1. The main circuit of the switching power supply is composed of an input electromagnetic interference filter (EMI), a rectifier filter circuit, a power conversion circuit, a PWM controller circuit, and an output rectifier filter circuit. The auxiliary circuits include input over-voltage and under-voltage protection circuits, output over-voltage and under-voltage protection circuits, output over-current protection circuits, and output short-circuit protection circuits.
The AC input loop includes electromagnetic interference filter (EMI), rectifier filter circuit circuit; CN1 is AC power socket, F1 (T3A250V) is 3A, 250V AC fuse, RT1 (471) is varistor, with lightning protection function: when there is a Lightning strikes, causing high voltage to be introduced into the power supply through the power grid. When the voltage applied to both sides of the varistor exceeds its working voltage, its resistance value decreases, so that the high voltage energy is consumed on the varistor. If the current is too large, F1 will burn out to protect the latter stage circuit. RT2 (471) is a negative temperature coefficient thermistor. At the moment of startup, the capacitor is almost short-circuited to the power supply, and its interference current is very large, causing the transformer to be overloaded.
The NTC thermistor is connected in series, and the charging current of the capacitor is limited by the NTC element at the moment of power-on. After 15 seconds of power on, the temperature of the NTC element is relatively stable, and the partial pressure on it also slowly drops to 0:00 V. This small voltage drop can be seen that this component is in a short-circuit state after completing the soft-start function, and will not interfere with the normal operation of the electrical appliance.
The double π-type filter input circuit composed of CX1, L1, CX2, and L2 mainly suppresses the electromagnetic noise and clutter signal of the input power supply, avoids the interference to the power supply, and also avoids the high-frequency clutter caused by the power supply itself. . When the power is turned on, it is necessary to charge CX1. Due to the large instantaneous current, adding RT1 (varistor) can effectively avoid inrush current. Because the instantaneous energy is all consumed on the RT1 resistor, the resistance value of RT1 decreases after the temperature rises after a certain period of time. At this time, the energy consumed by it is very small, and the subsequent circuit can operate normally. R1 and R2 are anti-surge resistors. CX1 and CX2 are safety capacitors, and L2 and L3 are common mode inductors.
After the AC voltage is rectified by D1, D2, D3, and D4, a relatively pure DC voltage is obtained after being filtered by C1. If the capacity of C1 becomes smaller, the output AC ripple will increase. The rectifier tube in the circuit uses a fast recovery diode HER207, with a reverse withstand voltage of 800V and a rated current of 2A. RT1 (5D-15) thermistor is to prevent the inrush current caused by the moment when the circuit is turned on. A power NTC thermistor is connected in series in the power input circuit, which can effectively suppress the inrush current when the power is turned on. , and after completing the function of suppressing the inrush current, the resistance value of the power NTC thermistor will drop to a very small level due to the continuous action of the current through it, so the power NTC thermistor is used in the power supply circuit. , is a relatively simple and effective measure to suppress the surge during startup to ensure that electronic products are not damaged.
2. Power conversion circuit
This power adapter uses a flyback switching transformer, which includes transformer T1, switch MOS transistor Q1 (20N60), driver chip U11 (OB2269), Schottky rectifier tubes D8, D9, D10 (MBR20100CT), and the flyback switching transformer is in When the switch tube is on, the energy is stored, and the discharge energy is explained to the load when the switch tube is turned off. In the figure, R3, R4, C2, and D5 form an RCD absorption circuit. When the switch tube Q1 is turned off, the primary coil of the transformer is likely to cause a peak voltage. Combined with the peak current, these components can effectively absorb the peak voltage and current, consume the leakage inductance energy of the transformer, and prevent the formation of a large peak voltage on the switch MOS tube, thereby protecting the switch tube.
Pin 1 of OB2269 is ground, pin 2 is feedback signal input, pin 3 is starting voltage input, pin 4 is oscillation resistance connection, pin 5 is overheat detection signal input, and pin 6 is overcurrent detection signal input. Pin 7 is the power input end after starting normally, and pin 8 is the output end of the switch tube driving signal. In the circuit shown in Figure 1, R5 (1M5/2W) is the starting resistance of OB2269. After 220V AC is rectified, a DC voltage of about 300V is obtained and filtered by R5 and C4 to provide a starting voltage of about 15V to OB2269. The choice of R5 resistance affects the circuit The start-up time should also be considered that the start-up circuit needs to be controlled within 5mA. In the circuit, R5 is 1M5/2W. After the OB2269 starts normally, the AC voltage output by the sampling coil is half-wave rectified by D6, R13, and C3 to provide the control chip with power supply voltage .
The OB22698 pin is connected to the output terminal of the switch tube driving signal. D11, R6, Q2, Q3 form a push-pull circuit to improve the output response time and output current. R7, R8 are the protection resistors for the control pole G of the switch MOS tube, so that the G pole is not floating And the current is within a certain range, Q1 (20N60) is an N-channel enhancement mode high-voltage power MOS field effect transistor, with the following characteristics
∗Drain-source voltage up to 600V;
∗Drain current can reach 20A at room temperature;
∗Low on-resistance RDS(on) (typ.) = 0.28Ω;
∗Low gate charge;
∗Low reverse transfer capacitance;
∗ Fast switching speed;
R11A and R11B are overcurrent detection resistors. The resistance of the connected resistors can be adjusted according to the overcurrent protection value. Since the output voltage is required to be 12V, and the rated current needs to have a load energy of 6A, the overcurrent protection of the switch tube on the primary side of the transformer is required. The value is set at 8A, that is, when the output load exceeds 8A, the light tube is turned off, and the current overcurrent detection resistor uses 4 chip resistors in a 1□1206 package. R10, R9, RV2 are oscillation resistance and overheat detection resistance respectively, generally take the recommended value of the typical circuit, R12, C5 and the output of the optocoupler form a feedback loop. Y capacitor CY2 (222M/3KV) is the primary and secondary isolation high-voltage capacitor of the transformer or safety capacitor.
3. Switching transformer secondary circuit
The secondary circuit of the switching transformer includes rectification and voltage regulation, voltage feedback, output voltage indication and output filter circuit.
The rectifier voltage regulator uses three Schottky diodes MBR20100CT. The Schottky diode is an N-type semiconductor device. It works under low voltage and high current state, and the reverse recovery time is short, only nanoseconds. The forward voltage drop is 0.3V or so, and the rectified current reaches tens of amps. It is one of the most widely used devices in switching power supplies recently. The way to distinguish between Schottky diodes and ultra-fast recovery diodes is that the forward voltage drops of the two are different. The forward voltage drop of Schottky diodes is 0.3V, and the forward voltage drop of ultra-fast recovery diodes is 0.6V. It is worth noting that the maximum reverse working voltage of the Schottky diode is generally not more than 100V, and it is suitable for use in low-voltage, high-current switching power supplies.
Therefore, in this design, the Schottky diode used is MBR20100CT, its rectified current is 20A, and the reverse withstand voltage is 100V. MBR20100CT contains two Schottky diodes connected by cathodes. Using the unidirectional conduction characteristics of diodes, the direction is continuously The changing AC current becomes the same direction. Three MBR20100CTs connected in parallel are used in the circuit to increase the instantaneous output current. R14 and C6 form a suction and discharge circuit, and C7, L3 and C8 form an LC filter circuit to ensure the output of 50Hz DC power supply voltage. R21 is a dummy load resistor, which can prevent the output voltage from jumping when the output is no-load. R15 and LED form an output indicator circuit. The selection of R15 needs to control the current of the LED within 10mA. U2 (PC817), U3 (TL431) and peripheral RC components form a voltage regulator feedback circuit. R19, R20, R18 are the output sampling resistors, so that the voltage of the reference pin REF of U3 (TL431) is controlled at 2.5V, and R16 provides the start-up current for the optocoupler. In order to ensure the stable output of the 12V voltage, the output voltage passes through R19, R20, R18 The output sampling resistor divides the voltage, and the result is sent to the error feedback network of the TL431REF pin. The output voltage of the error feedback is converted into a current signal proportional to the photodiode. Here the optocoupler plays the role of isolating the secondary side of the primary side, and Generates a collector current proportional to the diode current (that is, the collector of the optocoupler triode), because the optocoupler is connected to the FB pin of the control chip, the optocoupler output current is the Ifb current, and the feedback circuit uses TL431 and optocoupler PC817 as Core, the output voltage is divided by resistors R19, R20, R18 and sent to pin 1 of TL431. When the voltage is higher than 2.495V, TL431 is turned on and the optocoupler works, resulting in pin 2 of OB2269 being high. When the output PWM pulse is turned off, the output voltage decreases. Instead, the output voltage rises.
Summarize:
With the rapid development of electronic technology, more and more application fields of electronic systems, and more and more types of electronic equipment, any electronic equipment is inseparable from a reliable power supply, and the requirements for power supply are getting higher and higher. The power adapter of this design, input AC 220V, output 12V/6A DC regulated power supply, has over-current, over-voltage, short-circuit protection, and can meet the power supply requirements of 32-inch LCD TVs.
Follow Us
