Cost-effective power adapter application circuit design

　　The low-power power adapter discussed in this article is mainly for power systems with an output power of 5 to 15 watts. There are two main types of solutions, namely the integrated PWM controller solution and the discrete PWM controller solution. Figure 1 is a typical application diagram of the integrated PWM controller. U1 uses a DIP-8 package and integrates a PWM controller and a power MOSFET. The transformer input side circuit includes: an input filter circuit composed of an X capacitor CX and a common-mode inductor L-COM, a rectifier bridge circuit composed of BD, and a control and power circuit composed of U1. The transformer output side includes: an output rectifier filter current composed of a diode D10, etc.; a voltage stabilization feedback circuit composed of a fixed voltage reference U2, etc. Since the power device and the PWM device are integrated in one package, the solution has a high degree of integration, but the heat dissipation design is difficult. Figure 2 is a discrete PWM controller solution. U1 mostly uses SOIC-8 or SOT23-6, which only contains a PWM controller, and the power device Q1 is a MOSFET. The rest of the peripheral circuits are the same as the integrated PWM controller solution.

　　The common features of the PWM controllers of the above two solutions are: built-in fixed switching frequency, ramp compensation, automatic frequency hopping when light load, load short circuit and open circuit protection, all of which meet the low cost, low standby energy consumption and high reliability requirements of 5W~15W consumer power supply systems. The above two solutions and most of their extended application solutions have a dominant position in the fields of DVD power supply, computer auxiliary power supply, battery charger, network communication equipment, etc.

　　Both the integrated PWM control method in Figure 1 and the discrete PWM controller in Figure 2 can only be used with the power device MOSFET, so the cost is relatively high; in order to meet the requirements of electromagnetic compatibility, the input part of its application system must also contain an input filter circuit composed of X capacitors and common mode inductors L-COM, which is also expensive. According to a rough estimate, the cost of PWM control (including power MOSFET) and input filter circuits is 35% of the cost of the entire system components, which does not meet the trend of low cost of consumer electronics.

　　Therefore, only by seeking breakthroughs in the design concept of PWM controllers and maximizing the integration can the number of peripheral components be effectively reduced, thereby ultimately reducing system costs. This is exactly the design idea of ​​the newly launched PWM controller AP3710. AP3710 is an emitter-driven PWM controller chip. When it starts, it first obtains the initial current from the driver end OUT to supply the power supply end VCC, and the system starts working. When the system is working normally, it obtains enough energy from the auxiliary winding of the transformer to maintain the VCC voltage. The UVLO comparator ensures that the AP3710 operates reliably within a certain range of start-up voltage and shutdown voltage. The frequency of the built-in oscillator is fixed, but the switching frequency jitters within a certain range, which improves the system EMI. The ramp compensation function improves the stability of the system. The short-circuit protection function is implemented in the following way: when the system output is short-circuited, the VCC end will inevitably fall below the shutdown threshold. At this time, the chip does not start immediately, but pulls down the potential of the VA end through the discharge module, so that the VCC end of the AP3710 cannot get energy supply, thereby effectively reducing the input power when the system is short-circuited.

　　AP3710 power adapter solution

　　Figure 4 is the schematic diagram of the adapter solution of AP3710. The pulse output pin of AP3710 (U1) is directly connected to the emitter of transistor Q1. After the grid is powered on, the OUT pin of U1 first obtains energy from the emitter of Q1 to start up. C6, R7 and C5 are loop compensation components, and they are combined with the constant voltage component U2 to achieve stability regulation of the load end voltage. The overall solution has the best performance, such as standby power, EMI, conversion efficiency, dynamic characteristics and other performances meet the index requirements of high-performance power adapters. In addition, the solution has a small number of components, no input X capacitors and common-mode inductors, and Q1 is a low-priced crystal transistor, so this is an extremely cost-effective power adapter solution.