DC – DC CONVERTERS

DC – DC converters are electronic devices used whenever the need to change DC electrical efficiency, from one voltage level to another. They’re needed because unlike AC, DC can’t simply be stepped up or down using a transformer. In many ways, a DC –DC converter is the DC equivalent of a transformer.

Typical applications of DC-DC converters are where 24V DC from a truck battery must be stepped down to 12V DC  to operate a car radio, CB transceiver or mobile phone; where 12V DC from a car battery must be stepped down to 3V DC, to run a personal CD player.

In these applications we want to change the DC energy from one voltage level to another, while wasting as little as possible in the process. In other words, we want to perform the conversion with the highest possible efficiency.

An important point to remember about all DC-DC converters is that like a transformer; they essentially just change the input energy into a different impedance level.  So whatever the output voltage level, the output power all comes from the input; there’s no energy manufactured inside the converter. Quite the contrary, in fact – some is inevitably used up by the converter circuitry and components, in doing their job.

 Efficiency

The perfect DC-DC converter would be one where none of the incoming DC energy is wasted in the converter; it would all end up converted and fed to the output. Needless to say this doesn’t happen in the real world.

Inevitably practical converters have losses – voltage drops due to resistance in the inductor or transformer windings, ‘on resistance’ in the (metal-oxide semiconductor field-effect transistor) MOSFETs or forward voltage drop in the rectifier diodes.

MOSFETs switches are used in most of the converter circuits. That’s because modern MOSFETs make the most efficient electronic switches of high DC currents. When they’re turned off they are virtually an open circuit, and when they’re on they are very close to a short circuit – typically only a few milliohms. So they waste very little power.

DC-DC converters operate at a relatively high frequency, when compared with the 50-60Hz of the AC power mains.

When you use a high frequency, this allows the use of smaller inductors, transformers and capacitors in order to handle the same power level. This allows a reduction in both size and material cost of the converters.

Progress is being made all the time in developing materials and components that work efficiently at high frequencies.