Series switching power supply
After the 220V AC mains is rectified and filtered by CI, a 300V DC voltage is obtained on C (when the mains voltage is 220V, the maximum value after rectification can reach 220√2V=311V, where the 300V DC voltage refers to the approximate value), this voltage is The winding L1 is sent to the collector of the switch tube VT.
A pulse signal is applied to the base of the switch tube VT. When the high level of the pulse signal is sent to the base of VT, the VT is saturated and turned on, and the voltage of 300V is charged to the capacitor C2 through the collector and emitter of L1 and VT. When the charging current passes through L1, L1 will generate left positive and right negative electromotive force to block the current, and L2 will induce left positive and right negative electromotive force (the same name terminal has the same polarity) , the freewheeling diode VD1 is turned off; when the low level of the pulse signal is sent to the base of VT, VT is turned off, no current flows through L1, L1 immediately generates left negative and right positive electromotive force, and L2 induces left negative and right positive electromotive force. Electromotive force, diode VD1 is turned on, and the electromotive force on L2 charges C2.
The charging path is: the right positive of L2 → C2 → ground → VD1 → the left negative of L2, and the positive and lower negative voltages Uo are charged on C to supply the load RL. Voltage regulation process: If the mains voltage drops, the voltage across C1 will also drop. If the pulse width of the VT base remains unchanged, when the VT is turned on, the charging current will decrease due to the drop in the supply voltage. The voltage U at the terminal will drop.
In order to ensure that the voltage at both ends of C will not drop when the mains voltage drops, the pulse signal sent to the VT base can be widened (the high level lasts for a long time), the VT conduction time is long, the C2 charging time is long, and the C2 The voltage across both ends rises back to normal again.
