6.6 A low-voltage computer power supply with synchronous rectification. The trend in digital integrated cir- cuits is towards lower power supply voltages. It is difficult to construct a high-efficiency low-voltage power supply, because the conduction loss arising in the secondary-side diodes becomes very large. The objective of this problem is to estimate how the efficiency of a forward converter varies as the output voltage is reduced, and to investigate the use of synchronous rectifiers. The forward converter of Fig. 6.22 operates from a de input of V = 325 V, and supplies 20 A to its de load. Consider three cases: (i) V = 5 V. (ii) V= 3.3 V, and (iii) V= 1.5V. For each case, the turns ratio n/n, is chosen such that the converter produces the required output voltage at a transistor duty cycle of D=0.4. The MOSFET has on-resistance R5 2. The secondary-side schottky diodes have Problems (c) 181 forward voltage drops of V,=0.5 V. All other elements can be considered ideal. (a) Derive an equivalent circuit for the forward converter, which models the semiconductor conduc- tion losses described above. (b) Solve your model for cases (i), (ii), and (iii) described above. For each case, determine numeri- cal values of the turns ratio n/n, and for the efficiency n. The secondary-side Schottky diodes are replaced by MOSFETs operating as synchronous recti- fiers. The MOSFETs each have an on-resistance of 4 m2. Determine the new numerical values of the turns ratio ,/n, and the efficiency , forcases (i), (ii), and (iii). eee ieee reler 46 D Fig. 6.22 Single-transistor forward converter. D D, L ror R +