A lot of electronic devices need DC supply, they get the supply from battery or home power Electricity (which sometime is in AC form). If the electronic device need a DC supply which is in a different voltage level from the DC voltage that electronic device require, it would be need an extra amount device that can adjust the DC supply to match with the DC voltage which required by the electronic device. This an extra amount device is known as DC DC converter.

There are 2 kinds of DC DC converter based on voltage level. First is step down converter; output voltage is smaller than input voltage (Vin>Vout). Second is step up converter; output voltage is bigger than input voltage (Vin<Vout).

There are 2 methods for getting the desired voltage level: Linear and Switching.

Switching and linear regulators use fundamentally different techniques to produce a regulated output voltage from an unregulated input. Each technique has advantages and disadvantages, so the application will determine the most suitable choice. Linear power supplies can only step–down an input voltage to produce a lower output voltage. This is done by operating a bipolar transistor or MOSFET pass unit in its linear operating mode; that is, the drive to the pass unit is proportionally changed to maintain the required output voltage. Operating in this mode means that there is always a headroom voltage, Vdrop, between the input and the output. Consequently the regulator dissipates a considerable amount of power, given by (Vdrop Iload). This headroom loss causes the linear regulator to only be 35 to 65 percent efficient. However, linear regulators are cost–effective in step–down applications.

Designing with a linear regulator is simple and cheap, requiring few external components. A linear design is considerably quieter than a switcher since there is no high–frequency switching noise. You can read and learn the application of linear regulator from +5 VDC and +12 VDC power supply for TTL and CMOS

S W I T C H I N G :

The switching regulator consists of a stable reference and a high gain error amplifier identical to that of the linear regulator. This system differs in that a free running oscillator and a gated latch have been added. The error amplifier again monitors the output voltage, compares it to the reference level and generates a control signal. If the output voltage is below nominal, the control signal will go to a high state and turn on the gate, thus allowing the oscillator clock pulses to drive the series–pass element alternately from cutoff to saturation. This will continue until the output voltage is pumped up slightly above its nominal value. At this time, the control signal will go low and turn off the gate, terminating any further switching of the series–pass element. The output voltage will eventually decrease to below nominal due to the presence of an external load, and will initiate the switching process again.

The increase in conversion efficiency is primarily due to the operation of the series–pass element only in the saturated or cutoff state. The voltage drop across the element, when saturated, is small as is the dissipation. When in cutoff, the current through the element and likewise the power dissipation are also small.

Switching power supplies operate by rapidly switching the pass units between two efficient operating states:  cutoff, where there is a high voltage across the pass unit but no current flow; and saturation, where there is a high current through the pass unit but at a very small voltage drop. Essentially, the semiconductor power switch creates an AC voltage from the input DC voltage. This AC voltage can then be stepped–up or down by transformers and then finally filtered back to DC at its output. Switching power supplies are much more efficient, ranging from 65 to 95 percent. The downside of a switching design is that it is considerably more complex. In addition, the output voltage contains switching noise, which must be removed for many applications. Although there are clear differences between linear and switching regulators, many applications require both types to be used. For example, a switching regulator may provide the initial regulation, then a linear regulator may provide post–regulation for a noise–sensitive part of the design, such as a sensor interface circuit. You can read and learn the application of switching regulator from Switch Mode Power Supply (SMPS): Step-Up or BOOST DC DC Converter and Switch Mode Power Supply (SMPS): Step-Down or BUCK DC DC Converter

SMPS TOPOLOGY

The DC-DC converter topologies can be divided in two major parts, depending on whether or not they have galvanic isolation between the input supply and the output circuitry.

According to the position of the switch and the rectifier, different types of voltage converters can be made:

I.1 The "Buck" converter: Step down voltage regulator

The power device is switched at a frequency f = 1/T with a conduction duty cycle, d = t on /T. The output voltage can also be expressed as: V out = V in . d

In normal operation, the energy is fed from the inductor to the load, and then stored in the output capacitor. For this reason, the output capacitor is stressed a lot more than in the Buck converter.

 Actually there are many SMPS types which I can not write and explain here. I suggest you to read articles, application notes, power supply handbooks etc. I get SMPS knowledge mainly from application notes. You can find it in Semiconductor  website, such as National Semiconductor, Texas Instrument, etc. You may find those link in my Link.