Changhong DVB-2000 digital satellite receiver switching power supply circuit

Changhong DVB-2000 digital satellite receiver switching power supply circuit

The switching power supply circuit of the Changhong DVB-2000 digital satellite receiver box is mainly
composed of a power input circuit, a switching oscillation circuit, a DC output circuit, etc., as shown in the figure.
    (1) In the circuit shown in the power input
    circuit diagram , the 220 V AC power is rectified by the step-down transformer B and the bridge rectifier circuit VD502-VD505, and then
sent to the switching power supply circuit through the fuse F502 and the power switch K.
    The external input 12-24 V DC power supply is also sent to the switching power supply circuit
through F502 and K, and a rectifier diode VD501 is connected in series to the input circuit .
    The rectifier diode VD501 mainly plays two functions:
    ① The diode has unidirectional conductivity. It has an isolation effect on abnormal voltage.
    ② Isolate the input of AC 220 V and external input DC power supply. When there is AC 220 V input, the external DC voltage input will not work
, which can prevent the two inputs from working at the same time.
    (2) Switching oscillation circuit
    The switching oscillation circuit is composed of the switching transistor VD501, the switching oscillation integrated circuit Ul and the switching transformer T501. The fr<J@ pin of the integrated
circuit Ul SG3524 is the DC power supply end. After the power is turned on, the switching oscillation circuit starts to oscillate. The @ pin outputs the switching oscillation pulse, and the switching
power supply enters the working state. The voltage stabilizing circuit is connected to the +5 V output terminal. The +5 V output voltage is divided by the sampling resistors R501 and R502 to obtain
a +2.5 V sampling voltage and is sent to the ① pin of Ul as a negative feedback signal. When the output voltage rises , the internal error voltage of Ul (SG3524)
will rise, and the pulse width output by the uI@ pin will become narrower, thus causing the output voltage to drop, thereby achieving the purpose of voltage stabilization.
    (3) DC output circuit
    The DC output circuit has the same principle as the output circuit in a general digital satellite receiver . A +5V power supply composed of VD506
supplies power to audio D/A converters, tuners, etc., and at the same time outputs the required stable voltage through a low dropout (LDO) three-terminal voltage regulator integrator (IC502).
The input voltage of IC502 is Sv and the output is 3.3 V.