Power adapter soft switch technology.12w adapter

　　AC/DC and DC/DC converters, as the main devices that provide DC power to the equipment in the power adapter system, face many requirements such as smaller size, lighter weight, higher efficiency, and higher reliability. To meet the above requirements, the converter must realize the transformation of the operating frequency from low frequency to high frequency. As we all know, in the hard switching mode, continuously increasing the working frequency of the converter will cause the following problems.

　　①The loss of the switching power supply adapter is large. When it is turned on, the current rise and voltage drop of the switching device proceed simultaneously; when it is turned off, the voltage rise and current drop proceed simultaneously. The overlap of the voltage and current waveforms produces switching loss, which increases rapidly with the increase of the switching frequency. ②The inductive turn-off voltage spike is large. When the device is turned off, the inductive components in the circuit induce a spike voltage. 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 is easy to cause breakdown of the device. ③The capacitive turn-on current has a large peak. When the switching device is turned on at a very high voltage, the energy stored in the junction capacitance of the switching device will all be dissipated in the device in the form of current. The higher the frequency, the larger the turn-on current spike, which will cause overheating and damage to the device. In addition, there is a reverse recovery period when the diode changes from conduction to load stop, and the switch tube's turn-on action during this period is likely to produce a large inrush current. The higher the frequency, the larger the inrush current, which will harm the safe operation of the device.

　　④Serious electromagnetic interference. As the frequency increases, the di/dt and dv/dt in the circuit increase, which leads to an increase in electromagnetic interference (EMI), which affects the operation of the rectifier and surrounding electronic equipment

　　The above-mentioned problems seriously hinder the improvement of the operating frequency of the switching device. In recent years, the soft-switching power adapter technology research has provided an effective way to overcome the above-mentioned shortcomings. Unlike hard switching, the ideal soft turn-off process is that the current first drops to zero, and then 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 is turned off, the inductive turn-off problem is solved. The ideal soft turn-on process is that after the voltage first drops to zero, the current slowly rises to the on-state value, so the turn-on loss is approximately zero, and the voltage on the junction capacitance of the device is also zero, which solves the capacitive turn-on problem. At the same time, the diode reverse recovery process has ended when it is turned on, so the diode reverse recovery problem does not exist. The reduction of di/dt and du/dt enables the EMI problem to be solved. The soft switching technology actually uses the resonance of the capacitor and the inductance to make the current (or voltage) in the switching device change according to the sine or quasi-sine law. When the current crosses zero, the device is turned off; when the voltage crosses zero, the device is turned on to achieve zero switching loss. According to its control method, soft switching technology can be divided into pulse frequency modulation (Pulse Frequency Modulation, abbreviated as PFM), pulse width modulation (Pulse Width Modulation, abbreviated as PWM) and pulse phase shift control (Phase Shited, abbreviated as PS). ) Mode soft switching technology has obvious effects in improving the working state of power switching devices, making it possible to increase the frequency of the converter, and various soft switching technologies will also be widely used in switching power supply adapters. The PFM soft-switching converter has a simple circuit structure, but the operating frequency is not constant, which brings certain difficulties to the optimal design of magnetic components such as transformers and inductors. This type of converter is suitable for applications where the load and input voltage are relatively stable. The PWM soft-switching converter realizes constant frequency control, which greatly facilitates the optimal design of magnetic components. The PWM control method is the most widely used control method in the soft-switching converter. This type of converter is suitable for medium and small power applications. The phase-shifted full-bridge soft-switching converter has the advantages of low voltage and current ratings of the switching devices in the traditional full-bridge converter, high utilization rate of the power transformer, and large output power without adding or adding few components. At the same time, the soft switching of the switching device is realized, and the constant frequency control is adopted, the voltage and current stress are small, and the effect of reducing the volume and weight and reducing electromagnetic interference is obvious. Therefore, the phase-shifted full-bridge soft-switching converter is very suitable for medium and high power applications. But this type of converter has an obvious shortcoming, that is, there is a loss of duty cycle on the secondary side of the transformer, and the soft switching range of the lagging bridge arm is affected by multiple factors such as load and input voltage. How to minimize the loss of the duty cycle of the secondary side and increase the soft switching range of the lagging bridge arm is still a topic that needs to be studied in depth in the phase-shifted full-bridge soft-switching converter.

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