Mobile phone direct plug-in charger circuit

The input AC voltage of the mobile phone charger is 190~245V, 50Hz, 80mA. The output voltage is 5v, 500mA. The working principle of the circuit
is analyzed as follows: The circuit of
    a mobile phone plug-in charger is shown in Figure 12-39. The AC 220v voltage is half-wave rectified by the diode VD2 and filtered by the capacitor CZ,
forming a DC voltage of about 300V at both ends. This voltage is added to the c pole of the transistor VI1 through the secondary winding Wl of the switching transformer Tl. At the same time, this
voltage provides a forward bias voltage to the b pole of VT1 through the starting resistor island
, causing VT1 to conduct. At this time,
the intermittent oscillation circuit composed of VT1 and Tl starts to work, Tl - secondary
winding Wl has current flowing through it, and the voltage induced by the W2 winding
is added to the b pole of VT1 through the feedback resistor and capacitor C4
. Due to the positive feedback effect, the b-pole
conduction current of transistor VT1 increases, causing it to quickly enter the saturation region. At the same
time, the voltage induced by the W2 winding is rectified by the diode VD6
and filtered by the capacitor Ci, and then serves as the power supply for the photocoupler 1C1 ④
pin (collector of the light-receiving transistor). At the same time, the voltage
is charged towards 0. As the voltage across C4 continues to increase
, the b-pole voltage of vri gradually decreases, causing VT1 to gradually
exit the saturation zone (its collector current begins to decrease), and
the magnetic flux generated in the Tl winding WI also began to decrease.
The negative feedback voltage induced by the W2 winding causes Vn to quickly cut off and complete an oscillation cycle. Enter the deadline period in VT1. The voltage induced by the secondary winding W3 of Tl is rectified by the diode VD7 and filtered by the capacitor G, and outputs a DC voltage of about 5V.     When the charger output voltage rises, the diodes of pins ① and ⑦ of IC1 are turned on, causing the light-receiving transistors connected to pins ③ and ④ to be turned on, causing the transistor VT2 to be turned on - pulling down the base voltage of VTI. Make its conduction time shorter or cut off quickly. Coupled via T17. The secondary side output voltage decreases. On the contrary, the output voltage of the secondary side will increase, thereby ensuring the stability of the charger output voltage.   At the moment when the power supply is connected or when the L current of VT1 is too large for some reason, fr91f drops greatly on R, VT2 is turned on and VT1 is cut off . 1 This effectively prevents the switch from being damaged due to excessive impact current: VTI. VT2 is an over-current protection tube, and Rs is the sampling protection resistor of VT1.