ByPyramid Electronics


 Distribution Transformer rating 3KVA EL / EOG / 4 – 74 with CS-1

DC-DC converters are electronic devices used whenever we want to change DC electrical power efficiently 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; where 5V DC on a personal computer motherboard must be stepped down to 3V, 2V or less for one of the latest CPU chips; where the 340V DC obtained by rectifying 240V AC power must be stepped down to 5V, 12V and other DC voltages as part of a PC power supply; where 1.5V from a single cell must be stepped up to 5V or more, to operate electronic circuitry; where 6V or 9V DC must be stepped up to 500V DC or more, to provide an insulation testing voltage; where 12V DC must be stepped up to +/-40V or so, to run a car hifi amplifier’s circuitry; or where 12V DC must be stepped up to 650V DC or so, as part of a DC-AC sinewave inverter.

In all of 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.
We can therefore represent the basic power flow in a converter with this equation:

Pin= pout+ Plosses
where Pin is the power fed into the converter, Pout is theoutput power and Plosses is the power wasted inside the converter.


Of course if we had a ‘perfect’ converter, it would behave in the same way as a perfect transformer. There would be no losses, and Pout would be exactly the same as Pin. We could then say that:
Vin x Iin = Vout x Iout

or by re-arranging, we get:
Vout/Vin = In/Iout
In other words, if we step up the voltage we step down the current, and vice-versa.

Of course there’s no such thing as a perfect DC-DC converter, just as there are no perfect transformers. So we need the concept of efficiency, where:


Nowadays some types of converter achieve an efficiency of over 90%, using the latest components and circuit techniques. Most others achieve at least 80-85%, which as you can see compares very well with the efficiency of most standard AC transformers.
Many different types
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 various groups, however. For example some converters are only suitable for stepping down the voltage, while others are only suitable for stepping it up; a third group can be used for either.


Another important distinction is between converters which offer full dielectric isolation between their input and output circuits, and those which don’t. Needless to say this can be very important for some applications, although it may not be important in many others.


In this data sheet we’re going to look briefly at each of the main types of DC-DC converter in current use, to give you a good overview. We’ll start first with those which don’t offer input-output isolation, and then progress to those which do.


Non-isolating converters

The non-isolating type of converter is generally used where the voltage needs to be stepped up or down by a relatively small ratio (say less than 4:1), and there is no problem with the the output and input having no dielectric isolation. Examples are 24V/12V voltage reducers, 5V/3V reducers and 1.5V/5V step-up converters.
There are five main types of converter in this non-isolating group, usually called the buck,boost,busk-boost, cuk and charge-pump converters. 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 Cuk converter is named after its originator, Slobodan Cuk of Cal Tech university in California.)
The charge-pump converter is used for either voltage step-up or voltage inversion, but only in relatively low power applications.
Buck converter
The basic circuit configuration used in the buck converter is shown in Fig.1. As you can see there are only four main components: switching power MOSFET Q1, flywheel diode D1, inductor L and output filter capacitor C1. A control circuit (often a single IC) monitors the output voltage, and maintains it at the desired level by switching Q1 on and off at a fixed rate (the converter’s operating frequency), but with a varying duty cycle (the proportion of each switching period that Q1 is