Low power variable voltage converter

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Using this circuit diagram can be made a voltage converter that allows laps positive voltage of existing power sources, or converting a negative voltage.
New supply is electrically isolated from the source through the ferrite transformer wound on a torus G2-3FT12. Primary winding consists of 30 turns. Number of secondary turns, n, is calculated with the formula: n = 30 Uo / Ui, where Uo is the desired voltage and input voltage is Ui. Add to compensate loss 5/10/20 turns. If the output voltage is higher, it can always reduced by P1. Both coil can be from 0.3 mm of enamelled copper wire.
The transformer is controlled by a CMOS AND-NOT gate with Schmitt trigger transformed into a rectangular signal generator R1 and C1.
Additional current to charge C1 is provided by R2 and P1, which controls the duty cycle of the rectangular signal. The frequency of this signal is approximately 220 kHz and filling factor should be less than 0.5.
When T1 is open, some energy is transferred to the secondary winding and some is stored in the magnetic circuit. When T1 ceases conduction, energy is transferred to the magnetic field in the secondary winding.
Current through T1 will grow dangerously and if secondary task is too great. Average current through the primary should not exceed 150.
With this report in the scheme, secondary task can not be less than 80 ohms.
Besides excessive loads must also avoid working without load.
Converter efficiency at a supply voltage of 15 V is about 65%. With a low load current, it drops to about 50%. Efficiency also decreases when voltage is lower than indicated.

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Low power variable voltage converter

Current drawn from the source of 15 V for a load of 80 ohms is about 165 mA.

Circuit Diagram: 
Low power variable voltage converter


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