A Guide to Switched Mode Power Supplies - By: S.Roberts

Switch mode power supplies are much more efficient than linear power supplies and are available from very lower power (1W) through mass market (around 50-300W) and high power units above 1KW. Switched mode power supplies are often referred to as SMPS and power supplies may be seen by the term PSU.

AC/DC switch mode power supplies convert alternating current (e.g. Mains electricity) into a stable DC voltage. DC/DC converts either increase or reduce the input voltage to provide a new output voltage level. DC/DC converters are often used for converting a battery power source (e.g. car battery) to a stable DC output voltage to power some electronics. As the battery voltage may vary between 9 and 16V DC, the output voltage remains stable.

Both AC/DC and DC/DC power supply types may have one or more output voltages. Single outputs are commonly 3.3V, 5V, 12V and 24V. Dual output power supplies are usually configured as 5V/12V, 3.3V and 5V or dual +12V/-12V.

Each output has a maximum available output current Iol and for multi-output power supplies each output voltage may have a different value for the maximum output current. For instance, a power supply may have a main output at 3.3V @ 100W but a secondary output of 5v but at only 10W.

Ultimately, the power supply is limited by heat dissipation and therefore many higher power switched mode power supplied may include fan cooling. Modern switched mode power supplies typically run at efficiencies of around 90 of the required output power).

Power Supply Monitoring and Control
Modern switch mode power supplies include external monitoring and control features. Remote control is available in the form of an input which may enable/disable certain outputs or put the power supply into a low power 'sleep' mode.

Remote monitoring can test the output voltage is within a required range and provide a relay output if the output voltage is out of range. Overload, over voltage and other errors can also be signalled.

Bench Power Supplies
Bench power supplies are switched mode power supplies where the user has fine control over the output voltage and current limit. These are used by electronics/electrical engineers and scientists in design and testing of products.

Bench power supplies usually have an LED read out to show the current limit and output voltage and have one or more output voltage.

Linear Power Supplies
One of the big disadvantages of switch mode power supplies is all of the switching causes electrical noise. For applications which require very low noise outputs a linear power supply is preferable.

Linear power supplies are typically available up to 200W maximum and may have dual output +/-12V for instance.

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