This DC-to-DC converter delivers a maximum power of about 36 watts at an efficiency of 90%. Apart from a modern FET and a Schottky diode, this circuit is comprised entirely of familiar and inexpensive parts. In spite of this, the specifications are excellent:
Efficiency: approx 90%
Ripple voltage: max. 10 mV
Output current: max. 1.5 A
Switching frequency: 40 kHz
Input voltage: 12 V
Output voltage: 24 V regulated
The switching element is a fast power FET (T8). This FET has a relatively high input capacitance and is switched on and off by a push/pull stage consisting of two RF transistors (T5/T6). Schottky-diode D2 increases turn-off speed even further, which is crucial here because we are aiming to obtain the highest possible efficiency.
The switching signal is provided by a simple multivibrator, which is also made from two RF-transistors (T1/T2). Difference amplifier T3/T4 has been added to obtain a regulated output voltage of 24 V.
L2 is an off the shelf 5 A suppressor choke with a self-inductance of 65 µH. L1 is part of the output filter, the purpose of which is to eliminate RF noise. This is an air-cored coil, which you can easily make yourself by winding 25 turns of 0.5 mm dia. enameled copper wire around a 10 mm diameter drill. Because of the high efficiency, the dissipation of T8 remains smaller than about 3.6 W so a modest heatsink of about 10 K/W will suffice. It is advisable that the 12 V input supply includes a fast fuse, rated at about 3.5 A.
Considering that the duty cycle has a substantial effect on the efficiency, a second capacitor (C3) has been added in parallel with C2. The optimum setting can be determined by varying this additional capacitor.
The remaining components are not at all critical. Any 5 A suppressor choke will work for L2, any 5 A Schottky-diode for D3 and just about any power MOSFET for T8 (BUZ10, BUZ20, BUZ100).
