Power Supplies Tutorial No.1 (Switch-mode or Linear)

29.04.18

Linear v Switch mode

DC power supplies basically come in two types, either linear or switch-mode. While they both server the same purpose, to provide a stable DC supply, the way they generate this is quite different. Depending on your application they both can have advantages and disadvantages over each other. Lets compare the two.

Linear

A linear power supply design applies the Mains AC supply to a power transformer to step up or down the voltage to the required level prior to it being applied to the regulator circuitry. 

Linear power supply circuit

Switch-mode

In the case of a switch-mode design, the Mains AC supply is converted directly into DC voltage without using a transformer. This raw DC is then converted into a high frequency AC signal that is used by the regulator circuitry to produce the desired supply voltage. A switch-mode power supply also incorporates a transformer however as the size of a transformer is inversely proportional to the operating frequency the transformers are much smaller.

switch-mode power supply circuit

Comparisons

As a result of the difference in transformers, the most obvious difference between a linear and switch mode supply is weight. As the transformer in a linear supply is operating at 50/60 Hz the difference in size and weight is considerable. A switch-mode power supply can be as much as 80% smaller and lighter than a corresponding linear power supply. If weight and/or size is a factor, then a switch mode supply should be strongly considered.

Linear power supplies employ larger semiconductor devices in the regulator circuitry to regulate the output voltage. This therefore generates more heat, resulting in lower energy efficiency.  This coupled with the smaller high frequency transformers in switch mode supplies being considerably more efficient so this affects the overall efficiency of the power supply. A linear power supply normally operates around 60% efficiency for 24V outputs, whereas a switch-mode power supply operates at 80% or more.

One downside of the switch mode power supplier is because they utilise the high frequency AC signal in the regulation process they can generate high-frequency noise. If the supply is to be used with sensitive electronic equipment then consideration should be given to whether a linear supply may be a better choice. 

Another advantage of linear supplies is the ability to respond to transients. In the case of the linear supply the response time can be up to 100 times faster than switch mode designs.

Rule of Thumb

There are always exceptions to the rule and careful consideration needs to be given when selecting a power supply design however in general, switch mode supplies are well suited for portable equipment as they are lighter and more compact. Linear designs are generally better suited for powering sensitive equipment due to the inherently lower electrical noise characteristics.

 

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