(1) Switching Oscillation Circuit After the mains is rectified by D1~D4, the pulsating DC voltage is obtained at point A. The voltage is applied to the c-pole of the switching transistor Q1 via the 1-2 winding of the switching transformer T301, and the other is through the current limiting resistor. R3 is applied to the b pole of Q1 to provide the starting current for Q1. Q1 starts to conduct, and its collector current produces a positive and negative electromotive force in the 1-2 winding of T301. After T301 coupling, a positive electric potential of 3 positive and negative is induced in the 3-4 winding of T301. This electromotive force passes through R4. C1 is superimposed on the b pole of Q1 to make Q1 quickly saturate and conduct. Since the current flowing through the inductor cannot be abruptly changed, a positive and negative positive electromotive force is generated in the 1-2 winding of T301. After T301 coupling, 3 negative and 4 positive electromotive forces are induced in the 3-4 winding of T301, and Q1 is quickly turned into the off state through R4 and C1. As point A continues to charge C1 through R3, Q1 begins to conduct again, and then enters the next round of switching oscillation state. During the cut-off period, T301 releases the energy through D6 and its load circuit through the 5-6 winding of the secondary side to obtain the charging voltage required by MP3.
(2) The voltage regulator circuit is composed of Z1, Q2 and other components. When the load is reduced or the commercial power is increased, the voltage at point B is bound to rise. When the voltage is greater than 5.6V, Z1 breaks down, Q2 turns on quickly due to the positive junction of the junction, which causes Q1 to be cut off ahead of time, and thus the output voltage of the switching power supply tends to decrease; otherwise, the control process is reversed, so that the secondary side of T301 The output voltage is basically stable.
(3) The protection circuits R1 and R6 are current limiting resistors. When the load is too heavy, the collector-emitter current of Q1 will increase and the voltage drop across R6 will increase. When the voltage is greater than 0.7V, Q2 is saturated and turned on, which is equivalent to the short circuit of Q2's ce pole. Q1 is cut off immediately due to b-pole power loss, achieving the purpose of overcurrent protection. In order to avoid the spike high voltage breakdown Q1 induced by the 1-2 winding of the T301 during the off period, the spike absorbing resistor R2 is connected in parallel with the 1-2 winding of T301 to improve the switching characteristics of Q1.
(4) Charging circuit When the charging circuit is at no load. There is no current flowing on R8, the eb junction voltage of Q3 is basically equal, Q3 is cut off, LD2 (green light) is off, power indicator LD1 (red light) is on; when charging is connected to MP3, the charging current is generated on R8. Drop (ie V3e-b) causes Q3 to be forward biased and LD2 is lit, indicating that charging is in progress. As the battery is continuously charged, its charging current is gradually reduced, and the voltage drop across R8 is also reduced. When the eb junction bias voltage V3e-b of Q3 is less than 0.7V, Q3 is turned off and LD2 is extinguished, indicating that the electricity has been If it is full, stop charging in time to avoid damaging the rechargeable battery.
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