Design Ideas: April 11, 1996
A low-power, isolated 9V supply for LAN applications (Figure 1) delivers more than 250-mA load current. For inputs of 10.8 to 13.2V and load currents of 1 to 200 mA, the nominal 8.78V output offers approximately ±1% line and load regulation. IC1's transformer-driver outputs, D1 and D2, normally directly drive each end of a transformer primary. Each driver terminal (on turn-off) sees a flyback voltage equal to twice the center-tap voltage. The 24V flyback voltage in this application exceeds the 12V rating for IC1, so the circuit uses two MOSFETs in cascode to stand off the extra voltage and maintain the high switching frequency of IC 1 (typically 650 kHz).
Surface-mount transformer T1 has a split primary, a single secondary, and a turns ratio of 1:1:1. This single-secondary approach requires full-wave-bridge rectification and a two-diode-drop reduction in output voltage. However, the alternative—a split secondary, half-wave rectification at each end and a one-diode drop in output voltage—adds an extra winding that increases the transformer cost. The single-winding-primary inductance should be high (about 250 µH) to limit stored-energy losses. (The ideal would be infinite inductance, which would allow pure transformer action with no energy loss during the switching cycles.) The diode bridge drives a low-dropout linear regulator, IC3, which provides the 9V regulated output for inputs of 5 and 12V ±10%. (DI #1853)