Transformerless Voltage Booster Circuit: A DC-DC Step-Up Switching Regulator Using Transistors

A DC-to-DC step-up converter is traditionally implemented using transformer, working by converting the DC voltage to AC Voltage, step-up it using transformer, then rectify and filter the transformer’s output to  get a higher DC voltage. Using a switching method, we can step-up a voltage without a transformer. We just need an inductor which is driven by a switching transistor to boost the voltage.  This circuit store the electric energy in an inductor before it is added to the input voltage, so this circuit is different than capacitor  charge pump that store the electric energy using a capacitor.

The most interesting this is that circuit use a discrete components: no integrated chip is required, only few transistors with few passive components. Because the switching topology is a boost converter, this circuit cannot be operated as step-down regulator, so the output will always be higher than the input. The voltage output is depend on the load because the feedback mechanism, through the zener diode, will maintain the output at about 14 volt, regardless the voltage input variation and load current variation. The current from the voltage divider will flow through the zener diode if the output goes higher than the nominal value, and this condition will stop the oscillator built around the 2N3904 transistors. Stopping the oscillator will drop the output voltage and thus maintain the required voltage level at the output. This transistors (Q1, Q2, and Q3) form a  Schmidt trigger that drive the final transistor Q4 (the switching transistor 2N3053).

This circuit is suitable for battery booster, if you need to run your 12 volt equipments on your old car that provide only a 6V supply from the battery. The output of this voltage doubler can be adjusted by changing the voltage divider, or for easier adjustment, you can replace the 4,7K resistor with a 5K potentiometer. Using a good inductor (low resistance), you can achieve up to 80% efficiency, and up to 2 Watt power  can be  delivered to the load.   For the main diode, you can use a 3 Ampere  Schottky diode 1N5822. [Circuit’s schematic diagram source: Bill Bowden’s circuit collection]