Frequency modulated switching power supply part 2

The picture shows another example of FM switching power supply, which is the Telefunken 5016 color TV switching regulated power supply. Its frequency range is 38~70kHz. When the input voltage changes in the range of 90~300V, the output DC voltage can be kept stable. The power supply circuit consists of an input rectifier filter circuit, an oscillation circuit, an error amplification circuit, a push circuit, and an output rectifier. Filter circuit and other components. GR601 is a bridge rectifier, used for input rectification: C93, C94, and L714 form the input mains filter; C95~C98, R242, and R243 are used to suppress surge current. C99 is the input smoothing filter capacitor. The 300V DC voltage it outputs is provided to the switching power tube VT464 through the transformer T701. The oscillation circuit is composed of the switching tube VT464, the primary winding ①~③ terminals of the transformer T701 and the feedback winding ③~④ terminals, VD526, VD521, C109, C102, R265, R260, R250, etc. It uses the half-wave of 50Hz alternating current rectified by VD521 to trigger the oscillation. The oscillation state is formed and maintained by the positive feedback between the ①~⑧ end windings and the ③~④ end windings of T701 and the charge and discharge of C109. The error amplifier tube is VT463. The reference voltage circuit is composed of R254 and VD527. The sampling circuit is composed of voltage dividing resistors R251, R256, and R255. The ⑤~⑥ terminals of T701 are sampling windings. VT461, VT462, etc. form a driving circuit. The pulse signal voltage coupled from T701 at the ⑤~⑥ end of the sampling winding is rectified and filtered by the diode VD528 and the capacitor C108 and then a DC voltage of 20.4V is output. After being added to the base of VT463 by the sampling voltage dividing circuit, the error is compared with the reference voltage. The signal is amplified and then applied to the base of VT462. At the same time, the DC voltage obtained by the terminal windings of T701③~④ through VD525 and C107 is also added to VT462 as a trigger signal to turn on VT462 and VT461. In this way, the voltage error signal is added to the capacitor C103 through VT462 and VT461, and the base of the switching tube VT464 is controlled to change the oscillation frequency and keep the output stable. This power supply itself has an overcurrent protection function: when the load is short-circuited, the current in the secondary of transformer T701 is very large. This current can destroy the balance state of the intermittent oscillation circuit and stop the oscillation. From then on, the power supply stops working, so the power supply will not be damaged. VT465, R259 and R264 form an undervoltage protection circuit. When the input voltage drops below 150V, the collector voltage of VT464 decreases, the collector current decreases, the voltage of the ⑤~⑥ end winding of T70l decreases, the voltage on C108 decreases, the base potential of VT463 decreases, and the collectors of VT463 and VT461 Potential rises. As the base potential of VT464 rises, its collector current Ic464 increases, thereby maintaining the stability of the output voltage. When the input voltage is further reduced to about 110V, VT465 starts to work. As the internal resistance of VT465 becomes smaller, the oscillation frequency is changed, causing the output voltage to rise, while still maintaining the stability of the output voltage U0.