Linear vs. Switching Power Supply

Power supply is an important part of the circuit design, and the stability of the power supply determines the stability of the circuit to a large extent. Linear power supply and switching power supply are two common power supplies, and they have a great difference in the principles which determine the difference between the two applications.

Working principle
Linear power supply consists of industrial frequency transformer, output rectifier and filter, control circuit, and protective circuit, etc. The basic principle of linear power supply is that a commercial power is stepped down to a low voltage AC power through the industrial frequency transformer, then it is rectified and filtered to a DC, and finally a stable low-voltage DC is output through the voltage stabilizing circuit. Adjusting components in the circuit work in a linear state.

Switching power supply is composed of input power grid filter, input rectifier and filter, inverter, output rectifier and filter, control circuit, and protective circuit. The basic principle is that the AC is directly rectified into DC at the output side, and then under the action of high-frequency oscillating circuit, with the switching tube to control the current of the on-off, the high-frequency pulse current is formed. With the help of the inductor (high-frequency transformer), a stable low-voltage direct current is output.

Advantages and disadvantages

• Linear power supply
  Advantages: relatively simple structure, small output ripple, small high-frequency interference. Simple structure means easy maintenance, that is, maintenance of a linear power supply is often much less difficult than the switching power supply and maintenance success rate of linear power supply is also much higher than one of the switching power supply.
  Ripple is the AC component that is superimposed on the DC steady-state. The smaller the output ripple is, the higher the output DC purity is, which is an important symbol of DC power quality. High ripple DC will affect the normal operation of the transceiver. Now the high-grade linear power ripple can reach the level of 0.5mV, general products can reach 5mV level. The linear power supply does not have a device that operates at high frequencies so there is almost no high frequency interference or high frequency noise if the input filter does well.
  Disadvantages: as large and heavy transformer is necessary, the volume and weight of the required filter capacitor is quite large. The voltage feedback circuit works in a linear state and the adjusting tube has a certain voltage drop, which result in high power consumption and low conversion efficiency when outputting a larger operating current. It also needs to install a large heat sink. It is not suitable for computers and other equipment, which will be gradually replaced by switching power supply.
• Switching power supply
  Advantages: small size, light weight, high efficiency, low energy consumption, stronger anti-interference ability, wide regulation range, and modularity.
  Disadvantages: the high-frequency voltage can be produced in the inverter circuit, which has some interference to the surrounding equipment, so good shielding and grounding are required. After rectification, AC can become DC. However, due to changes in AC voltage and load current, the rectified DC voltage typically results in voltage changes of 20% to 40%. In order to get a stable DC voltage, voltage regulator circuit should be used to achieve voltage regulation.

Applications
Linear power supply operates in linear status. In other words, the power devices never stop once started, so they perform poorly in terms of their working efficiency which proves to be 50-60%. And it is also in big size, low efficiency and large heating. However, the linear power supply also has advantages. It generates less noise due to less ripples and satisfactory adjustment rate. The linear power supply is applicable to artificial circuit and various amplifiers.
Switching power supply is in small size, high efficiency, but with some ripple and interference. With the continuous development of electronic technology, switching power supply design is more and more scientific. Nowadays switching power supplies are applied more widely than linear power supply and become the mainstream in the all kinds of applications.