Switching power supply charger overall circuit design

　　The switching power supply charger has a supply voltage of 110V, which can be easily changed to 90-250V and continue to work; the output voltage is 5V, which can be changed to 5-12V, which is especially suitable for fast charging of 3.6V (or 4-9V) batteries of cordless phones or mobile phones.

　　Working principle of the circuit: As can be seen from the figure, VC1, L5 and C2 form a mains input rectifier filter circuit, and a DC high voltage of about 300V is generated at both ends of C2. VT1, VT2, L1, L2, etc. form a self-excited oscillation circuit, and R3 and R4 provide a starting bias current, so that VT1 is turned on when powered on. When current flows through the main circuit L1, an induced electromotive force is generated on L2. When its peak value exceeds 3V, VD5 is broken down, and a bias current is provided to VT2 through R8, so that VT2 is saturated and turned on, and VT1 is turned off because the bias voltage is short-circuited. When the current in L1 is turned off, the polarity of the induced electromotive force of L2 is reversed, and a reverse bias is applied to the base of VT2 through VD5 and R8, VT2 is transformed into a cut-off state, and VT1 is turned on again through the bias current provided by R3 and R4. This cycle repeats, forming intermittent self-excited oscillation. C5 and R6 are used to improve the oscillation waveform, and the photoelectric coupler OPT1 is used to adjust the pulse width of the oscillator.

　　L3, L4, C7 and other components form a rectifier output circuit, diode 3S90 is used for half-wave rectification, RK14 is used for charging isolation, and R18 is used as an output current sampling resistor. When the output current is overloaded (greater than 0.8A) or short-circuited, a large voltage drop occurs on R18, causing the output potential of OP1 to drop sharply, the photocoupler controls the oscillation pulse to become narrower, and the average energy coupled from L1 to L3 is also greatly reduced. Even if the output is short-circuited, the output current is only a dozen milliamperes, thus avoiding the threat of overload or even short circuit at the output end to the switching power supply itself. The voltage stabilization part is composed of peripheral circuits such as TL431. The voltage sampling points are taken from both ends of the charged battery. According to the parameter values ​​of R13+R14 in the figure, the no-load output voltage is 5.25V. The maximum charging current for 3.6V rechargeable batteries is 0.95A, which is suitable for direct charging of nickel-cadmium or lithium batteries above 2A·h. If it is used to charge 0.7-1A·h nickel-cadmium or lithium batteries, a current-limiting resistor with a resistance of 1.5-2.5Ω and a power of 0.5W can be connected in series in the charging circuit to limit the charging current to 0.3-0.4A.