Mobile phone USB charger production plan

　　In the figure, a 1 ohm resistor F1 acts as a fuse, and a diode D1 is used to complete the rectification. After the power is turned on, C1 will have a DC voltage of about 300V, which provides current to the base of Q1 through R2. The emitter of Q1 has a current detection resistor R1. After the base of Q1 is energized, it will generate collector current through T1's (3, 4) and simultaneously generate induced voltage on T1's (5, 6) (1, 2). These two secondary insulated coils have the same number of turns. The output of T1 (1, 2) is rectified by D7 and filtered by C5, and then supplied to the load through the USB socket; T1 (5, 6) is rectified by D6 and filtered by C2, and then passes through IC1 (actually a 4.3V voltage regulator) and Q2 to form a sampling comparison circuit to detect the output voltage; T1 (5, 6), C3, and R4 also form the positive feedback circuit of the Q1 transistor, allowing Q1 to work at a high frequency oscillation and continuously supply power to the T1 (3, 4) switch. When the output voltage increases due to any reason such as the load becoming lighter or the power supply voltage becoming higher, T1 (5, 6) and IC1 sampling and comparison cause Q2 to turn on, Q1 base current decreases, collector current decreases, and load capacity decreases, thereby causing the output voltage to decrease; when the output voltage decreases, Q2 will be cut off after sampling, Q1's load capacity will become stronger, and the output voltage will increase again; this plays an automatic voltage stabilization role.

　　Although this circuit has few components, it is also designed with overcurrent, overload and short-circuit protection functions. When the load is overloaded or short-circuited, the collector current of Q1 increases greatly, and the emitter resistor R1 of Q1 will produce a higher voltage drop. The high voltage generated by this overload or short circuit will pass through R3 to saturate Q2 and turn on, thereby stopping Q1 from outputting to prevent overload damage. Therefore, changing the size of R1 can change the load capacity. If a small output current is required, for example, only 5V100MA needs to be output, the resistance value of R1 can be increased. Of course, if 5V500MA needs to be output, R1 needs to be appropriately reduced. Note: Reducing R1 will increase the possibility of burning Q1. If a large current output is required, it is recommended to replace the high-power tube 13003 or 13007.

　　What are the functions of C4, R5, and D5? Transformer T1 is an inductor. Q1 works in a switching state. When Q1 is turned off, a very high voltage will be induced at the collector. This voltage may be as high as 1000 volts or more, which will cause Q1 to break down and be damaged. Now there is a high-speed switch tube D5. This voltage can charge C4 and absorb this high voltage. After C4 is charged, it can be discharged immediately through R5. In this way, Q1 will not be damaged by the high voltage breakdown of the collector. Therefore, if these three components are switched or damaged, Q1 is very dangerous and may be damaged at any time.

　　1N4007 is a low-frequency diode, FR107 is a high-frequency high-voltage diode, and 1N5819 is a low-voltage high-frequency Schottky diode. (Substitution relationship: FR107 can replace 1N4007, but not vice versa; 1N5819 cannot be replaced by other diodes. The conduction voltage of 1N5819 is very low, equivalent to the conduction voltage of germanium tube. Therefore, the low-voltage rectification efficiency is very high. If other diodes must be used instead, the output power will be reduced, the heat will be serious, and the efficiency will be low.)

　　Remember: FR104 (7) is a high-frequency output rectifier diode, and 1N4007 is a power rectifier diode.