DC to DC step up

I want to build a step up dc - dc converter to run some amps in a car. Therefore it would need to step 12 volts dc up to about +/- 20 volts dc, and provide about 2-4 amps per rail.
Can anyone offer suggestions on what type of transformer, mosfets and switching arrangement I would need.
What sort of waveform needs to enter the primary of the transformer (square/sine/modified sine) and how is this acheived. What is the most suitable frequency of switching.
My idea was to build a sinewave generator, and pump this signal into a circuit or PIC to provide the mosfets a PWM signal which varied in duty cycle according to the sine wave, thus effectively providing the primary with a sine wave.

 

[johnday]

You will basically need to use a switch mode design at high frequency, about 25kHz or higher. These use square waves run by special purpose regulator IC's. The transformers are ferrite cored and will usually have to use either heavy copper wire or copper strap to handle the current. High current schottky diodes are used for rectification.

I cannot point you to specific data but Linear Technology may make the sort of controller IC's you need. Magazines such as ELektor may have published a design. I know I have seen these types of convertors in the magazines.

DO a search on SMPS (Switch Mode Power SUpply) and see what you come up with.

 

[Quique]

I have used the Motorola MC34063, which handles 500 mA with the built-in power transistor. You can use an external transistor, like the 2N2222 to boost current output.
With this device, you can build a step-up, step-down or negative switching power supply, with a single power supply of up to +40V.

With regards to the inductance, I have used a Coilcraft one. They manufacture a lot of models. You can choose the power you require, with your desired PCB footprint.

As this device switches power at 100 KHz, you should have care with the PCB design.

I hope this could help.

Enrique Schilling