The power supply of Venus D2902, D2912 and other models adopts the power supply thick film block STR-S6708 of three companies. The power supply has the characteristics of adapting to the grid voltage width (90V-270V), perfect protection circuit and few external components. The circuit can change the switch. The power pulse width is operated in a narrow pulse mode during standby, and operates in a wide pulse mode during normal power-on, so there is no need to set an auxiliary power supply during standby.
Switch circuit oscillation process
The (9) pin of the STR-S6708 is the power supply pin. Only when the (9) pin is powered normally, the thick film circuit will work normally.
The VD908 is directly rectified from 220V AC. After R903, R917 current limit and C909 filter, DC voltage of about 8V is obtained, which is applied to the (9) pin of IC901. IC901 starts to work, the Switching Power Supply starts to oscillate, and the voltage energy obtained by VD908 rectification is obtained. Smaller, can not 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 voltage. After VD903 rectification and C909 filtering, a stable 8V voltage can be obtained to supply power to IC901.
The voltage after VD903 rectification is still not good, because when the TV enters the standby state, the main voltage of the whole machine will drop from 127V to 30V. At this time, the output voltage of the (V2) pin of the switching transformer will also be greatly increased. Down, the voltage after rectification by VD903 can not reach 8V at all, then it is necessary to rely on the V901 circuit to continue to maintain power. 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 applied to the C pole of V901. However, since the emitter voltage of V901 is 8V at this time. The base is connected to the voltage regulator VD920, the voltage regulation value of VD920 is 7.2V, so the base voltage of V901 is lower than the emitter voltage, V901 will not be turned on, and the power supply of (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, since the voltage of the VD903 output is very low, the V901 is positively biased and starts to conduct, and its emitter The output voltage is about 6.7V and continues to supply power to the (9) pin of the IC901.
Another function of the V901 circuit is that when the grid voltage is reduced, the voltage after rectification of the VD903 will also decrease. When it is lowered below 6.6V, the V901 will be turned on and continue to supply power to the (9) pin of the STR-S6708. This switching power supply adapts to a wide range of grid voltages.
The (9) pin of IC901 starts to oscillate after receiving the voltage, and the pulse frequency and pulse width of the oscillation are determined by the RC time constant inside the IC901. The oscillation pulse is output from the (5) pin of IC901, and then divided into two paths: one is sent as a negative feedback signal to the (4) pin of IC901 via R906 and R910 to control the working state of the internal proportional drive circuit of IC901; the other is via C911. Return to the (3) pin of IC901. The (3) pin of STR-S6708 is the base of the internal switch tube, (1) the pin is the collector, and the (2) pin is the emitter. The oscillation starts under the action of the switching pulse, the switching tube generates an oscillating current between C and E, and the induced voltage is generated in the (P1) and (P2) windings of the switching transformer, and after coupling, the corresponding induced voltage is also generated in the other windings. After rectification and filtering, it is used for various levels of load.
The switching power supply consisting of STR-S6708 can be oscillated as long as the voltage of (9) is increased by more than 6V, and no positive feedback circuit is required.
Voltage regulation process
The STR-S6708 controls the output voltage according to the current of the (7) pin. (7) The larger the pin current is, the shorter the on-time of the switch is, and the lower the output voltage is. On the contrary, the more the foot current is (7) Small, the switch tube conduction time becomes longer, and the output voltage is higher.
The output voltage of the whole machine is controlled by V951. V951 is the sampling transistor. R955 is the sampling resistor and samples directly from the main power supply. If there is some reason for the main power supply voltage to rise, the following series of control processes will occur, eventually making The output voltage is stable:
+B rises → V pole B voltage ↑ → V951 C pole voltage ↓ → photocoupler IC902 (1), (2) pin current ↑ → optocoupler IC902 (4), (5) pin current ↑ → STR-S6708 (7) foot current ↑ → output voltage is reduced.
Conversely, if the output voltage is lowered, the reverse process described above occurs.
Standby/boot control
The D2902's standby/boot design is unique and complex, so let's analyze this part of the circuit.
There are two 5Vs in the power output of the D2902, but the 5V on the (2) pin of the X306 is the supply voltage of the CPU. It is not controlled by the power on or standby. The output voltage is always 5V, so it can continue in standby mode. CPU power supply.
At the time of power-on, the CPU's POWER pin outputs a high level, which is applied to the base of V956, and V956 is saturated. At this time, for VD959, since its positive terminal voltage is 0V, VD959 is cut off, and the VD953 base is derived. V952 and V958 are also cut off without offset. The collector voltage of V957 is obtained by VD953 rectification and filtering (about 9V). After regulation, it outputs 5V-STB voltage from the emitter for CPU use.
During standby, the CPU's POWER pin outputs a low level, which is applied to the base of V955, V955 is turned off, and V954 is also turned off due to no bias. Therefore, IC5's 5V stop output is added; the base of V956 is applied, V956 is cut off, and the other is On the other hand, the voltage output by VD952 rectifies about 15V (80V at startup), is added to the positive pole of VD959 via R952 and VD972, and is applied to the base of VD953 via VD959, so that VD953 is turned on, the current of IC902 is increased, and the voltage of each channel is increased. Substantially reduced, the voltage obtained by VD953 rectification and filtering is also greatly reduced (about 3V), and it is impossible to supply voltage to the collector of V957. However, since VD953 is turned on, V952 and V958 are also turned on, and continue to the collector of V957. Power supply, after V957 voltage regulation, continue to output 5V voltage to supply power to the CPU.
Switching power supply protection
D2902 power supply has perfect protection function. STR-S6708 can detect the working state of switching power supply, and can protect against overcurrent, overvoltage and spike of switching tube. For the subsequent load of each stage has complex and comprehensive protection, we will introduce various protections below.
One: protection of the thick film of the power supply.
STR-S6708 power thick film block has three protections: overheat, overvoltage and overcurrent.
1. Overheat protection: When the temperature of the STR-S6708 rises due to abnormal power supply operation, the internal temperature Sensor detects that the temperature reaches 150 °C, the temperature protection circuit operates, the internal oscillator stops, and the switch has no drive pulse and stops working. Temperature protection is a function of the STR-S6708 itself, eliminating the need for peripheral circuitry.
2. Overvoltage protection: (9) of STR-S6708 is both the power supply pin and the overvoltage protection circuit detection terminal. It is connected with overvoltage protection circuit. When the voltage of (9) is greater than 11V, the protection circuit operates and internal oscillation The device stops vibrating, and the switch tube stops working without a drive pulse.
The (8) pin of the STR-S6708 also has overvoltage protection. When the voltage of the (8) pin is greater than 1.5V, the internal oscillation circuit stops working and the switching power supply has no output.
3. Overcurrent protection: (2) of the STR-S6708 is the emitter of the internal switch, and (2) is grounded by connecting R907 and R921 in parallel. Obviously, the current flowing through R907 and R921 is the emitter current of the switch. The voltage drop generated on R907 and R921 can reflect the current of the switch tube. The voltage drop generated on R907 and R921 is applied to the (6) pin via R911. When the voltage of the (6) pin is lower than the voltage of the (2) pin by more than 1V, the internal oscillation circuit stops operating and the switching power supply has no output.
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