One of the width modulated and frequency modulated hybrid switching regulated power supplies

The circuit shown in the figure is a switching stabilized power supply with mixed pulse width and pulse frequency modulation using an integrated voltage regulator W724 as the control circuit. Its main technical indicators are as follows: Input voltage: 220V±10%; Output voltage: 4.5~5.5V; Output current: 0~5A; Voltage adjustment rate: 0.05%; Load adjustment rate: 0.2%; Working frequency 9~20kHz; Output Ripple <20mV (peak to peak); Efficiency: 62%. The self-oscillation of this circuit is realized by R13C1. As soon as the power supply is started, 220V AC is applied to the primary of transformer T1. After secondary transformation, bridge rectification and filter capacitor C6, an unstable smooth DC voltage of about 15V is output, which is applied to the input of composite switching power tubes VTl2 ~ VT14. terminal; at the same time, this voltage also provides a stable operating voltage to the integrated voltage regulator W724 through the resistor R10 and the voltage regulator tube VD6. At this time, since the output voltage Uo has not yet been established, there is no voltage across the sampling voltage dividing resistors R17, R24, and R18, and VT5 in the error amplifier is cut off because there is no base bias voltage. The current of VT3, which acts as a constant current source, passes through VT6 to turn on VT7. The voltage drop generated by the collector current Ic7 on R11 causes the composite switching tubes VTl2 to VTl4 to conduct. As the emitter potential of VTl4 increases, the freewheeling diode VD7 is reverse-biased and cut off. This current flows through the inductor L, gradually building up the output voltage Uo. In addition, the emitter current of VTl4 also charges capacitor C1 through the base-emitter of R13, VT6, and VT7. With the output voltage U. is established, the base potential of VT5 gradually increases, and almost all the collector current of VT3 flows through VT5. The base current of VT6 decreases sharply, close to zero and tends to cut off. No current flows through R11, and the voltage drop decreases close to zero. , causing the composite switch adjustment tubes VTl2~VTl4 to be cut off. Coupled with the positive feedback effect of R13 and C1, the base potential of VT6 drops faster, causing VTl4 to completely cut off. At this time, the polarity of the voltage on the inductor L changes from the original left positive and right negative to the left negative and right positive, causing the freewheeling diode VD7 to conduct due to forward bias, and the energy stored in the inductor L supplies power to the load through VD7. When Uo is lower than the voltage of capacitor C7, C7 continues to discharge to the load. Uo continues to decrease as the energy storage of L and C7 decreases, causing the base potential of VT5 to also decrease, causing the collector current of VT5 to decrease, causing the current injected into the base of VT6 to increase; in addition, during the cut-off period of VT4, Constant current source VT3 reversely charges C1 through R13. The polarity of charging is positive down and negative up. The base potential of VT6 further accelerates and increases, causing VT6 and VT7 to conduct quickly, and controlling the composite switch tubes VTl2~VTl4 also quickly. conduction. When this happens over and over again, self-oscillation begins. It is not difficult to see from this that if the output voltage becomes lower and the output current becomes larger, the oscillation frequency will automatically accelerate, about 20kHz when the rated output current is 5A; conversely, if the output voltage becomes higher and the output current becomes smaller, the oscillation frequency will The frequency will automatically slow down, and the oscillation frequency is about 9kHz at no load. In addition, when the output voltage Uo changes and the base potential of VT5 also changes, the change in the collector current of VT5 controls the base potential of VT6 and the charge and discharge time of C1, thereby controlling the pulse width of the switching tube VTl4, that is, When Uo rises, the pulse width becomes narrower, causing U. decrease; conversely, when Uo decreases, the pulse width becomes wider, causing Uo to increase, thus maintaining the stability of the output voltage. R14, VTl5, VTl6, and VTl0 form an overcurrent protection circuit. When the output is overloaded, the voltage drop across R14 increases, causing VTl5, VTl6, and VTl0 to conduct, the base potential of VT6 decreases, and the output pulse width narrows, limiting the output current below the rated value, thus limiting the current. Protective effects. C4 and C5 are used to eliminate possible high-frequency oscillation; R15, C2 and C3, R16 are used to reduce ripple and radio frequency interference.