Width modulation, frequency modulation hybrid switching regulated power supply part 2

The picture shows another example of a mixed frequency modulation and width modulation switching regulated power supply, which is a power supply for Shanghai brand Z237-1 color TV. Its output is 110V, and the mains voltage can vary within the range of 90~275V. This power supply adopts self-oscillation, and VT90l is a switching power tube and self-oscillation tube. The control circuit consists of VT902~VT904, etc. When the power supply is working, the induced voltage on the ⑦~⑧ terminals of the sampling winding of the transformer T901 is rectified by the diode VD911 and then supplied to the error amplifier for sampling and amplification. Its error voltage is output from the collector to the base of VT903 to control the bias voltage of VT903. In addition, the signal differentiated by R919 and C913 from the pulse output by the ⑦~⑧ end winding is also added to the base of VT903. These two signals are superimposed to control the on and off of VT902 and VT903. When the two tubes are turned off, the rectangular pulse voltage on the terminals ⑨~⑩ of the feedback winding is rectified by VD901, and an upper negative and lower positive voltage is established on the capacitor C910, causing VD910 to be reverse-biased and turned on, and VT90l is turned on. When VT902 and VT903 are turned on, C910 discharges through VT902, and the potential of the lower end of C910 decreases. VD910 is forward-biased and turned on, and the emitter potential of VT902 decreases, causing VT90l to change from conduction to cut-off. Changing the bias of VT903 can change the pulse frequency of the oscillation circuit; changing the conduction time of VT903 can change the pulse width of VT901, thereby achieving mixed modulation of pulse width and pulse frequency. If the output voltage Uo rises, the voltage on the sampling winding ⑦~⑧ also rises, and the voltage provided to the error amplifier VT904 through VD911 rectification also rises, causing its collector current to increase, the base voltage of VT903 to rise, and VT902 to , the cut-off time of VT903 becomes shorter, the conduction time of the switch tube VT901 is shortened, and the output pulse duty cycle is reduced, which causes the output voltage Uo to decrease, thereby maintaining the stability of the output voltage. This power supply also has an overvoltage protection circuit composed of a silicon controlled SCR, a Zener diode VD913, and a diode VD906. In case of overvoltage, it can cause the entire power supply to stop working.