6.PWM circuit (DC-DC conversion)
The chip of the PWM circuit is UC3845, the parameters are shown in Table 3, and the PWM circuit is shown in Figure 7. After power factor correction and boosting, the output voltage of 380V is passed through the switching transformer T15. The primary 1 and 2 pins are sent to the D pole of the Q150. At the same time, the VCC3 (15V) voltage is supplied from the chip power supply circuit to the 7th pin of the IC150 as the startup power of the IC150. After the working conditions are met, the internal circuit starts to oscillate, and the 6-pin output PWM positive pulse, Q152 is cut off, Q15. Turn on, at this time, the Q150 source external current detection circuit R151, R152 converts the current signal of Q150 into a voltage signal and feeds back to the 3 pin. When the voltage of the 3 pin to ground reaches 0.9V-1.2V, the power supply enters the overcurrent protection state. The power supply adopts direct sampling and voltage stabilization mode, and the output voltage is stabilized by the optical disaster PC150. When the switching power supply causes 24V voltage rise for some reason, the 24V voltage is higher than 2.5V of the R terminal of IC200 through R200 and R202, which causes the resistance between IC200 and A to decrease, and the K terminal potential to drop and flow through PC15. . The current of the LEDs 1 and 2 increases, the illumination is enhanced, and the internal photosensitive tube of the PC150 is controlled to conduct. On the one hand, the feedback voltage is sent to the inside of the 2 pin of the IC150, and compared with the reference voltage, the PWM duty cycle is controlled, and the output voltage is lowered. ,vice versa. On the other hand, the base potential of Q151 is lowered and turned on, and the voltage output from the collector is sent to pin 4 of IC150 to control the oscillation frequency of the power supply and adjust the output voltage of 24V DC.
7. Protection circuit The protection circuit of this power supply is perfect, with high sensitivity, and there are spike absorption, overcurrent, overvoltage and undervoltage protection circuits.
(1) spike absorption
1) 5V sub-power supply circuit peak absorption by ZD600, R600, D600, C600 constitutes a peak absorption circuit, eliminating the impact of T600 self-induced electromotive force on IC601 when U601 internal switch tube is cut off, to protect the safety of the switch tube. In addition, D121 is designed to prevent C120 surge protection.
2) 24V forming circuit spike absorption circuit consists of C151, R150, D150, R159 to avoid the impact of spikes on Q150.
(2) The overcurrent protection circuit of the whole power supply is composed of R127, R127B, R126 and ZD131. When the current of the whole machine is too large, the voltage of the upper end of the shunt resistor flowing through R127 and R127B rises, and the voltage of IC1204 rises through R126. When the voltage of this point is higher than 0.8V, the internal detection circuit of IC120 starts, stops the oscillation, 7 The foot output is 0V, no 380V output, the whole machine stops working.
(3) backlight voltage forming circuit (24V forming circuit)
Power drive switch tube Q15. The source external current detecting circuit, R151, R152 converts the current signal of Q150 into a voltage signal and feeds back to the 3 pin. When the voltage of the 3 pin to ground reaches 0.9V~1.2V, the power supply enters the overcurrent protection state.
(4) Sub power supply circuit
The IC601's 4 pin is the internal circuit to start the power supply terminal, and an external undervoltage detection circuit is connected. When the mains rectification voltage is lower than 120V, the sub power supply does not start. The 3 pin is externally connected with an overcurrent protection sampling resistor. When the voltage of the pin to ground reaches 0.77V, the auxiliary power supply stops working.
(5) Overvoltage protection circuit
1) The input overvoltage protection circuit is used by VA100. When the grid voltage is higher than 300V, the resistance value drops suddenly, burning insurance and protecting safety.
2) The auxiliary power supply overvoltage protection circuit consists of Q100, Q101 and peripheral components. Through R105, R106, R107, R108, R109 sampling and partial pressure, after D101 is added to Q100 G pole, after the grid voltage exceeds 270V, Q100 is saturated and turned on, and VCCO is short-circuited into the ground through R111, the auxiliary power supply is forced to stop vibration, and the whole machine is protected. Safety. When the rectified output voltage does not reach the protection voltage, Q100 is turned off. Whether Q101 is turned on or not depends on the level of VCCO. If VCCO is too high for some reason, Q101 can be turned on through R111 to correct the sub-power circuit composition.
Second, the fault repair example fault phenomenon 1: plug in the power plug, the panel indicator light is not bright, key control and remote control can not boot.
Analysis and overhaul: After removing the back cover, observe that the power board of the machine has no obvious burnt marks, and the insurance is intact. First check the resistance of the road. The positive and negative resistances of the output terminals of 300V, 380V, 5V and 24V are not obviously abnormal. The voltage at the 5V output terminal is 1.8V, indicating that the secondary power supply is not normal, resulting in the whole machine not working.
Next, check the auxiliary power supply section, power-on test IC601 pin voltage, 1 pin voltage is 320V, 4 pin voltage fluctuates between 10V~14V, indicating that the chip main power supply and power supply are basically normal, and the eyes are transferred to the output, if The +5V load short circuit or the secondary short circuit and the voltage regulation link can cause the output voltage to be low, but one by one, all components are normal. Remove the chip and replace it with a three-terminal thick film block. No matter how the voltage regulator is adjusted, the output voltage is as before. Put the chip back again, check the ZD600 of the spike absorption circuit, the DC resistance at both ends is 0, then measure the D600 resistance to 0, and disconnect the D600 from the circuit board. The result is the same. At this point, the fault culprit appears, replace it with the boot, +5V has Build, manual second boot everything works fine.
Summary: After the D600 breakdown in the peak absorbing circuit, although the rest of the chip is normal, the power supply is normal, and the ZD600 clamps the potential at the primary end of the switching transformer, the induced electromotive force cannot be increased, causing the secondary output to drop, and the CPU cannot receive normal power supply. The whole machine can't work, it's more concealed.
Symptom 2: Plug in the power plug, the panel indicator does not light, and the key control and remote control cannot be turned on.
Analysis and repair: open the back cover, fast test input circuit, F100 insurance burned, current limiting resistor TH120 breaks, the forward and reverse resistances of the rectified input are normal, the forward and reverse resistance of the rectified output is normal, and the forward and reverse resistance of the PFC output are 0, D120 D121 has normal line resistance. Preliminary judgment only has a PWM switch tube as the key object, and its DS forward and reverse resistance is 0. Its model is 12N65C3, replace it with new ones, check the source over-current detection circuit, and the ZD151 breaks down. After replacing the new product, the DC resistance of the PFC output is restored. The fuse replacement is turned on and the +5V is established, the indicator light is on, and the manual start-up picture is good. .
Many people do not know what kind of cameras are invisible cameras, and do not know how to distinguish invisible cameras, so here is how to distinguish invisible cameras.
Invisibility, as the name suggests, means that it is not easy to be seen or found, so where do such cameras exist? In fact, invisible things are hidden by the external environment, so they will not be discovered. Such invisible cameras are generally hidden in things that are more difficult to see, such as: inside the socket, inside the TV, inside the stereo, inside the fan, etc. Cameras can be hidden in various household appliances. Why should they be hidden in electrical equipment? The main reason is that these equipments have power supply and will not be used because the camera cannot be powered.
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