Parallel switching power supply for televisions (a)

It is a parallel switching regulated power supply commonly used in televisions. It does not have a power transformer. It directly rectifies the mains
power to obtain a DC voltage of about 300V, and then transforms it through a parallel switching power supply circuit with a pulse transformer
to obtain two sets of stable low voltages (6-3V and 35V) and a set of 100V. In the medium voltage output port diagram (a), VTi is the switching tube and T is
the pulse transformer. Its primary winding ni is equivalent to the energy storage inductor on2, n3 and n4 are three secondary windings with different numbers of turns,
corresponding to three different the output voltage. The winding code is the feedback winding used to provide positive feedback voltage when the switching tube performs intermittent oscillation
. The 35V output voltage in the circuit is also used for sampling. MA, Rio, and Rio are sampling resistors, VD6 is the reference voltage
regulator tube, VT2, and VT3 are the comparison amplifier ports. Among the three sets of outputs, VD, VD4, and VI are respectively are the freewheeling diodes of each group
, and C7, Island, and C9 are the filter capacitors of each group.
    According to the same terminal of the transformer, when the switch tube VTi is turned on, the induced voltage on the winding is negative up and positive down, and island VI is
reverse-biased and cut off. At this time, T stores energy, and the load current is supplied by island discharge, which is equivalent to Figure 7- 30 (C) When VTi is cut off
, the induced voltage on La is positive up and negative down, the freewheeling diode VI Ga is turned on, T releases energy to charge the island, and
supplies power to the load at the same time, which is equivalent to Figure 7-30 (d) oIn
    this circuit, VTi is both a switching tube and an oscillation tube. Its oscillation is established like this: after the power is turned on,
the DC current obtained after rectification and filtering of the 220V mains power is added to the base of VTi through R2. Under the action of the base voltage, t
 VTi generates a collector current and is driven by a small The column size gradually increases, so a self-induced electromotive force is generated in the winding ni in the collector circuit
to prevent the current from increasing. Its polarity is positive up and negative down. At the same time
, an induced potential with the same polarity as ni is induced in the secondary winding M of T , that is, positive up and negative down. Since the voltage across the capacitor cannot
change suddenly, C is not charged immediately when n5 generates the induced electromotive force. Instead,
the base potential of vTi is increased through the coupling of Crs, which means a positive feedback is formed. Due to the effect of this positive feedback, the base potential of VTi
increases, the collector current increases, the induced electromotive force further increases, and the collector current continues to increase until VTi enters
the saturated conduction state.