Design points of RCC switching power supply

　　RCC is widely used in small and medium power conversion applications due to its simple circuit topology, electrical isolation of output and input voltage, no need for output filter inductor, ability to efficiently provide multiple sets of DC outputs, and wide voltage rise and fall range. It is also a converter frequently used in power supplies with a capacity generally below 50 W.

　　Linear regulated power supplies have always been popular in low-power applications due to their simple circuits and low cost. This linear regulated power supply requires only a few components and is easier to design and manufacture than a switching mode power supply (SMPS). However, linear power supplies have gradually been replaced in recent years for two reasons: First, many linear power supplies are sold as external power supplies (EPS) for products such as PDAs, cordless phones, and mobile phones. Today's EPS must comply with strict new energy-saving standards, which almost exclude linear power supplies because linear power supplies usually cannot meet the standards in terms of operating efficiency and no-load power consumption; second, most advanced low-power SMPS are comparable to linear power supplies in terms of cost and simplicity. This article will explore the shortcomings of low-power SMPS in the initial application stage and discuss a feasible method to help design engineers design products that meet the new EPS energy-saving standards in terms of cost-effectiveness, while reducing design time and simplifying design work.

　　Self-oscillating flyback converter. RCC (Ring Choke Converter) is widely used in small and medium power conversion occasions due to its simple circuit topology, electrical isolation of output and input voltage, no need for output filter inductor, high efficiency in providing multiple sets of DC output, wide voltage rise and fall range, etc. It is also a converter commonly used in power supplies with a capacity generally below 50 W. It is widely used in mobile phone chargers and notebook adapters and other equipment. RCC adopts a driving method opposite to the PWM converter. The on and off of the switch does not require a special trigger circuit, but is completely completed by the circuit. This converter has its unique advantages, that is, the circuit is simple and has a high cost performance. However, if the RCC circuit is composed of discrete components, the typical number of circuit components is as high as more than 50, so designing an integrated RCC power supply device has become a trend.

　　Here, the circuit principle is first analyzed and designed in detail, and the circuit is simulated by computer simulation. Secondly, the RCC device is applied to the charger for actual testing, which is verified with the theoretical value, and then the device test results and the problems that need to be further solved are analyzed. Finally, a conclusion is given.

Figure 1: Typical RCC application circuit 　　Application circuit of RCC device

　　A typical RCC circuit requires about 50 discrete components, which is very difficult to design and debug, and the reliability is not high enough. In order to solve this problem, an RCC integrated device is designed, and Figure 1 is its typical application circuit. As can be seen from the figure, there are only 8 discrete components on the input side of the discrete device and 2 discrete components on the output side. If the transistor 13001, diode VD2 and capacitor C4 are packaged into the device, the discrete components will be reduced to 7, which improves the integration and will be the simplest RCC circuit. The rectifier and filter circuit of this application circuit is composed of diode VD5 and capacitor C5; the converter adopts a double-winding flyback converter, the power tube model selected is 13001, the startup circuit is composed of resistor R6 and capacitor C6 in series, the pin FB of the flyback switching power supply integrated circuit is connected to the secondary coil in the converter, the pin SW is connected to the emitter of the power tube 13001, the collector of the power tube 13001 is connected to the main coil, the pin VCC is connected to the positive pole of the capacitor C6, and the pin GND is grounded.

Figure 2: Schematic diagram of the internal structure of RCC

　　The 85-220 V AC input first passes through VD5 and C5, and the waveform is converted from AC to DC voltage with relatively large ripple. Since the voltage of capacitor C6 is 0V when powered on, the output tube of pin SW is in the off state, and the power supply charges capacitor C6 through resistor R6. When capacitor C6 is charged to the starting voltage of the flyback switching power supply integrated circuit, the flyback switching power supply integrated circuit starts to work normally, and its internal oscillator starts to start. SW outputs a large duty cycle switching signal to control the output power tube 13001, so that the power tube 13001 is also turned on and off. When the power tube 13001 is turned on, the power tube 13001 The voltage of the collector is low voltage, so the voltage of the output and pin FB sensed by the transformer are both negative voltages. When 13001 is turned off, since the current of the inductor cannot change suddenly, a kickback voltage will be generated on the main coil of the power tube 13001, and the output coil and auxiliary coil of the transformer will couple out a positive voltage. At this time, the output rectifier diode VD7 is turned on, and capacitors C6 and C8 are charged. When the power tube 13001 is turned on once, the coupling voltage on the output coil and the auxiliary coil is a negative voltage, and the voltage on capacitors C6 and C8 can maintain the operating current of the flyback switching power supply integrated circuit and the operating current of the output load. In this way, the system can continue to work; the voltage control at the output end is controlled by the overvoltage protection voltage inside the flyback switching power supply integrated circuit. When the output load decreases, the voltage of VCC rises to the overvoltage point, and the flyback switching power supply integrated circuit will shut down the SW. At this time, the power tube 13001 will not conduct until the VCC voltage discharges below the overvoltage point. The SW will not turn on, so that the flyback switching power supply integrated circuit will enter the intermittent working mode (working for several cycles and not working for several cycles), and the operating frequency will be reduced. The output voltage can be maintained at a constant value. 　　The internal structure of the RCC device

　　Figure 2 is a schematic diagram of the internal structure of RCC. The flyback switching power supply integrated circuit includes an oscillator, a small duty cycle generating circuit, a duty cycle selecting circuit and a blanking circuit. The oscillator is connected to the small duty cycle generating circuit, the oscillator and the small duty cycle generating circuit are respectively connected to the duty cycle selecting circuit, the duty cycle selecting circuit is connected to the blanking circuit, the undervoltage lockout (UVLO) is the startup circuit of the entire flyback switching power supply integrated circuit, and controls the startup and shutdown of the flyback switching power supply integrated circuit. The protection circuit is connected to the output drive tube VMO, and the blanking circuit also controls the output drive tube VMO. The diode VD8 is directly connected to the pin FB and the pin VCC, and forms a rectifier and filter circuit with the capacitor outside the flyback switching power supply integrated circuit (i.e., C6 in Figure 1).

　　Device working process

　　当电源电压VCC上升到欠压锁定(UVL0)电路的开启电压时，电路开始工作，振荡器、小占空比产生电路、占空比选择电路、消隐电路启动，此时SW端口跳变，后备电源启动，对引脚FB充电，随着引脚FB电压的上升，当超过VCC电压时，二极管VD8导通，后备电源对VCC提供工作电流。振荡器提供一个占空比为12％振荡频率为40 kHz方波，随着VCC电压继续上升，当上升到钳位电路的箝位电压点时，反激式开关电源集成电路会切换到小占空比(4％)状态下工作，这时输出电压将会下降，但是不会马上切换到大占空比状态，直到VCC电压低于过压点时，才会回到大占空比状态，这时工作频率会上升，可以避免反激式开关电源集成电路的工作频率低于20 kHz；当反激式开关电源集成电路的输出负载增加时，电感反激时的能量不足以提供系统输出的能量，VCC电压会下降，当电压下降到反激式开关电源集成电路的欠压点时，反激式开关电源集成电路将会全部关断，等待重启，这时系统进入打嗝模式。如果反激式开关电源集成电路的工作温度过高时，反激式开关电源集成电路的过温保护会将输出SW关断，这时VCC电压会持续下降，一直下降到欠压点电压，反激式开关电源集成电路关断，等待重启，反激式开关电源集成电路也会进入打嗝模式。

　　Experimental data and processing

　　According to the application circuit of Figure 1, the test data of a single-cell lithium battery charger are shown in Table 1 and Table 2. Figure 3 is a transient characteristic diagram of the current.

　　From the data in Table 1 and Table 2, it can be seen that the device has basically met the design standards, but there are still the following problems: 1) The starting current is too large; 2) The overvoltage is too close to the starting voltage; 3) The operating frequency is too low and needs to be improved through subsequent design.

Figure 3: Load dynamic characteristics A typical RCC contains 5 to 10 times the number of components of an equivalent linear power supply. Although most of the components are very cheap, the design and manufacturing costs are high due to the large absolute number. The more components there are, the more complex the PCB routing becomes, the longer it takes to optimize the layout, and the higher the possibility of errors when mounting components. Mounting SMD components also requires additional manufacturing steps, which increases production time and cost. The performance of the RCC depends on the interaction between the values ​​of parasitic components that are difficult to control and the combined tolerances of a large number of discrete components. Continuous monitoring and adjustment are required during the manufacturing process to keep the yield at an acceptable level, so an RCC integrated device must be designed to effectively improve the advantages of the RCC circuit.

　　This scheme designs the internal structure of the device, including the rectifier filter circuit, converter and output circuit connected in sequence, and the rectifier filter circuit is connected to the startup circuit. The rectifier filter circuit, converter and startup circuit are respectively connected to the flyback switching power supply integrated circuit. The device was simulated and actually tested. The test results show that although there are three problems such as "large startup current", this scheme basically overcomes the shortcomings of the separate RCC scheme, and the efficiency is greater than 65%, which is one of the more ideal RCC switching power supply devices at present.