The simple dc/dc converter in Fig 1 has an efficiency of 90% min and output ripple measured at 10 mV p-p.
The simple dc/dc converter in Fig 1 has an efficiency of 90% min and output ripple measured at 10 mV p-p.
This circuit employs an inexpensive 78L15 regulator in an unusual manner. The regulator, IC2, operates as a voltage-level shifter. Because the regulator's normal output is tied to ground, the regulator tries to maintain its input at its characteristic 15V above its voltage-reference (VR) pin's level by drawing the appropriate current through biasing resistor R1. You can use this trick with any member of the 78LXX family.
The regulator is part of a feedback loop that controls the circuit's pulse-width modulator. The pulse-width modulator comprises Q1, gate IC1B, and associated components. The Cúk configuration of the output stage, comprising Q2, Q3, and the transformer, has inherently low input and output ripple.
Oscillator IC1A and associated components, which feed the pulse-width modulator, operate at a fixed frequency of 350 kHz, simplifying output-noise filtering. Because of this relatively high frequency, use low-ESR, tantalum-chip capacitors instead of electrolytic capacitors. Similarly, use a fast epitaxial diode for D1.
A 74HC14 is preferable for IC1, but the brave old 74LS14 works (component values shown are for the LS version). Select a fast switching transistor for Q3, which can sustain IC>1A, VCE>60V.
Transformer T1 is a toroid, 11 mm in diameter having 0.3-mm enameled wire for the primary and secondary windings. Your circuit's layout must be as compact as possible, having no long runs. (DI #1579)