12-12-2012, 02:23 PM
DC-DC converter to step up input voltage
A simple step-up converter.
This step-up converter is intended for use in a '67 Citroen 2CV. This car, and I use the word loosely, has a 6V battery and won't support a modern radio that needs 12V. The circuit described here converts 6V to 12V at 1A sustained load current.
It works something like this:
When the switch is closed an extra current flows through the inductance and stores energy there. The capacitor supplies the load with current during this time.
After the switch closes the capacitor is charged by the energy stored in the inductance and an extra current starts flowing through the load, causing the output voltage to rise (energy is supplied directly from the input source also as long as the diode is forward biased). During this time, the system behaves like a RLC-circuit, so, after a while, the current decreases. The switch is then closed again and the cycle repeats. One could say that charge is pumped from input to output, increasing the output voltage up to the point where there is an equilibrium between the discharging of the capacitor while the switch is closed and the charging by the inductor while the switch is open.
Input voltage versus output voltage
The first setup of this circuit used the solenoid and a 9v battery. The output was around 41v with no load. With the 60 turn coil, it exceeded 80v! (With a 50v rated capacitor for C2 I didn't leave this running for long...)
What I found more interesting was using a single AA cell (1.5v). You may get mixed results because the 74C14/40106 schmitt trigger inverter may not be able to work at voltages this low. One trick you can do is to power the 74C14/40106 from the output voltage. The only problem is that the circuit needs a kick start, since it can't step up voltage until the oscillator is running. I found that a 10k resistor across the diode (normally 0.6v drop) provided sufficient voltage to get the oscillator started, without affecting the output too much. Startup is not instantaneous because the output capacitor needs to charge. Another way to get it started is to temporarily connect a 3-9v battery in series with a 10k resistor to the supply rails of the 74C14/40106. This method is more reliable but it's not so practical since you need another power source. However, if you are using this circuit to do something like charge a 6v battery from a 1.5v solar cell, then you can simply power the 74C14/40106 from the battery.
Output stability and regulation
There is none. The output voltage will dip as you load it more. While you can use a series regulator such as a 7812 or 7805 on the output (take care not to exceed their maximum input voltage) I'm guessing that pulse width modulation (PWM) of the 40KHz square wave would be a more efficient method of regulation. I haven't looked into this yet as it adds more complexity to the circuit, requiring some sort of feedback to vary the duty cycle. A PIC or Atmel with an A/D converter and a spare output, plus some fancy programming, would probably do the trick.
Conclusion
This is a basic circuit only and it can be improved in many ways. Building it on a breadboard makes it easy to fiddle with values of components. Make sure you don't accidentally exceed the ratings of any components if you manage to find a variation that steps up 12v by 10 times!