One of the self-excited switching regulated power supplies

The picture shows a self-excited switching regulated power supply circuit, which is composed of switching power tube VT2, pulse width modulation tube VT3, error amplifier tube VT4, overcurrent protection thyristor switch VTl and pulse transformer Tl. Among them, the switching power tube also serves as an intermittent oscillation tube. VT2, Tl, R2~R5, etc. constitute a deformed intermittent oscillator. After the power is turned on, the DC high voltage output by the rectifier is stepped down by R2~R5 and then provides an appropriate positive bias voltage to the base of VT2 to turn it on. Its collector current Ic2 passes through the primary coil N1, generating upper positive and lower negative induced voltages at both ends, and is coupled to the secondary feedback coil N5 through T1. The polarity of the induced voltage at both ends of N5 just makes the base of VT2 positive, causing Ic2 to increase. It can be seen that this is a positive feedback process, and VT2 quickly enters the saturated conduction state. At this time, the base loses control and the intermittent oscillator enters a relatively stable stage. The charging voltage at both ends of capacitor C5 is positive on the left and negative on the right, which is opposite to the polarity of the feedback voltage of N5. Therefore, the base power supply Ib2 gradually decreases, VT2 returns from the saturation area to the amplification area, J& decreases, causing the Nl induced electromotive force to change direction, and the potential on N5 also changes direction, Ib2 drops sharply, and Ic2 also drops sharply. Due to the effect of positive feedback, VT2 quickly turns to the cut-off state. At this time, the magnetic energy in the transformer cannot disappear immediately, and a higher voltage is induced in the collector circuit, which superimposes with the power supply voltage, causing the voltage between the collector and emitter of VT2 to exceed the input voltage. When VT2 is cut off, the voltage on the primary coil of the transformer is reversed and induced in the secondary. When the secondary voltage rises to a certain level, the output rectifiers VD2, VD3, and VD4 are turned on, and there is an output voltage at the output end. When VT2 is turned on again, the current on N1 will increase linearly, coupling to the secondary voltage and causing the output rectifier diode to be reverse biased and cut off. At this time, the output voltage is provided by the energy stored in the LC filter circuit. If the output voltage drops for some reason, the voltage applied to the emitter of VT4 through the Zener diode VD5 will also drop, and the voltage applied to the base of VT4 through the voltage divider R15, Rl7, and R16 will also decrease. Obviously, the emitter voltage decreases much more than the base, so the collector current of VT4 decreases, and the voltage drop across R11 also decreases. The base voltage of VT3 decreases. If the collector current decreases, R2, R3, The voltage drop across R4 and R5 decreases, the base potential of the switch tube VT2 increases, and the switch tube conduction time is lengthened, causing the output voltage to increase, thereby maintaining the stability of the output voltage. Since this power supply adopts self-excited working mode, when the load is short-circuited, the feedback voltage of the feedback coil is greatly reduced or dropped to zero, causing the intermittent oscillator to stop vibrating and the power supply to stop working, thus automatically playing a protective role. However, since it takes a certain amount of time to stop working after a short circuit occurs, the transistor may still be damaged. For this reason, a switching transistor with sufficient back pressure and power must be used. This power supply also uses a silicon controlled switch overcurrent and short circuit protection circuit. When the load is short-circuited, the emitter current of the switch VT2 increases significantly, and the voltage across the detection resistor R10 also increases significantly. This voltage is applied between the control electrode and cathode of the silicon controlled switch SCR, turning it on. Once the thyristor switch is turned on, the base of VT2 is grounded, causing the switch tube to stop working immediately, thus playing a protective role. As long as the resistance of the detection resistor R10 is appropriately selected, when the VT2 emitter current reaches a certain limit value, the SCR can be triggered and turned on to play an overcurrent protection role. In order to reset quickly, a reset switch S2 is specially designed. As long as S2 is pressed, the SCR can quickly return to the cut-off state.