transistor can be integrated with a CMOS PWM controller to serve as a pre-regulator stage. This reduces the number of external components by permitting the power controller IC to interface directly to the power bus. The second advantage of MOS is speed. Bipolar PWM controllers can be made fast, but only with a significant increase in supply current. Logic gate delays of 5 ns are readily achievable using 5-µm CMOS, comparator propagation delays...