Two designs of smartphone charging circuit modules

　The first circuit principle:   The AC220V voltage is half-wave rectified by D3 and filtered by C1 to obtain a voltage of about +300V. One path is added to the switch tube Q2 c pole through the primary winding L1 of the switching transformer T, and the other path is added to the Q2 b pole through the starting resistor R3. Q2 enters the micro-conduction state, and an induced electromotive force with positive top and negative bottom is generated in L1, and an induced electromotive force with negative top and positive bottom is generated in L2. The induced electromotive force in L2 is positively fed back to the Q2 b pole through R8 and C2, and Q2 quickly enters the saturation state. During the saturation of Q2, since the current in L1 increases approximately linearly, a stable induced electromotive force is generated in L2. This electromotive force charges C2 through the be junction of R8, R6, and Q2. As C2 is charged, the voltage of Q2 b pole gradually decreases. When it drops to a certain value, Q2 exits the saturation state, the current flowing through L1 decreases, and the polarity of the induced electromotive force in L1 and L2 is reversed. Under the positive feedback of R8 and C2, Q2 quickly retreats from the saturation state to the cut-off state. At this time, the +300V voltage reversely charges C2 through R3, R8, L2, and R16, and the potential at the right end of C2 gradually rises. When it rises to a certain value, under the action of R3, Q2 turns on again, and the above process is repeated, and this cycle repeats to form self-excited oscillation.

　　During the conduction period of Q2, the polarity of the induced electromotive force in L3 is negative at the top and positive at the bottom, and D7 is cut off; during the cut-off period of Q2, the polarity of the induced electromotive force in L3 is positive at the top and negative at the bottom, and D7 is turned on to supply power to the outside. In Figure 1, components such as VD1 and Q1 form a regulated voltage. If the output voltage is too high, the induced voltage of the L2 winding will also increase, and the voltage obtained by D1 rectification and C4 filtering will increase. Since the two ends of VD1 always maintain a regulated voltage value of 5.6V, the voltage of Q1 b pole increases, and the conduction process of Q1 deepens, that is, the shunt effect on the current of Q2 b pole is enhanced, Q2 is cut off early, and the output voltage drops. If the output voltage drops, its voltage regulation control process is the opposite of the above. In addition, R6, R4, and Q1 form an overcurrent protection circuit. If the current flowing through Q2 is too large, the voltage drop on R6 increases, Q1 is turned on, and Q2 is cut off to prevent Q2 from being damaged by overcurrent.

　　The second circuit principle: 220V AC input, one end passes through a 4007 half-wave rectifier, the other end passes through a 10 ohm resistor, and is filtered by a 10uF capacitor. This 10 ohm resistor is used for protection. If a fault occurs later and causes overcurrent, 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 the high voltage from being applied to the switch tube 13003 and causing breakdown. 13003 is a switch tube (the full name should be MJE13003), which is used to control the on and off between the primary winding and the power supply. When the primary winding is constantly on and off, a changing magnetic field will be formed in the switching transformer, thereby generating an induced voltage in the secondary winding.

　　Since the same-name end of the winding is not marked in the figure, it is not clear whether it is a forward or flyback type. However, from the structure of this circuit, it can be inferred that this power supply should be a flyback type. The 510KΩ on the left is a starting resistor, which provides the base current for the switch tube to start. The 10Ω resistor below 13003 is a current sampling resistor. After sampling, the current becomes a voltage (its value is 10*I). This voltage is added to the base of transistor C945 after passing through diode 4148. When the sampling voltage is approximately greater than 1.4V, that is, the switch tube current is greater than 0.14A, transistor C945 is turned on, thereby lowering the base voltage of the switch tube 13003, thereby reducing the collector current, thus limiting the current of the switch to prevent excessive current from burning (in fact, this is a constant current structure, which limits the maximum current of the switch tube to about 140mA).

　　The induced voltage of the winding (sampling winding) at the lower left of the transformer is rectified by the rectifier diode 4148 and filtered by the 22uF capacitor to form a sampling voltage. For the convenience of analysis, we take one end of the emitter of the transistor C945 as the ground. Then the sampling voltage is negative (about -4V), and the higher the output voltage, the more negative the sampling voltage. After the sampling voltage passes through the 6.2V voltage regulator diode, it is added to the base of the switch tube 13003. As mentioned earlier, when the output voltage is higher, the sampling voltage is more negative. When it is negative to a certain extent, the 6.2V voltage regulator diode is broken down, thereby lowering the base potential of the switch 13003, which will cause the switch tube to disconnect or delay the conduction of the switch, thereby controlling the energy input into the transformer, and controlling the increase of the output voltage, realizing the function of voltage regulation output. The 1KΩ resistor and the 2700pF capacitor in series below are positive feedback branches, which take out the induced voltage from the sampling winding and add it to the base of the switch tube to maintain oscillation.