A practical switching power supply production (schematic diagram + PCB)

　　1. Working Principle

　　Let's first familiarize ourselves with the working principle of a switching power supply that can output 5V voltage, as shown in Figure 1.

　　1. Anti-interference circuit

　　First, an NTC5D-9 negative temperature coefficient thermistor is set at the input end of the power grid to protect the rectifier bridge behind. When the power is just turned on, the thermistor is in a cold state and has a relatively large resistance value, which can limit the input current. During normal operation, the resistance is relatively small, which effectively buffers the surge current when the power is turned on.

　　Capacitors CY1, CY2, CY3, and CY4 are used to filter out asymmetric stray signals entering the switching power supply from the power frequency grid and from the switching power supply into the power frequency grid. Capacitors CX1 and CX2 are used to filter out symmetrical stray signals entering the switching power supply from the power frequency grid and from the switching power supply into the power frequency grid. Inductor L1 is used to suppress stray interference current signals with the same frequency and opposite phases entering the switching power supply from the power frequency grid and from the switching power supply into the power frequency grid.

　　By using ceramic capacitors and iron core inductors with good high-frequency characteristics, the high-frequency radiation in the switching voltage-stabilized power supply circuit will not pollute the power frequency power grid, and the stray electromagnetic waves on the power frequency power grid will not penetrate into the switching voltage-stabilized power supply circuit to interfere with and affect its operation. It has the function of sharply blocking the passage of high-frequency components or harmonic components of the power frequency, and is approximately a short-circuit line for low-frequency components below a few hundred Hz.

Figure 1 Working principle diagram of switching power supply

　　2. Rectification and filtering circuit

　　In the circuit, D1, D2, D3, and D4 form a full-bridge rectifier circuit, which full-wave rectifies the input AC voltage, then filters it with C1, and finally converts it into a DC output power supply voltage to power the subsequent power converter. The voltage after rectification and filtering is about 300V.

　　3. UC3842 power supply and oscillation

　　The pulsating DC voltage of 300V is stepped down by R12 to charge C4 and supply power to pin 7 of UC3842. When the voltage of C4 reaches the starting voltage threshold of UC3842, UC3842 starts to work and provides driving pulses, which drive the switch tube to work through the output of pin 6. Once the switch tube works, the energy of the feedback winding is rectified by D6, filtered by C4, and supplied to pin 7 of UC3842. At this time, the startup of R12 is no longer required.

　　C9 and R11 are connected to the timing terminal of UC3842, and together with the internal circuit form an oscillation circuit. The oscillation operating frequency is calculated as:

　　f=1.8/(Rt*Ct)

　　Substituting the data, the operating frequency can be calculated:

　　f=68.18K

　　4. Voltage stabilization circuit

　　The circuit is mainly composed of precision voltage regulator TL431 and linear optocoupler PC817. Assume that the output voltage ↑ → the sampling voltage through R16, R19, R20, and RES3 ↑ → the voltage of pin 1 of TL431 ↑. When the voltage of this pin is greater than the reference voltage of TL431 2.5V, pins 2 and 3 of TL431 are turned on → through photoelectric coupling to pin 2 of UC3842, so the duty cycle of the driving pulse of pin 6 of UC3842 ↓ → the energy on the winding of switching transformer T1 ↓ → the output voltage ↓, achieving the voltage regulation effect; conversely, assuming that the output voltage drops, the voltage regulation process is the opposite of the above.

　　R9 obtains the feedback voltage input to UC3842, and R8, R10, and C6 are used to change the internal gain and frequency characteristics of UC3842.

　　5. Overcurrent, overvoltage and undervoltage protection circuits

　　Due to the instability of input voltage or some other external factors, sometimes the circuit may experience short circuit, overvoltage, undervoltage and other phenomena that are not conducive to the operation of the circuit. Therefore, the circuit must have certain protection functions.

　　(1) Overcurrent protection. If for some reason the output is short-circuited and overcurrent is generated, the energy of the switching transformer winding will be quickly discharged. In order to replenish this energy, the switch tube must extend the on-time. The drain current of the switch tube will increase significantly, the voltage across R5 will increase, and the voltage on pin 3 of UC3842 will also increase. When the voltage of this pin exceeds the normal value of 0.3V and reaches 1V, the PWM comparator inside UC3842 outputs a high level, resetting the PWM latch and turning off the output. There is no output on pin 6 of UC3842, the switch tube is cut off, and overcurrent protection is achieved.

　　(2) Overvoltage protection. If the power supply voltage is overvoltage (above 260V), the primary winding voltage of the transformer will increase greatly. After the voltage on the sampling winding is rectified and filtered, the supply voltage of pin 7 of UC3842 will also rise sharply. The voltage of its pin 2 will also rise, and the output of pin 6 will be turned off, the switch tube will stop oscillating, and overvoltage protection will be achieved.

　　(3) Undervoltage protection. If the voltage of the power grid is lower than 90V, the voltage of pin 1 of UC3842 will drop. When it drops below 1V, the PWM comparator inside UC3842 will output a high level, reset the PWM latch, and turn off the output of pin 6, thus achieving undervoltage protection.

　　6. Output rectifier filter circuit

　　D9, D10, C13, C14, and C15 are the rectifier and filter circuits at the output end. This part of the circuit belongs to the high-frequency filtering part.

　　D9 and D10 are Schottky diodes, which have the characteristics of high-frequency fast recovery switching diodes, and have low forward voltage drop, fast switching speed, and small reverse leakage current when cut off, which is beneficial to improving the efficiency of the power supply. Their reverse recovery time is short, which is beneficial to reducing high-frequency noise.

　　2. Plate making design

　　First, draw the schematic diagram, here we use protel dx p2004 software. Enter the component parameters in the SCH interface, then create the schematic circuit network table, load the schematic circuit network table in the PCB interface, then create the design parameters, and finally perform manual layout and manual wiring of components.

　　1. PCB board size setting

　　When the PCB size is too large, the printed circuit is long, the impedance increases, the anti-noise ability decreases, and the cost increases. If it is too small, the heat dissipation is poor, and the adjacent lines are thin and easily interfered. Here, a rectangular plate with a length-to-width ratio of 3:2 and a length-to-width of 0.150.10m is used.

　　2. Component Layout

　　The layout is centered around the core components of each function. Here, the power input stage, power conversion, power output stage, and PWM control unit are wired. The components are arranged evenly, neatly, and compactly. The positions of each functional circuit unit are arranged according to the circuit flow, so that the layout is convenient for signal flow and the signal is kept in the same direction as much as possible. In addition, the components in the circuit are arranged in parallel as much as possible to facilitate the subsequent welding work.

　　3. Component location considerations

　　From the perspective of the overall reliability of the power supply, the electrolytic capacitor is an important component in the power supply circuit. This component is greatly affected by temperature and should be kept away from heat-generating power switching devices.

　　The PWM controller chip UC3842 is located far away from the secondary side of the switching transformer and the AC grid input terminal, because the power switching converter is both a heat source and a high-frequency radiation source in the power supply. The PCB diagram is shown in Figure 2.

Figure 2 PCB diagram