How does the mobile phone charger circuit work?
For mobile phone chargers everywhere in the market, universal chargers are constantly increasing, but the quality is not very high, and problems often occur. It is a pity that I will teach you a few tricks to analyze the principle of mobile phone chargers and hope to repair them. Bring some help.
When analyzing a power supply, it often starts with the input. 220V AC input, one end is subjected to a 4007 half-wave rectification, and the other end is filtered through a 10uF capacitor after a 10 ohm resistor. This 10 ohm resistor is used for protection. If an overcurrent occurs due to a fault later, this resistor will be burned out to avoid causing a larger fault. The 4007, 4700pF capacitor and 82KΩ resistor on the right constitute a high-voltage absorption circuit. When the switch tube 13003 is turned off, it is responsible for absorbing the induced voltage on the coil, thereby preventing high voltage from being applied to the switch tube 13003 and causing breakdown. 13003 is a switching tube (the complete name should be MJE13003), withstand voltage of 400V, maximum collector current, and maximum collector power consumption of 14W, which is used to control the on and off between the primary winding and the power supply. When the primary winding is continuously turned on and off, a changing magnetic field is formed in the switching transformer, and an induced voltage is generated in the secondary winding. The left end of 510KΩ is the starting resistance, which provides the base current for the switching tube to start. The 10Ω resistor under 13003 is a current sampling resistor. After the current is sampled, it becomes a voltage (its value is 10 * I). This voltage is applied to the base of the transistor C945 after the diode 4148.
When the sampling voltage is greater than 1.4V, that is, when the switching tube current is greater, the transistor C945 is turned on, thereby pulling down the base voltage of the switching tube 13003, thereby reducing the collector current, which limits the switching current and prevents the current from being excessive. Large and burned (actually this is a constant current structure, limiting the maximum current of the switch tube to about 140mA). The induced voltage of the winding at the bottom left of the transformer (sampling winding) is rectified by a rectifier diode 4148, and the 22uF capacitor is filtered to form a sampling voltage.
For the convenience of analysis, we take the C945 emitter end as the ground. Then the sampling voltage is negative (about -4V), and the higher the output voltage, the more negative the sampling voltage is. After the sampling voltage passes the 6.2V Zener diode, it is added to the base of the switch 13003. As mentioned earlier, when the output voltage is higher, the sampling voltage becomes more negative. When the negative voltage reaches a certain level, the 6.2V Zener diode is broken down, thereby pulling down the base potential of the switch 13003, which will cause the switch tube Opening or delaying the conduction of the switch, thereby controlling the input of energy into the transformer, controlling the increase of the output voltage, and realizing the function of regulated output. The lower 1KΩ resistor and the 2700pF capacitor in series are positive feedback branches. The induced voltage is taken from the sampling winding and added to the base of the switch to maintain oscillation. There is not much to say about the secondary winding on the right. It is rectified by the diode RF93 and filtered by a 220uF capacitor to output a voltage of 6V. No information on the diode RF93 was found, it is estimated that it is a fast recovery tube, such as a Schottky diode, etc. Because the switching power supply has a higher operating frequency, a diode with an operating frequency is needed.
Here you can use common 1N5816, 1N5817 and other Schottky diodes instead. Also because of the high frequency, the transformer must also use a high-frequency switching transformer. The iron core is generally a high-frequency ferrite core, which has a high resistivity to reduce eddy currents.
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