6. PWM Circuit (DC-DC Conversion)
The PWM circuit uses the UC3845 chip, with parameters detailed in Table 3, and the circuit diagram is shown in Figure 7. After power factor correction and boost, the 380V output voltage passes through the switching transformer T15. Pins 1 and 2 of the primary side are sent to the D pole of Q150. Simultaneously, the VCC3 (15V) voltage is supplied from the chip power supply circuit to pin 7 of IC150 as the startup power for IC150. Once the operating conditions are satisfied, the internal circuit begins to oscillate, producing a 6-pin PWM positive pulse. Q152 is then turned off while Q150 is turned on. At this point, the source external current detection circuit R151 and R152 convert the current signal of Q150 into a voltage signal and feed it back to pin 3. When the voltage at pin 3 reaches 0.9V-1.2V, the power supply enters the overcurrent protection state. The power supply employs direct sampling and voltage stabilization mode, with the output voltage regulated by the optocoupler PC150. If the switching power supply causes a 24V voltage increase for any reason, the 24V voltage surpasses 2.5V at the R terminal of IC200 via R200 and R202. This results in a decrease in resistance between IC200 and A, causing the K terminal potential to drop and flow through PC150. The current of LEDs 1 and 2 increases, enhancing the illumination, and the internal photosensitive tube of PC150 conducts. On one hand, the feedback voltage is sent to pin 2 of IC150, compared with the reference voltage, and the PWM duty cycle is controlled to regulate the output voltage; conversely, the base potential of Q151 drops and turns on, sending the voltage output from the collector 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 comprehensive, highly sensitive, and includes spike absorption, overcurrent, overvoltage, and undervoltage protection circuits.
(1) Spike Absorption
1) The 5V sub-power supply circuit's peak absorption is achieved by ZD600, R600, D600, and C600, forming a peak absorption circuit that eliminates the impact of T600’s self-induced electromotive force when the internal switch tube of U601 is turned off, protecting the safety of the switch tube. Additionally, D121 is designed to prevent C120 surge protection.
2) The 24V forming circuit's spike absorption circuit comprises C151, R150, D150, and R159 to avoid the impact of spikes on Q150.
(2) The entire power supply's overcurrent protection circuit is formed by R127, R127B, R126, and ZD131. When the overall machine current becomes too large, the voltage at the upper end of the shunt resistor flowing through R127 and R127B rises, and the voltage at IC1204 rises through R126. When this voltage exceeds 0.8V, the internal detection circuit of IC120 starts, halting oscillation, making the 7th foot output 0V, ceasing the 380V output, and causing the entire machine to stop working.
(3) Backlight Voltage Forming Circuit (24V Forming Circuit)
The power drive switch tube Q150’s source external current detecting circuit, R151 and R152, converts the current signal of Q150 into a voltage signal and feeds it back to pin 3. When the voltage at pin 3 reaches 0.9V~1.2V, the power supply enters the overcurrent protection state.
(4) Sub-Power Supply Circuit
Pin 4 of IC601 is the internal circuit starting terminal of the power supply, and an external undervoltage detection circuit is connected. When the mains rectification voltage falls below 120V, the sub-power supply does not start. Pin 3 is externally connected with an overcurrent protection sampling resistor. When the voltage at this pin to ground reaches 0.77V, the auxiliary power supply stops working.
(5) Overvoltage Protection Circuit
1) The input overvoltage protection circuit uses VA100. When the grid voltage exceeds 300V, the resistance value drops sharply, blowing the fuse and ensuring safety.
2) The auxiliary power supply overvoltage protection circuit consists of Q100, Q101, and surrounding components. Sampling and partial pressure are performed through R105, R106, R107, R108, and R109. After adding D101 to the G pole of Q100, when the grid voltage exceeds 270V, Q100 saturates and turns on, short-circuiting VCCO to the ground through R111, forcing the auxiliary power supply to stop oscillating, and protecting the entire machine safely. When the rectified output voltage does not reach the protection voltage, Q100 turns off. Whether Q101 turns on or not depends on the level of VCCO. If VCCO is too high for some reason, Q101 can turn on through R111 to correct the sub-power circuit composition.
Second, Fault Repair Example
Fault Phenomenon 1: Plugging in the power plug, the panel indicator light does not light up, and the key control and remote control cannot start the machine.
Analysis and Overhaul: After removing the back cover, observe that the power board of the machine shows no obvious burnt marks, and the fuse is intact. First, check the resistance of the circuit. 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, causing the entire machine to fail to work.
Next, check the auxiliary power supply section. During power-on testing, the voltage at IC601 pin 1 is 320V, and pin 4 fluctuates between 10V~14V, indicating that the main power supply chip and power supply are basically normal. Attention shifts to the output. If the +5V load is short-circuited or the secondary short circuit and the voltage regulation link cause the output voltage to be low, but one by one, all components are normal. Removing the chip and replacing it with a three-terminal thick-film block. Regardless of how the voltage regulator is adjusted, the output voltage remains the same. Putting the chip back, checking the ZD600 of the spike absorption circuit, the DC resistance at both ends is 0, then measuring the D600 resistance to 0, and disconnecting D600 from the circuit board. The result is the same. At this point, the fault culprit appears. Replacing it boots up, +5V is established, and manual second boot works fine.
Summary: After the D600 breakdown in the peak absorption circuit, even though the rest of the chip is normal, the power supply is normal, and ZD600 clamps the potential at the primary end of the switching transformer, preventing the induced electromotive force from increasing, causing the secondary output to drop, and the CPU cannot receive a normal power supply. The entire machine cannot work, which is more hidden.
Symptom 2: Plugging in the power plug, the panel indicator does not light, and the key control and remote control cannot turn on.
Analysis and Repair: Open the back cover, quickly test the input circuit, F100 fuse burns out, current-limiting resistor TH120 breaks, the forward and reverse resistances of the rectified input are normal, the forward and reverse resistances of the rectified output are normal, and the forward and reverse resistances of the PFC output are 0, D120 D121 have normal line resistance. Preliminary judgment only has a PWM switch tube as the key object, and its DS forward and reverse resistances are 0. Its model is 12N65C3, replaced with new ones, check the source over-current detection circuit, and ZD151 breaks down. After replacing the new product, the DC resistance of the PFC output is restored. The fuse replacement is turned on, +5V is established, the indicator light is on, and the manual start-up picture is good.
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