Absolutely usable switching power supply: Venus C7458 power supply

Input circuit: The
220V alternating current is filtered by L801 and L802 to filter out the interference signals in the power grid, and then sent to BD801 for full-wave rectification. After being filtered by C808 and C809, a DC voltage of about 300V is obtained, which is used as the working voltage of the switching power supply. L801, L802 and the capacitors in the input circuit also have the function of preventing the pulse signal of the switching power supply from entering the power grid and polluting the power supply.

Oscillation of switching power supply:
The oscillation chip of switching power supply adopts TDA4605-2. This circuit can complete the oscillation, voltage stabilization and various protection of switching power supply.
After the 220V AC voltage is rectified by BD801, current limited by R804, and filtered by C810, a voltage of about 10V is obtained, which is added to pin (6) of IC801 as the starting voltage of the switching power supply. IC801 starts to oscillate, and the oscillation pulse is output from pin (5), which is added to the gate of switching tube Q801 through R813. Each winding of the switching transformer starts to output voltage, completing the starting process. The oscillation pulse frequency output by IC801 (5) pin is related to the weight of the load.

Voltage stabilization of switching power supply:
Pin (1) of TDA4605-2 is the error amplification sampling current input terminal. The voltage output by the winding of pin (5) of the switching transformer is rectified by D802, filtered by C815, and then divided by R824 + R812 and R810 before being sent to pin (1) of IC801. At the same time, the C and C of the triode inside the optocoupler IC803 The E pole is connected in parallel to R824, so the conduction degree of the transistor inside IC803 affects the input current of pin (1), and the light-emitting diode inside the optocoupler is connected to the main power output through RR825, VR801, R826, and R827. The voltage is sampled at 120V, thus forming a voltage stabilizing control loop. By adjusting VR801, the output voltage can be controlled at 120V.

Protection circuit:
Mains overvoltage protection:
Pin (2) of IC801 is directly connected to 300V through R807+R808. If the grid voltage is too high, 300V will inevitably increase. Once the voltage of pin (2) of IC801 is greater than 3V, the internal The protection circuit operates, (5) pin has no excitation pulse output, and the switching power supply stops vibrating.

Mains undervoltage protection:
If the grid voltage is too low, the voltage of the positive pole of C808 and C809 will also be low, and the voltage after dividing by R840+R805 and R806 will also be low. Once the voltage of pin (3) of IC801 is lower than 1.8V, then The internal undervoltage protection circuit operates, (5) pin has no excitation pulse output, and the switching power supply stops oscillating.

Switching power supply output overvoltage protection:
For some reason, the output voltage of the switching power supply is too high, causing the voltage output by the switching transformer (5) pin to be rectified by D801 and then added to the IC801 pin (6) to increase the voltage. Once the IC801 ( 6) If the pin voltage is greater than 18V, its internal circuit will automatically cut off the excitation output pulse of pin (5), causing the switching power supply to stop oscillating.

Switching power supply overload protection: There is a drive power detection circuit inside pin (5) of IC801. Once the load is overloaded or short-circuited, causing the drive power of pin (5) to be too large, the internal protection circuit will act, and pin (5) will have no excitation pulse output, and the switch will The power supply stops vibrating.

There is also a "zero-crossing detection circuit" inside pin (8) of TDA4605-2. Simply put, pin (8) detects the output pulse of pin (5) of the switching transformer through R811 and R815. Once no continuous pulse or detection is detected, The received pulse amplitude is too small, and the zero-crossing detection circuit will cause no excitation pulse output at pin (5).
Pin (7) of TDA4605-2 is the input terminal of the soft-start integrating capacitor. It is connected to the external integrating capacitor C811. The charging of C811 can be used to achieve soft starting of the switching power supply when the power is turned on.

Output circuit:
The output voltage of the (16) and (18) windings of the switching transformer is rectified by D806 and filtered by C833 to obtain a 120V DC voltage, which is used as the line scan working voltage and also as the sampling voltage. It is added to the optocoupler IC803 through R825, VR801, etc. . During standby, relay RL802 is disconnected, so there is no 120V power supply for the row scanning circuit.
After the output voltage of the (14) and (15) windings of the switching transformer is rectified by D805 and filtered by V820, a 12V voltage is obtained, which is used as the horizontal oscillation supply voltage. During standby, relay RL802 is disconnected, the horizontal oscillation supply voltage is cut off, and the horizontal oscillation stops. Vibrate. In addition, the 12V voltage is also stabilized by IC802 to obtain a 5V voltage, which is the power supply voltage for the CPU.
After the output voltage of the (12) and (13) windings of the switching transformer is rectified by D804 and filtered by V818, a 24V voltage is obtained, which is the power supply voltage of the audio amplifier circuit.