In many electronic devices, there is a need for a conversion in voltage level. Many of these devices use voltages that are much less than the mains electricity level. Step down converters should be employed to achieve suitable voltages and direct current which can be used in electronic devices. Step down converters can be designed by two major topologies, Linear and switching.
The older linear converters are based on dropping the voltage difference between the input voltage and the desired output voltage on some active devices such as power BJTs and MOSFETs. The whole load current would pass these transistors, so the input voltage should be close enough to output voltage to avoid extensive heat and high power loss which lead to low efficiency.
Using stepping down main-frequency transformers is common in linear converters to convert the input voltage to a few volts above the desired output voltage. This difference is called Head room voltage. There is a minimum for this voltage to allow the regulator to work properly. It is about 2.5 volts in commercial integrated linear regulators such as the 78xx series.
Linear voltage converters have many drawbacks. Using Heavy large low frequency transformers, converting only higher voltages to lower ones, extensive power dissipation in series-pass transistors and the need of large output capacitors to limit the ripple voltage, are among these drawbacks.
Although linear regulators are yet in use, especially in small low power onboard regulators and integrated circuits, but most of the converters today are of the switching type.
If we consider a switching converter as a system (Fig 1), it contains an additional control input along with its input and output ports. The control block compares the output with a reference which is proportional to desired output and produces a control signal for power blockto achieve the designed output. In addition to this feedback from output, in most cases there is need for a feed forward signal from input to let the system "knows" the variations in input.
FIG1. A Switching converter's blocks