Design Ideas: July 7, 1994
The low-noise switcher in Fig 1 works much like a TV's horizontal-deflection circuit to provide clean power for a 16-bit A/D converter.
In operation, Q3 is a logic-level gate-drive MOSFET driven by the same 100-kHz square wave that clocks the A/D converter. When Q3 turns off, the primary of T1 resonates with C3 and flies back to 25V. Energy then transfers to the transformer's secondary. When the flyback voltage attempts to go below ground, the integral body diode of the power MOSFET Q3 clamps the flyback voltage, causing excess resonant energy in the transformer to flow back through its primary, recharging C1. While C1 is recharging and the drain of Q3 is nearly at 0V, the gate of Q3 gets turned on again, repeating the cycle.
Q1 adjusts T1's input voltage to achieve regulation in response to the error amplifier's comparing the output voltage to a 2.5V reference (not shown). IC1, Q2, and associated components form the error amplifier. Because Q1 is a linear regulator, you must trim C3's maximum voltage to achieve the best efficiency. Trim C3 to raise T1's input voltage to above 4V. At 4.5V, 70% efficiency is possible.
T1's construction is not critical. The primary consists of seven turns of 24 AWG, and the secondary has 12 turns of 24 AWG with the ground tap at six turns. The core is a Philips E187 3C85, and the gap in the outer legs of the core measures 2 mils. The primary's inductance should be around 12 µH. C3 must be a high-quality, low-inductance electrolytic, suitable for switching supplies. (DI #1451)