Toshiba ultra-thin DVD player switching voltage stabilizing circuit

Toshiba ultra-thin DVD player switching voltage stabilizing circuit

Shown is the switching regulated power supply circuit of Toshiba 's ultra-thin DVD player. It mainly consists of AC input circuit, bridge rectifier filter
circuit , switching oscillation circuit, switching transformer T802 and secondary rectification, filtering, voltage stabilization and output circuits, etc. Partial composition.
    1. AC input circuit
    The AC input circuit is composed of fuse F801, mutual inductance filter T801, filter capacitors C801-C804, etc. Its main function
is to filter out noise and pulse interference in the AC circuit.
    2. The 220 V AC voltage filtered by the rectifier filter circuit
    is rectified by the bridge rectifier stack D803 and becomes a DC voltage of about 300 V, and then added to
the @ pin of the switching transformer T802 to provide DC bias for the switching oscillation circuit.
    3. Switching oscillation circuit
    The switching oscillation circuit is mainly composed of switching field effect transistor VT801, switching oscillation control integrated circuit VT802 and related circuits. When starting
up, the 300 V DC voltage rectified and output by the 220 V AC voltage is added to
the drain D of the switching transistor VT801 through the primary winding @~③ of the switching transformer. The source of the switching transistor VT801 is grounded, and the gate is controlled by the ⑦ pin of the switching oscillation integrated circuit VT802. The 300 V DC voltage at startup
provides a starting voltage to the ① pin of VT802 through the starting resistor R806, causing the oscillator in VT802 to start oscillating and
providing an oscillating signal to the gate of the switching tube VT801, so the switching tube VT801 starts to oscillate, and the switching transformer A switching current is generated in the primary coil of T802. The induced current is generated in the secondary windings ①~② of the switch
transformer and is output by pin ②. After rectification and filtering, a positive feedback voltage is formed and added to
pin ① of VT802, thereby maintaining the operation of the oscillation circuit and bringing the switching power supply into normal working condition. .
    4． After the switching power supply of the secondary output circuit
    is started, the secondary coil of the switching transformer T802 outputs switching pulse signals respectively. Each output end is connected with a rectifier filter
circuit , which outputs +12V, +5V, +9V, +3.3V, 34V respectively. V and other voltages.
    The error detection circuit is set in the +5 V output circuit. R824 and R825 form a voltage dividing circuit, and its voltage dividing point is connected to
the input terminal R of the error detection circuit VT821 as a sampling point.
    If the output voltage is unstable, the voltage at the sampling point will change proportionally, and this change will cause the impedance of VT821 to change.
VT821 is connected to the cathode of the light-emitting diode in the photocoupler VT803. If the impedance of VT821 becomes smaller,
the luminous intensity of the light-emitting diode in VT803 will increase, otherwise it will weaken. The impedance of the phototransistor in the VT803 will also change. The output of VT803 is connected to the voltage stabilizing negative feedback terminal ④ pin of VT802
. Through this negative feedback loop, the output of VT802 is controlled, thereby stabilizing the output of the switching power supply.
    5. The power-on and standby control circuits
    add the power-on/standby control signal output by the emblem processor to the bases of VT830 and VT824. The ③
pin of the switching oscillation integrated circuit VT802 is the power-on and standby control terminal. VT830 pulls the VT802 pin through the photoelectric coupler VT804. ③Feet. VT824 controls the +8 V output to put the whole
machine in standby mode.