Small power converter
The converter enables an existing positive supply voltage to be raised, to be lowered, or to be changed into negative potential.
The new voltage is electrically isolated from source by a DIY transformer wound on a G2-3FT12 toroid. The primary winding consists of 30 turns. The number of secondary turns, n, is calculated from
where U0 is the wanted voltage and U1 is the input voltage. Increase the number of turns so found by 10-20 to compensate for losses. the output voltage is somewhat too large, an always be reduced with P1 Both windings may he wound with 0.3 mm Dia. enameled copper wire. Make sure that the turns both are evenly distributed along the core. The transformer is driven by a CMOS Schmitt trigger NAND gate that has been convened to a rectangular-wave generator by R1 and C1. MOSFET T1 serves as an output stage. Additional charging current for C1 is provided via R2 and P1, which control the duty factor of the rectangular signal. The frequency of this signal is about 220 kHz and its duty factor must be smaller than 0.5.
When T1 is switched on, some energy is transferred to the secondary winding and some are stored in the magnetic field. When T1 stops conducting, the energy in the magnetic field is transferred to the secondary winding.
The idea is to make the duty factor small enough to ensure that all energy stored in the field is transferred before T1 switches on again. If not, there is the risk that the residual magnetic field becomes stronger and stronger, which causes the core to become saturated and that in turn lowers the efficiency. Also, owing to the reduced inductance at the primary side, the current through T1 will rise appreciably, which means the end of the transistor.
The current through T1 will also rise dangerously when the secondary load is too heavy. The average current through the primary should not exceed 150 mA (the peak current may be several times larger). From the turns ratio, it is easily calculated what the maximum secondary load current should be. With the ratio shown in the drawing (10% extra turns at the secondary side), the secondary load should be not smaller than 80 Q.
Apart from a heavy load, no load is also to be avoided. In that case, the energy stored in the magnetic field can only move to C2 where it is stored in the electric field. This means that the charge on C2, and thus the voltage across it, increases to a level where it can have a serious effect on the circuit to which the converter will be connected. As a rule of thumb (just as with the maximum load current), the indicated value of 1.5 is directly proportional to the turns ratio.
Type 1N4148 rectifier diodes are fast enough to cope with the frequency of 220 kHz (1N400x types are not). These diodes can stand a constant current of 200 mA (400 mA peak).
The efficiency of the converter with a supply voltage of 15 V is about 65%. When the load current is small, this drops to about 50%. The efficiency also drops when the supply voltage is lower than indicated.
The maximum input voltage is 15 V since the supply to neither IC1 nor T1 must exceed this value. The current drawn from a 15 V source by a load of 80 Q is about 165 mA. The prototype worked fine with the wiper of P1 turned completely to +; it may even be possible to lower the value of R2 slightly.
If necessary, the duty factor of the rectangular voltage may be increased somewhat with P1. While this is being done, the current through T1 should be watched carefully, preferably on an oscilloscope. If the current suddenly rises too fast, the core is becoming saturated and this means that P1 must be turned back slightly. Bear in mind that when P1 is the set to a critical position, a slight change in 11 loads can drive the core into saturation, with all the consequences of this.