What is a linear power supply and how does it work? what are the difference between linear and switching power supply

What is a linear power supply

All electronic circuit needs a source of voltage, especially the electronic devices which uses AC main supply. So they must be provided with a circuit that will operate on a different voltage rather than the standard AC supply, and this is achieved with a special kind of circuit known as power supply. This power supply is categorized into two groups. The linear power supply is the most common type of design, they are used for years for powering resistive, capacitive and inductive loads. Large filter capacitors are used for DC filtering, reserve current can provide extra currents for short periods of time without any damage. The output is also free of high-frequency noise which would make it practical to use with access controls. But the heat generated by the regulator component of the supply due to its moderate efficiency is a major disadvantage of the linear power supply. This generated heat would badly affect the heat-sensitive components.


How does a linear power supply work

The AC supply is routed into a transformer, which converts or steps down the voltage to the required level. The rating of the transformer is depended on the current requirement of the load so the transformer would be bulky. The voltage which is stepped down is rectified and filtered with the help of a bridge rectifier and electrolytic capacitors.

The power transistor would get the input from the filter. The output voltage is fed back to a comparator, which compares it with a standard reference and gives an error to the base of a transistor such that its conduction is increased or decreased. The major advantage of using a linear power supply is that its design is basic and simple, some disadvantages are their large physical size, their weight, heat generation, and tendency to be inefficient no more than 50 percent.

What is unregulated linear power supply

This type of power supply has four basic components they are transformer, rectifier, filter capacitor, and bleeder resistor. Unregulated linear power supply, due to its simplicity is less costly and reliable for low power requirements. The major disadvantage of this supply is that the output voltage is not constant, it will vary with the input voltage and the load current, and the ripple is not suitable for electronic applications. The ripple can be reduced by changing the filter capacitor to an IC filter but the cost of making this change would make the use of regulated linear power supply more economical.

What is regulated linear power supply

A regulated linear power supply is similar to the unregulated power supply, except that the three-terminal regulators are used in place of the bleeder resistor. The regulated linear power supply solves all the problems of the unregulated supply but is not as efficient because the dissipation of extra power in the three-terminal regulator is done in the form of heat. The output of the linear regulated power supply has a small ripple, load regulation is also small, and has high reliability so that it can be used for low power electronic application.

What is the difference between linear and switched power supply and what are the advantages of SMPS over linear power supply

SMPS design has a number of advantages over linear. Its high efficiency of switching transistors prevents significant energy wastage in the form of heat. SMPS has a higher operating frequency so lighter and smaller transformer is used instead of a bulky transformer. If AC mains supply is not possible, a high-frequency transformer with the correct turns ratio for the required output voltage must be used.

In a linear regulated circuit, the excess voltage from the unregulated DC input supply drops across a series element, so there will be power loss due to this voltage drop. While in the case of SMPS, by modulating the switch duty ratio the unregulated voltage portion can be removed. The switching losses in the SMPS is very small when compared to a linear lower supply. In an SMPS, it is possible to insert a high-frequency transformer to isolate the output and to scale the output voltage magnitude. But in the case of linear power supply, the voltage scaling transformer can only be put across the low-frequency supply. Low-frequency transformer is heavier and bulky while compared to a high-frequency transformer of a similar VA rating. Similarly, the low-frequency ripples are much bulkier than the high-frequency ripples. SMPS produces ripples of high frequency that can be removed easily by using a smaller volume of filtering elements.

There are some advantages for linear power supply over SMSP the control of the linear power supply circuit is much simpler than the SMPS circuit. In the linear power supply, there is no electromagnetic interference because there is no high-frequency switching, but is of some concern in the SMPS circuit. Output voltage regulation of the linear power supply is better than the SMPS.

What are the advantages of linear power supply

  • Very low output noise
  • Very low ripple

What are the disadvantages of linear power supply

  • Due to the bulky transformer the power supply unit is mostly bulky
  • Narrow input voltage range
  • Very low output hold uptime
  • Low efficiency
  • Dissipation of heat is high