MP3 charger principle and maintenance
Source: InternetPublisher:newlandmark Keywords: Switching power supply charger Updated: 2024/07/11
Working Principle
(1) Switch Oscillator Circuit After the AC power is rectified by D1~D4, a pulsating DC voltage is obtained at point A. This voltage
is added to the c pole of the switch tube Q1 through the ①-② windings of the switch transformer T301, and is added to the b pole of Q1 through the current limiting resistor R3, providing a starting current for Q1. Q1 starts to conduct, and its collector current generates ① positive ② negative electromotive force in the ①-② windings of T301. After coupling with T301, ③ positive ④ negative electromotive force is induced in the ③-④ windings of T301. This electromotive force is superimposed on the b pole of Q1 through R4 and C1, causing Q1 to quickly saturate and conduct. Since the current flowing through the inductor cannot change suddenly, ① negative ② positive electromotive force is generated in the ①-② windings of T301. After coupling with T301, ③ negative ④ positive electromotive force is induced in the ③-④ windings of T301, and through R4 and C1, Q1 quickly enters the cut-off state. As point A continues to charge C1 through R3, Q1 starts to conduct again, and then enters the next round of switching oscillation state. During the cut-off period, T301 releases energy through the secondary winding ⑤-⑥, through D6 and its load circuit, and obtains the charging voltage required by MP3.
(2) Voltage regulator circuit The voltage regulator circuit consists of components such as Z1 and Q2. When the load is reduced or the mains power increases, the voltage at point B is bound to rise. When the voltage is greater than 5.6V, Z1 breaks down, and Q2 quickly conducts due to the forward bias of the be junction, causing Q1 to be cut off in advance, thereby causing the output voltage of the switching power supply to tend to decrease; conversely, the control process is reversed, so that the output voltage of the secondary side of T301 is basically stable.
(3) Protection circuit R1 and R6 are current limiting resistors. When the load is too heavy, the collector-emitter current of Q1 is bound to increase, and the voltage drop on R6 also increases accordingly. When the voltage is greater than 0.7V, Q2 is saturated and turned on, which is equivalent to the ce pole of Q2 being short-circuited. Q1 is immediately cut off due to the loss of power to the b pole, achieving the purpose of overcurrent protection. In order to prevent the spike pulse high voltage induced by the ①-② winding of T301 from breaking down Q1 during the cut-off period, a spike pulse absorption resistor R2 is connected in parallel to the ①-② winding of T301 to improve the switching characteristics of Q1.
(4) Charging circuit When the charging circuit is unloaded. No current flows through R8, the eb junction voltage of Q3 is basically equal, Q3 is cut off, LD2 (green light) is off, and the power indicator LD1 (red light) is on; when MP3 is connected for charging, the voltage drop (i.e. V3e-b) generated by the charging current on R8 makes Q3 forward biased and LD2 is on, indicating that charging is in progress. As the battery is continuously charged, its charging current gradually decreases, and the voltage drop on R8 also decreases accordingly. When the eb junction bias voltage V3e-b of Q3 is less than 0.7V, Q3 is cut off and LD2 is off, indicating that the battery is fully charged. Charging should be stopped in time to avoid damaging the rechargeable battery.
Common faults and repairs
(1) If the MP3 cannot be charged, check whether the power indicator LD1 is lit in the no-load state. If it is not lit, it means that there is no charging voltage output. If the AC power connection is normal, the fault is in the charger itself and the charger needs to be opened for repair. If LD1 is on but cannot charge, it is mostly because the USB connection cable between the charger and the MP3 is broken. Replace the USB connection cable.
(2) If there is no output voltage, first check whether R1 and D6 are normal. It is common that Q1 is damaged by breakdown, causing R1 to burn out and open the circuit. The reason for Q1 damage, in addition to abnormal voltage regulation control loop, is also caused by short-circuit damage of one of D1~D4. When D6 breaks down, the switching power supply will also have no output voltage due to load short-circuit protection.
(3) If the output voltage is low, check whether C1 and C2 are invalid and whether Z1 is leaking. It is common that Z1 reverse leakage and R6 resistance value increase, resulting in low output voltage.
During maintenance, it is recommended to replace Q1 with a medium-power tube such as MJE13003. When R1 is open, it is recommended to use the fuse in a 0.2A fuse tube and directly solder it to the original R1 pad to replace R1 to improve the protection performance.
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