MP3 power adapter circuit principle

The circuit of this adapter is shown in the attached figure. The serial numbers of the components in the figure are consistent with the real object. The transformer pin numbers and the same-name terminals and interface symbols are marked by the author.

The power adapter adopts a typical switching power supply structure. The working principle is as follows: AC220V AC mains electricity is added to the main winding of the switching transformer T1 after DI~D4 bridge rectification and C1 filtering, ①→②→the c pole of the switching tube Q1; the other way is through the starting resistor R2→R4 (R4 mainly plays a current limiting role)→Q1b pole, so that Q1 is forward biased and turned on. At the same time, the polarity of the induced voltage generated on the feedback winding is ③ positive and ④ negative. This voltage passes through the acceleration capacitor C3//R6→current limiting resistor R5→R4→Q1b pole. Under the joint action of the above voltages, Q1 quickly saturates. At this moment, the magnetic flux Ψ in the switching transformer reaches the maximum; the magnetic flux increment △Ψ is zero. This is the process of "electricity" generating "magnetism". After that, the magnetic field disappears rapidly. The magnetic flux change △Ψ is the maximum, and the polarity of the induced voltage generated in each winding is reversed. This is the process of "magnetism" generating "electricity". The polarity of the induced voltage generated in each winding of T1 in this process is: ① negative ② positive; ③ negative ④ positive; ⑤ negative ⑦ positive. The induced voltage of ① negative ② positive is very easy to break down the switch tube Ql. To prevent the breakdown of Q1, the D2, C4, R3 clipping suppression circuit is added to the circuit. The ③ negative ④ positive feedback voltage prompts the rapid reverse bias cutoff of Q1. In the ③→C3//R6→R5→R4→b--e emitter junction of Q1→④→③ loop of T1, the polarity of the induced voltage generated in the ③～④ windings of T1 keeps changing forward, reverse, forward, reverse... over and over again, causing Q1 to repeatedly work in the switching state of saturation, cutoff, saturation, cutoff... The ⑤ negative ⑦ positive induced voltage is rectified by D7 and filtered by C5 for battery charging. Z1, C2, and D5 in the circuit form a clamping circuit, so that the voltage at the intersection of R2, R4, and R5 when power is just turned on is not higher than the algebraic sum of the regulated voltage value of Z1 and the forward conduction voltage drop value of D5 (6.8V+0.7V in this circuit), in order to prevent Q1 from being in a state of conduction and loss of control.

Charging indicator: After the power is turned on, the red working indicator lights up. After the MP3 is plugged in, the green charging indicator lights up, indicating that it is in the charging state. As the charging voltage increases, the voltage of the b pole of Q2 also increases. When it rises to Q2's Vbe≤-0.7V, Q2 is cut off and the green charging indicator goes out, indicating that the battery voltage is fully charged. After that, it enters the trickle charging working state, and the current flows as follows: C5's positive pole → Q2's e → b → R10 → charged battery → C5's negative pole. The measured voltage across C5 is 6.8V when it is unloaded; when charging, it is 6.1V, the maximum charging current is 150mA, and the trickle charging current is about 5mA.