Switch Mode Power Supplies.

Introduction - Some Definitions.

Switch Mode Power Supplies are the current state of the art in high efficiency power supplies. Conventional series-regulated linear power supplies maintain a constant voltage by varying their resistance to cope with input voltage changes or load current demand changes. The linear regulator can, therefore, tend to be very inefficient. The switch mode power supply, however, uses a high frequency switch (in practice a transistor) with varying duty cycle to maintain the output voltage. The output voltage variations caused by the switching are filtered out by an LC filter.

SMPSs can be used to step-down a supply voltage, just as linear supplies do. Unlike a linear regulator, however, an SMPS can also provide a step-up function and an inverted output function. Typical applications are given below.

Typical application for a ste-down switching regulator:

Generation of 5V for TTL-based circuits from a 12V battery (particularly suitable if the 12V battery has limited capacity, as switching regulators are far more efficient than linear regulators).

Typical application for a step-up switching regulator:

Generation of 25V from a 5V supply in an EPROM programmer.

Typical application for an inverting switching regulator:

Generation of a double-ended supply from a single-ended for OP-AMP.

Generation of a negative bias for MOS devices eg Dynamic RAMS.

The term switch mode regulator is used to describe a circuit which takes a DC input and provides a DC output of the same or opposite polarity, and of a lower or higher voltage. Switch Mode regulators use an inductor and there is no input to output regulation.

The term switch mode converter is used to describe a circuit which takes a DC input and provides a single or multiple DC outputs, again of same or opposite polarity and lower or higher voltage. Converters use a transformer and may provide input to output isolation.

The term Switch Mode Power Supply or SMPS is used to describe switch mode regulators and converters.

Source http://www.hills2.u-net.com/electron/smps.htm