Introduction and principle analysis of switching regulated power supply

What is a switching power supply?

According to the working state of the adjustment tube, we often divide the voltage stabilized power supply into two categories: linear voltage stabilized power supply and switching voltage stabilized power supply. Switching power supply is a relatively new type of power supply. After the advent of switching power supply (hereinafter referred to as switching power supply), it has gradually replaced linear voltage stabilized power supply and thyristor phase-controlled power supply in many fields. In the early days, the series switching power supply appeared, and its main circuit topology was similar to that of the linear power supply, but the power transistor worked in the switching state. With the development of pulse width modulation (PWM) technology, PWM switching power supply came out. Its characteristic is that it uses a 20kHz carrier for pulse width modulation, and the efficiency of the power supply can reach 65%~70%, while the efficiency of the linear power supply is only 30%~40%. Therefore, replacing the linear power supply with a PWM switching power supply with an operating frequency of 20 kHz can greatly save energy, which has attracted widespread attention and is known as the 20kHz revolution in the history of power supply technology development.

As the size of ultra-large-scale integrated (ULSI) chips continues to decrease, the size of power supplies is much larger than that of microprocessors; and aerospace, submarines, military switching power supplies, and portable electronic devices using batteries (such as laptops, mobile phones, etc.) require miniaturized and lightweight power supplies. Therefore, small and lightweight requirements are put forward for switching power supplies, including small volume and weight of magnetic components and capacitors. In addition, switching power supplies are also required to have higher efficiency, better performance, and higher reliability. All these high-tech requirements have promoted the continuous development and progress of switching power supplies.

Principle of Switching Power Supply

Through the figure below, let's briefly talk about the working principle of the step-down switching power supply. As shown in the figure, the circuit consists of switch K (a triode or field effect tube in the actual circuit), freewheeling diode D, energy storage inductor L, filter capacitor C, etc. When the switch is closed, the power supply supplies power to the load through switch K and inductor L, and stores part of the electric energy in inductor L and capacitor C. Due to the self-inductance of inductor L, the current increases slowly after the switch is turned on, that is, the output cannot reach the power supply voltage value immediately. After a certain period of time, the switch is disconnected. Due to the self-inductance of inductor L (it can be more vividly considered that the current in the inductor has an inertia effect), the current in the circuit will remain unchanged, that is, it will continue to flow from left to right. This current flows through the load, returns from the ground wire, flows to the positive pole of the freewheeling diode D, passes through diode D, and returns to the left end of inductor L, thus forming a loop. By controlling the time of switch closing and opening (i.e. PWM-pulse width modulation), the output voltage can be controlled. If the time of opening and closing is controlled by detecting the output voltage to keep the output voltage unchanged, the purpose of voltage regulation is achieved.