ECRI Microelectronics

The working principle of step-down switching power supply

A linear regulated power supply is a regulated power supply in which the regulating tube operates in a linear state. In the switching power supply is different, the switch tube (in the switching power supply, we generally call the adjustment tube called the switch tube) is working in the open and closed state: open - the resistance is small; off - the resistance is very Big.

Switching power supply is a relatively new type of power supply. It has the advantages of high efficiency, light weight, high and low voltage, and high output power. However, since the circuit operates in the switching state, the noise is relatively large. Through the following figure, let's briefly talk about the working principle of the step-down switching power supply. As shown in the figure, the circuit is composed of a switch K (a triode or a field effect transistor in the actual circuit), a freewheeling diode D, a storage inductor L, and a filter capacitor C. When the switch is closed, the power supply supplies power to the load through the switch K and the inductor L, and stores part of the electrical energy in the inductor L and the capacitor C. Due to the self-inductance of the inductor L, after the switch is turned on, the current increases relatively slowly, that is, the output cannot immediately reach the power supply voltage value. After a certain period of time, the switch is turned off. Due to the self-inductance of the inductor L (which can be compared with the image, the current in the inductor has an inertial effect), the current in the circuit will remain unchanged, that is, the flow will continue from left to right. This current flows through the load, returns from the ground, flows to the positive terminal of the freewheeling diode D, passes through the diode D, and returns to the left end of the inductor L, thereby forming a loop. The output voltage can be controlled by controlling the time the switch is closed and open (ie, PWM - Pulse Width Modulation). If the output voltage is controlled by detecting the output voltage to keep the output voltage constant, this achieves the purpose of voltage regulation.

During the closing of the switch, the inductor stores energy; during the opening of the switch, the inductor releases energy, so the inductor L is called the energy storage inductor. Diode D is responsible for providing a current path to inductor L during the off period of the switch, so diode D is called a freewheeling diode.

In the actual switching power supply, the switch K is replaced by a triode or a field effect transistor. When the switch is off, the current is small; when the switch is closed, the voltage is small, so the heating power U& Times; I will be small. This is why the switching power supply is efficient.