● What is a switching power supply?

        With the development and innovation of power electronics technology, switching power supply technology is also constantly innovating.

        At present, switching power supplies are widely used in almost all electronic devices with the characteristics of small size, light weight and high efficiency. It is an indispensable power supply method for the rapid development of today's electronic information industry.

        A switching power supply is a power supply that uses modern power electronics technology to control the time ratio of the switch tube to open and close to maintain a stable output voltage. A switching power supply is generally composed of a pulse width modulation (PWM) control IC and a MOSFET.

        A switching power supply is relative to a linear power supply. Its input end directly rectifies the AC power into DC power, and then under the action of a high-frequency oscillation circuit, the switch tube is used to control the on and off of the current to form a high-frequency pulse current.

        With the help of an inductor (high-frequency transformer), a stable low-voltage DC power is output. Since the size of the transformer's magnetic core is inversely proportional to the square of the operating frequency of the switching power supply, the higher the frequency, the smaller the core.

        In this way, the transformer can be greatly reduced, making the power supply lighter in weight and volume. And because it directly controls the DC, the efficiency of this power supply is much higher than that of a linear power supply.

        This saves energy, so it is favored by people. But it also has disadvantages, that is, the circuit is complex, difficult to maintain, and seriously pollutes the circuit. The power supply noise is large and is not suitable for some low-noise circuits.

● Characteristics of switching power supplies

        Switching power supplies are generally composed of pulse width modulation (PWM) control IC and MOSFET.

        With the development and innovation of power electronics technology, switching power supplies are currently widely used in almost all electronic devices with the characteristics of small size, light weight and high efficiency, and their importance can be seen.

● Classification of switching power supplies

        According to the way the switching device is connected in the circuit, switching power supplies can be generally divided into three categories: series switching power supplies, parallel switching power supplies, and transformer switching power supplies.

        Among them, transformer switching power supplies can be further divided into: push-pull, half-bridge, full-bridge, etc.

        According to the excitation of the transformer and the phase of the output voltage, it can be divided into: forward, flyback, single-excitation and dual-excitation, etc.

● The difference between switching power supply and ordinary power supply

        Ordinary power supply is generally linear power supply. Linear power supply refers to the power supply in which the adjustment tube works in linear state. But in switching power supply, it is different. 

        The switch tube (in switching power supply, we generally call the adjustment tube the switch tube) works in two states: on - very small resistance, off - very large resistance. 

        Switching power supply is a relatively new type of power supply. It has the advantages of high efficiency, light weight, voltage increase and decrease, and high output power. 

        However, since the circuit works in the switching state, the noise is relatively large. 

● Example: Buck switching power supply

        Let's briefly talk about the working principle of buck switching power supply: the circuit is composed of a switch (a triode or field effect tube in the actual circuit), a freewheeling diode, an energy storage inductor, a filter capacitor, etc.

        When the switch is closed, the power supply supplies power to the load through the switch and inductor, and stores part of the electrical energy in the inductor and capacitor. Due to the self-inductance of the inductor, the current increases slowly after the switch is turned on, that is, the output cannot immediately reach the power supply voltage value.

        After a certain period of time, the switch is disconnected. Due to the self-inductance of the inductor (it can be more vividly considered that the current in the inductor has an inertial 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, passes through the diode, and returns to the left end of the inductor, 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.

        The same thing between ordinary power supplies and switching power supplies is that they both have voltage adjustment tubes and use the feedback principle to stabilize the voltage. The difference is that the switching power supply uses the switching tube for adjustment, and the ordinary power supply generally uses the linear amplification area of the transistor for adjustment.

        In comparison, the switching power supply has low energy consumption, a wider range of application for AC voltage, and a better ripple factor for output DC. The disadvantage is switching pulse interference.

        The main working principle of an ordinary half-bridge switching power supply is that the switch tubes of the upper bridge and the lower bridge (the switch tube is VMOS when the frequency is high) are turned on in turn. 

        First, the current flows in through the upper bridge switch tube, and the storage function of the inductor coil is used to gather the electric energy in the coil. 

        Finally, the upper bridge switch tube is turned off, and the lower bridge switch tube is turned on. The inductor coil and the capacitor continue to supply power to the outside.

        Then the lower bridge switch tube is turned off, and the upper bridge is turned on to let the current in. This is repeated. Because the two switch tubes are turned on and off in turn, it is called a switching power supply.

         The linear power supply is different. Since there is no switch intervention, the upper water pipe is always draining water. If there is too much, it will leak out. 

        This is why we often see that the adjustment tube of some linear power supplies generates a lot of heat, and the unused electric energy is all converted into heat energy.

        From this point of view, the conversion efficiency of the linear power supply is very low, and when the heat is high, the life of the component is bound to decrease, affecting the final use effect.

● The main difference: working mode

        The power adjustment tube of the linear power supply always works in the amplification area, and the current flowing through is continuous. Due to the large power loss on the adjustment tube, a large power adjustment tube is required and a large heat sink is installed. The heat is serious and the efficiency is very low, generally 40%~60% (it must be said that it is a very good linear power supply).

        The working mode of the linear power supply is that there must be a pressure device to change from high voltage to low voltage. Generally, it is a transformer, and there are others like KX power supply, and then the DC voltage is output after rectification.

        In this way, the volume is large, relatively bulky, inefficient, and heat generation is also large; but there are also advantages: small ripple, good adjustment rate, small external interference, suitable for analog circuits/various amplifiers, etc.

        The power device of the switching power supply works in the switching state. When the voltage is adjusted, the energy is temporarily stored through the inductor coil, so its loss is small, the efficiency is high, and the heat dissipation requirement is low, but it also has higher requirements for transformers and energy storage inductors, and they must be made of low-loss and high-permeability materials.

        Its transformer is small. The total efficiency is between 80% and 98%. The switching power supply has high efficiency but small size. However, compared with the linear power supply, its ripple and voltage and current regulation rate are somewhat discounted.