Switching power supply circuit in set-top box

Switching power supply circuit in set-top box

The switching power supply circuit in the digital cable TV set-top box is shown in the figure. As can be seen from the figure, it is mainly composed of AC input circuit, rectifier
filter circuit, switching oscillation circuit, switching transformer T602, secondary rectifier filter and voltage stabilizing circuit.
    (l) AC input circuit
    The AC input circuit is composed of fuse F601, mutual inductance filter T601, filter capacitors C601, C603, C604, etc. Its function
is to filter out pulse interference in the AC input signal.
    (2) The AC 220 V voltage of the rectifier and filter circuit
    is filtered by the bridge rectifier stack D601, the rectifier and filter capacitor C605, and the
approximately 300 V DC voltage output by the filter and bridge rectifier circuit is sent to the ④ pin of the switching transformer T602.
    (3) Switching oscillation circuit '.
    The switching oscillation circuit is mainly composed of JC601 and peripheral circuits. The oscillation circuit, voltage stabilizing control circuit and switching field effect transistor
are all integrated into IC 6(tl (E2A265)). After starting up, the 300 V DC voltage output by the AC 220 V rectifier is added through the primary winding
of the switching transformer T601 ④~@ to the solid and ⑤ pins of IC601. The ④ and ⑤ pins of IC601 are internally connected to the drain of the switching field effect transistor. When turning on the machine, DC 300V passes through R603 to form a driving voltage and is added to the ⑦ pin of IC601 to supply power to the circuit in IC601. The oscillation circuit in IC601 starts to oscillate, and the switching current begins to flow in the primary winding ④ of the switching transformer T601. The     switching signal is induced in the secondary windings ④~② of T601, and the output of pin ① is rectified, filtered and stabilized. The circuit forms a positive feedback signal that is superimposed on pin ⑦ of 1C601. Keeping pin ⑦ with sufficient DC voltage can maintain the oscillation in IC601 and make the switching circuit enter a stable oscillation state.     (4) The secondary winding of the secondary rectifier output circuit     has There are multiple taps, and the output end of each winding tap is connected with a rectification and filtering circuit, which outputs voltages such as 3.3 V, Sv and 12 V respectively.     The error detection circuit is set in the 3.3 V output circuit, and 3.3 V is connected in series with ground. Connect two voltage dividing resistors. The voltage dividing point of R622 and R623 is used as the voltage sampling terminal, and the sampling voltage is added to the input terminal R of the error amplifier IC603. The photocoupler IC602② is connected to the C terminal of IC603. When the output voltage changes It will cause changes in the luminous intensity of the light-emitting diode in IC602, which will change the impedance between the collector and emitter of the photosensitive transistor in IC602. The ④ terminal of IC602 is connected to the ② pin of IC601, and the @ pin is the voltage stabilizing negative feedback signal of IC601 Input terminal. This negative feedback loop can stabilize the output voltage.     IC604 (7805) is a 5v three-terminal voltage regulator. IC 60s is a 12V three-terminal voltage regulator.