The Difference Between a Linear Supply and a Switched Mode Power Supply

There are two broad types of DC Power supplies: linear and switched-mode. Both designs use different techniques to achieve the same result: convert AC power to DC power. Depending on the application, each power supply type enjoys a few advantages. This article will briefly go through the difference between linear and switched-mode power supplies.

Linear power supply

A large transformer (as per power needs) is used in a linear supply to reduce voltage from the AC line to a lower AC voltage. A clean DC voltage is subsequently produced through a rectifier circuitry and the filtering process.  These power supplies are designed for extreme low noise due to absence of high frequency switching.

The linear power supply has a few drawbacks: the sink, an intrinsic part of the voltage regulator, increases the power supply size. The voltage regulator dissipates power leading to ohmic losses with the consequent rise in temperature. Dissipation due to a variable resistor reduces the efficiency of the linear power supply. They are used where low ripple, low electromagnetic emissions, and excellent regulation is needed. Since transformer size is indirectly proportional to operational frequency, this kind of power supply has a large footprint and is heavy.

Switch Mode Power Supply (SMPS)

Switched-mode power supplies (SMPS for short) are designed to be small and are extremely efficient. A switching regulator is included for better conversion of electrical power.

SMPSs use Pulse Width Modulation (PWM) to regulate the output voltage. PWM permits multiple topologies such as forward converter, flyback, buck, boost, or a half bridge rectifier, depending on output power needs. The PWM process leads to high frequency noise, but also enables the building of switching power supplies with excellent power efficiency and small form factor.

A switched mode power supply can enjoy superb line and load regulation. An SMPS has better efficiency compared to linear regulators as negligible power is dissipated by the switching transistor. This switching, however, can generate noise which can be reduced by filtering.

The defining factor that differentiates an SMPS and a linear power supply is the working procedure. High voltage AC is converted, using a transformer, into low voltage in the linear power supply. It is subsequently converted into DC voltages, while the SMPS first converts AC into DC and then transforms the DC voltage to the desired voltage level.

SMPSs find wide application in DC Motors and mobile chargers. In contrast, a linear power supply is used in high frequency applications such as a Radio Frequency application. Size is another significant differentiating factor between an SMPS and a linear power supply. The SMPS is light while the word “bulky” is the best description for a linear power supply. The SMPS is portable and can be effortlessly used anywhere while the linear power supply may be used solely for electronic, laboratory, or facility-sized electrical power circuits.

If the equipment is to be used globally, then the SMPS works all over the world. A linear power supply, in contrast, must be adjusted by manual means so that it can work with any foreign power grid. The list of applications which prefer an SMPS are the general purpose ones utilized in R&D, high power/high current applications, manufacturing, testing, and communication systems. The technology finds broad preference when it comes to network equipment, mobile stations, electroplating, anodizing, electroforming, electrophoresis, electrolysis, waste treatment, hydrogen generator, fuel cell applications, DC motors, aviation, and shipboard/boat applications. The following compression chart describes the relation between an SMPS and a linear power supply.