Absolutely usable switching power supply: Venus D2902 power supply

The power supply of Jinxing D2902, D2912 and other models uses the thick film power supply block STR-S6708 of Sangen Company. This power supply has the characteristics of adapting to a wide grid voltage (90V-270V), complete protection circuit, and few peripheral components. The circuit can change the switch. The power pulse width adopts narrow pulse mode in standby mode and wide pulse mode in normal startup mode, so there is no need to set up an auxiliary power supply in standby mode.

During the oscillation process of the switching circuit,
pin (9) of STR-S6708 is the power supply pin. Only when the power supply to pin (9) is normal, the thick film circuit will work normally.
VD908 directly rectifies the 220V AC power. After R903, R917 current limiting and C909 filtering, it obtains a DC voltage of about 8V. It is added to pin (9) of IC901. IC901 starts to work, and the switching power supply starts to oscillate. The voltage energy obtained by the rectification of VD908 It is too small to maintain the normal operation of IC901, but when the switching power supply starts to oscillate, the (V2) pin of the switching transformer T901 will output the voltage. After being rectified by VD903 and filtered by C909, a stable 8V voltage can be obtained to supply power to IC901.
The voltage rectified by VD903 alone is still not enough, because when the TV enters the standby state, the main voltage of the whole machine will drop from 127V to about 30V. At this time, the output voltage of the (V2) pin of the switching transformer will also increase significantly. drop, the voltage rectified by VD903 cannot reach 8V at all. At this time, it is necessary to rely on the V901 circuit to continue to maintain the power supply. In the normal power-on state, the output voltage of the (V3) pin of the switching transformer is rectified by VD902 and filtered by C908 to obtain a DC voltage of about 45V, which is added to the C pole of V901. However, since the emitter voltage of V901 at this time is 8V , and the base is connected to the voltage stabilizing tube VD920. The voltage stabilization value of VD920 is 7.2V, so the base voltage of V901 is lower than the emitter voltage, V901 will not conduct, and the power supply of pin (9) of IC901 is provided by VD903. When the whole machine enters the standby state, the output voltage of the (V3) pin of the switching transformer is rectified by VD902 to a voltage of about 11V. At this time, because the voltage output by VD903 is very low, V901 is forward biased and begins to conduct, and its emitter The output voltage is about 6.7V, which continues to provide power to pin (9) of IC901.
Another function of the V901 circuit is that when the grid voltage decreases, the rectified voltage of VD903 will also decrease. When it decreases below 6.6V, V901 will conduct and continue to supply power to pin (9) of STR-S6708. Therefore, This kind of switching power supply can adapt to a wide range of grid voltage.
Pin (9) of IC901 starts to oscillate after receiving voltage. The pulse frequency and pulse width of the oscillation are determined by the RC time constant inside IC901. The oscillation pulse is output from pin (5) of IC901 and then divided into two channels: one channel is used as a negative feedback signal and sent back to pin (4) of IC901 via R906 and R910 to control the working status of the internal proportional drive circuit of IC901; the other channel is sent via C911 Back to pin (3) of IC901, pin (3) of STR-S6708 is the base of the internal switch tube, pin (1) is the collector, and pin (2) is the emitter. Under the action of the switching pulse, it starts to oscillate. The switching tube generates an oscillating current between C and E. It generates an induced voltage in the (P1) and (P2) windings of the switching transformer. After coupling, it also generates corresponding induced voltages in other windings. After rectification and filtering, it can be used by all levels of loads.
The switching power supply composed of STR-S6708, as long as the voltage above 6V is applied to its (9) pin, the circuit can start to oscillate, and there is no need to set up a positive feedback circuit.

The voltage stabilization process
of STR-S6708 is based on changing the current of pin (7) to control the output voltage. The greater the current of pin (7), the shorter the switch conduction time is and the lower the output voltage is; conversely, (7) The smaller the pin current is, the longer the switch conduction time is and the higher the output voltage is.
The output voltage of the whole machine is controlled by V951. V951 is a sampling transistor and R955 is a sampling resistor. It is sampled directly from the main power supply. If there is some reason that the main power supply voltage rises, the following series of control processes will occur, and finally the The output voltage is stable:
+B rises → B pole voltage of V951 ↑ → C pole voltage of V951 ↓ → Current of (1) and (2) pins of optocoupler IC902 ↑ → (4) and (5) of optocoupler IC902 Pin current ↑ → STR-S6708 (7) Pin current ↑ → output voltage decreases.
On the contrary, if the output voltage decreases, the above-mentioned opposite process occurs.

Standby/on control
The standby/on design of D2902 is unique and complex. Let’s analyze this part of the circuit.
There are two 5V outputs in the power supply of D2902, but the 5V on pin (2) of the power strip X306 is the power supply voltage of the CPU. It is not controlled by power on or standby. The output voltage always remains 5V, so it can continue to provide power during standby. CPU power supply.
When starting up, the POWER pin of the CPU outputs a high level, which is added to the base of V956, and V956 is saturated and turned on. At this time, for VD959, since its positive terminal voltage is 0V, VD959 is cut off, and the base of VD953 is When it is not biased and is cut off, V952 and V958 are also cut off. The collector voltage of V957 is obtained after rectification and filtering by VD953 (about 9V). After voltage stabilization, the 5V-STB voltage is output from the emitter for use by the CPU.
When in standby, the POWER pin of the CPU outputs a low level, which is added to the V955 base, V9