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　　Analysis of soft switching technology in switching power supply

　　Hard switching and soft switching in switching power supplies are for switching transistors. Hard switching is to force the switch tube to be turned on or off regardless of the voltage or current on the switch tube. When the voltage and current of the switch tube (between the drain and the source, or between the collector and the emitter) are large, the switch tube is switched. Due to the switching between switch tube states (from on to off, or from off) It takes a certain amount of time to turn on), which will cause a crossover area between voltage and current during a certain period of time when the switching tube state is switched. The switching tube loss caused by this crossover (switching loss of the switching tube) increases with the switch. Increases rapidly with the increase in frequency.

　　If it is an inductive load, a spike voltage will be induced when the switching transistor is turned off. The higher the switching frequency, the faster the turn-off, and the higher the induced voltage. This voltage is applied to both ends of the switching device, which can easily cause device breakdown.

　　If it is a capacitive load, the peak current at the moment when the switching transistor is turned on is large. Therefore, when the switching transistor is turned on at a very high voltage, all the energy stored in the junction capacitance of the switching transistor is dissipated within the device in the form of current. The higher the frequency, the larger the turn-on current peak, which may cause overheating damage to the switching tube.

　　In addition, the diode in the secondary high-frequency rectification circuit has a reverse recovery period when it changes from conduction to cutoff. When the switching transistor is turned on during this period, it is easy to generate a large inrush current. Obviously, the higher the frequency, the greater the inrush current, which will harm the safe operation of the switching transistor.

　　Finally, in switching power supplies used for hard switching, the switching transistors will produce serious electromagnetic disturbance. As the frequency increases and di/dt and du/dt in the circuit increase, the electromagnetic disturbance generated also increases, affecting the normal operation of the switching power supply itself and surrounding electronic equipment.

　　The above problems seriously hinder the improvement of the operating frequency of switching devices (switching transistors and high-frequency rectifier diodes). Research on soft switching technology carried out in recent years provides an effective way to overcome the above shortcomings. Different from the working principle of hard switching, the ideal soft turn-off process is that the current first drops to zero, and the voltage slowly rises to the off-state value, so the turn-off loss is approximately zero. Since the current has dropped to zero before the device turns off, the inductive turn-off problem is solved. The ideal soft turn-on process is that the voltage first drops to zero, and the current slowly rises to the on-state value, so the turn-on loss is approximately zero, and the voltage of the device junction capacitance is also zero, solving the capacitive turn-on problem. At the same time, when it is turned on, the diode reverse recovery process has ended, so the diode reverse recovery problem does not exist.

　　Soft switching technology also helps to reduce the level of electromagnetic disturbance. The reason is that the switching transistor is turned on at zero voltage and turned off at zero current. At the same time, the fast recovery diode is also soft turned off, which can significantly reduce di/dt and du/dt of low-power devices, thereby reducing the level of electromagnetic interference.

　　Generally speaking, the efficiency of soft switching is higher (because there is no switching loss); the operating frequency is higher, and the size of the PFC or transformer can be reduced, so the size of the switching power supply can be smaller. However, the cost is relatively high and the design is complex.

　　Basic working principle of switching power supply

　　1-1. Several basic types of switching power supplies

　　As the name suggests, switching power supply uses electronic switching devices (such as transistors, field effect tubes, thyristors, etc.) to control the circuit to continuously turn on and off the electronic switching devices, allowing the electronic switching devices to respond to the input voltage. Perform pulse modulation to achieve DC/AC, DC/DC voltage conversion, as well as adjustable output voltage and automatic voltage stabilization.

　　Switching power supplies generally have three working modes: fixed frequency and pulse width mode, fixed frequency and variable pulse width mode, and variable frequency and pulse width mode. The former working mode is mostly used for DC/AC inverter power supply, or DC/DC voltage conversion; the latter two working modes are mostly used for switching regulated power supply. In addition, the switching power supply output voltage also has three working modes: direct output voltage mode, average output voltage mode, and amplitude output voltage mode. Similarly, the former working mode is mostly used for DC/AC inverter power supply or DC/DC voltage conversion; the latter two working modes are mostly used for switching regulated power supply.

　　According to the way switching devices are connected in the circuit, the currently widely used switching power supplies can be roughly divided into three categories: series switching power supplies, parallel switching power supplies, and transformer-type switching power supplies. Among them, transformer switching power supply (hereinafter referred to as transformer switching power supply) can be further divided into: push-pull type, half-bridge type, full-bridge type, etc.; according to the excitation of the transformer and the phase of the output voltage, it can be further divided into: forward type , flyback type, single-excited type and double-excited type, etc.; if divided according to the use, it can also be divided into more types.

　　Next, we will first briefly introduce the working principles of the three most basic switching power supplies, such as series type, parallel type, and transformer type. Other types of switching power supplies will also be analyzed in detail step by step.

　　1-2. Series switching power supply

　　1-2-1. Working principle of series switching power supply

　　Figure 1-1-a is the simplest working principle diagram of a series switching power supply. In Figure 1-1-a, Ui is the working voltage of the switching power supply, that is, the DC input voltage; K is the control switch, and R is the load. When the control switch K is turned on, the switching power supply outputs a pulse voltage Up with a pulse width Ton and an amplitude Ui to the load R; when the control switch K is turned off, it is equivalent to the switching power supply outputting a pulse to the load R. A pulse voltage with width Toff and amplitude 0. In this way, the control switch K is constantly turned on and off, and a pulse modulated output voltage uo can be obtained at both ends of the load.

　　Figure 1-1-b is the waveform of the output voltage of the series switching power supply. It can be seen from the figure that the output voltage uo of the control switch K is a pulse modulated square wave. The pulse amplitude Up is equal to the input voltage Ui, and the pulse width is equal to the control switch K. The on-time Ton, from which the average value Ua of the series switching power supply output voltage uo can be obtained is:

　　The amplitude Up of the output voltage uo of the series switching power supply is equal to the input voltage Ui, and the average value Ua of the output voltage uo is always smaller than the input voltage Ui. Therefore, the series switching power supply generally uses the average value Ua as the variable output voltage. Therefore, the series switching power supply is a step-down switching power supply.

　　Some people also call the series switching power supply a chopper. Because of its simple working principle and high working efficiency, it is widely used in output power control. For example, electric motorcycle speed controllers and light brightness controllers are applications of series switching power supplies. If the series switching power supply is only used for power output control, the voltage output does not need to be connected to a rectification and filtering circuit, but directly provides power output to the load; but if it is used for voltage stabilization output, it must be rectified and filtered.

　　The disadvantage of the series switching power supply is that the input and output share the same ground. Therefore, it is easy to cause EMI interference and the base plate is electrified. When the input voltage is the mains rectified output voltage, it is easy to cause electric shock and is not safe for people.

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