Transistor switching regulated power supply application circuit example

Transistor switching regulated power supplies are widely used, as shown in the figure is a specific circuit example. In the figure, transformer T, diodes VD1 ~ VD4 and capacitor C5 form a rectifier filter circuit; resistors R7, R8 and potentiometer R9 are connected in series to form a sampling circuit; transistor Vr4 is used as an error amplifier; transistors Vr2 and Vr3 are combined to form a switching regulator; VT1 is a pulse Wide adjustment tube; VTl, VT2, VT3 and R3, C8 form a self-oscillation circuit; resistor R6 and Zener diode VD6 form a reference voltage source; VD5 is a freewheeling diode; L is an energy storage inductor; VD5, L and C12 Forming an output filter; the loop composed of R5 and C10 can make the freewheeling diode VD5 have good recovery characteristics. When the AC grid power switch S is turned on, the bridge rectifier outputs DC voltage. Its positive terminal is connected to the emitter of the V T2 tube, and its negative terminal provides a bias voltage to the base of the Vr2 tube through the resistor R2, so that the composite switch adjustment tube is saturated and turned on. When the output current passes through the energy storage inductor L, it generates positive left and negative induced potentials on the right. At this time, the emitter of the VB tube is at a high potential, the freewheeling diode VD5 is cut off, the output current is supplied to the load, and the capacitor C12 is charged at the same time. When the voltage rises to a certain level, the error amplifier VT4 starts to work. After VT4 works, the output current of the switching regulator charges the capacitor C8, and the voltage on the capacitor C8 is positive on the upper side and negative on the lower side. When C8 is charged to a certain level, transistor VT1 is saturated and turned on. Its tube pressure drop Uce1 is very small. The base potential of VT2 increases, forcing the composite switching tubes VT2 and VT3 to cut off. At this time, the current on the energy storage inductor has risen to the maximum value. However, due to the turn-off of the switch tube, the current on L will decrease. The induced potential generated by this changed current on L is negative on the left and positive on the right, which will prevent the current from decreasing and at the same time make the freewheeling diode VD5 conductive. The energy on L forms a path with the load through VD5, allowing it to continue to supply power to the load. When the voltage provided by L to the load is lower than the voltage across C12, C12 will supplement the power supply to make up for the lack of power released by L, so that the output voltage remains a smooth DC. Once the switch tube VT3 enters the cut-off state, C8 changes from the charging state to the discharging state, and then develops into the reverse charging state. The voltage on C8 is negative up and positive down. When reverse charging reaches a certain level, VT1 is cut off due to the increase in its base potential. The composite switch tube is turned on because the base of VT2 regains the low potential, and the self-oscillation cycle continues like this. Its oscillation frequency is mainly determined by resistor R3 and capacitor C8. When the output voltage rises for some reason, the potential provided to the base of the error amplifier tube VT4 through the sampling circuit increases, causing its collector current to increase and the tube voltage drop Uce4 to decrease, thereby accelerating the charging of C8. The voltage across C8 increases rapidly, and the collector potential of VT4 decreases rapidly, causing the pulse width modulation tube VTl to quickly switch from cutoff to conduction, and increase the conduction time. The composite switching tubes VT2 and VT3 accordingly extend the cut-off time, narrow the output pulse width, and reduce the increased output voltage. On the contrary, when the output voltage drops, the adjustment process is the same and in the opposite direction, raising the dropped output voltage again, thereby maintaining the stability of the output voltage. Transistors can be used as switching adjustment devices in many types of switching power supplies. For the sake of simplicity, they are not listed here.