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.

Different types of Converters

There are many different types of DC-DC converter, each of which tends to be more suitable for some types of application than for others. For convenience they can be classified into a various groups, however.  For example some converters are only suitable for stepping down the voltage, while others are only suitable for stepping it up.

Non-isolating converters

The non-isolating type of converter is generally used where the needs to be stepped up or down by a relatively small ratio (say less than 4:1), and there is no problem with the output and input having no dielectric isolation.

Examples are 24V/12V voltage reduces, 5V/3V reduces and 1.5V/5V step-up converters.

There are five main types of converter in this non-isolating group, usually called the buck, boost, buck-boost, Cuk and charge-pump converter.

The buck converter is used for voltage step-down/reduction, while the boost converter is used for voltage step-up. The buck-boost and Cuk converters can be used for either step-down or step-up, but are essentially voltage polarity reversers or ‘inverters’ as well.

The charge-pump converter is used for either voltage step-up or voltage inversion, but only in relatively low power application.

 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.