Absolutely usable switching power supply: Venus C7428 power supply

The C7428 power supply adopts a wide voltage design and can adapt to the grid voltage of 110V-245V. It is very similar to the Toshiba 2500XH power supply.

Grid input switching conversion circuit
In order to adapt to the grid voltage of 110V-245V, the power supply is designed to automatically switch between voltage doubler rectifier/bridge rectifier. When the grid voltage is lower than 145V, voltage doubler rectification is used; when the grid voltage is higher than 145V, bridge rectification is used.
After the grid input voltage is low-pass filtered, it is first rectified by V801 and V802, and the identified voltage is obtained on C808, which is added to the (2) and (5) pins of STR81145. If the grid input voltage is lower than 145V, the voltage at both ends of C808 will be higher. Low, pins (2) and (3) of STR81145 are connected. At this time, V803, C818, and C819 form a voltage doubler rectifier. After the voltages of C818 and C819 are superimposed, a voltage of about 285V is obtained; if the grid input voltage is higher than 145V, both C808 and The voltage at the terminal is relatively high, and pins (2) and (3) of STR81145 are disconnected. At this time, V803, C818, and C819 form a bridge rectifier. After the voltages of C818 and C819 are superimposed, a voltage of more than 250V is obtained.
In order to prevent misidentification of STR81145, there is an overvoltage protection circuit in the circuit. Under normal circumstances, the voltage at both ends of R808 is lower than 22V, V804 is cut off, and the thyristor V808 is also cut off. If the output voltage is too high, the voltage at both ends of R808 exceeds 22V. When, V804 is turned on, V808 is also turned on, and the fuse is burned out.

During the oscillation process
, 300V DC voltage is added to pin (16) of Q803 through R810, Q803 starts to oscillate, and the excitation pulse is output from pin (14) to the B pole of the switch tube V884, and the switching power supply starts to work. At this time, T803 (8), ( 9) The induced voltage on the winding is rectified by V810 and filtered by C824 to generate a 12V voltage to supply power to pin (16) of Q803. At the same time, after rectification by V833 and V807 and filtering by C820, the voltage at the positive terminal of C820 turns on V832, V897 turns on, and V896 turns off.
In the standby state, N807 stops working and relies on the pulse width modulation generating circuit of Q803 itself. Pin (14) still outputs excitation pulses, but at this time the oscillation frequency of the switching power supply drops to 30HZ, and the output voltage of each winding also drops to the original value. About half of the time, the voltage at the positive terminal of C820 is not enough to turn on V832. In this way, V897 is cut off and V896 is turned on. V896 continues to supply power to pin Q803 (16), and the voltage source of V896 is obtained by rectifying pin (7) of T803 through V809. , (7), (9) windings are twice as many as (8), (9) windings, so in the standby state V896 can still output 12V voltage to supply pin (16) of O803. In addition, the voltage at pin (16) of Q803 is divided by R815 and R816 to obtain a 3.3V voltage, which is sent to pin (9) of Q803 to monitor the width of the excitation pulse at pin (14) to ensure that Q803 still has a stable voltage in the standby state. effect.
C829 is the excitation pulse input coupling capacitor of V884. In the saturated state of V880, C829 is charged, and the result of charging is that the left side of C829 is positive and the right side is negative; in the cut-off state of V884, the negative pulse of Q803 (14) pin turns on V812, so the right end of C829 The negative voltage is added to the B pole of V884 to maintain the cut-off of V884. The function of V813-V816 is to ensure that the voltage across C829 is around 2.4V.

Voltage stabilizing circuit
The voltage stabilizing circuit of C7428 is completed by N807 (TEA5170). The 115V voltage is sampled by R833, R851, and R834 and sent to pin (5) of N807. It is amplified by the internal reference comparison to control the pulse width output by pin (3). The pulse output by pin (3) is coupled by T804 and sent to Q803. (6) pin, adjust the pulse width output by (14) pin through Q803 to control the stability of the output voltage. Pin (7) of TEA5170 is an oscillation pin. By charging and discharging C838, it generates a 32KHZ, 1V peak-to-peak sawtooth wave signal.

Standby control circuit
In the standby state, the POWER output of the CPU is high, and is added to the B pole of V881 through R860. V881 is cut off, V889 is cut off, and V894 is cut off, so that the power supply to pin (2) of TEA5170 is cut off. On the other hand, the cut-off of V881 causes the cut-off of V830, the cut-off of V895, the conduction of V805, and the power supply of the horizontal oscillation circuit is cut off.