Three examples of inverter switching power supply failure
Example:
The 2844 chip was powered on separately with no pulse at pin 6, and the voltage at pins 1, 2, and 3 was tested to be normal. The voltage at pin 4 is 1.8V, and the oscilloscope measures that there are no sawtooth pulses at pin 4; The previous repairman replaced four chips in a row. Measure the resistance of pin 4 to ground, which is significantly lower than that of other circuits, with a resistance of tens of thousands of ohms. This machine costs around 3000 euros. Remove the chip and its peripheral components, suspend the four pins, and measure the resistance to ground to be 9 kiloohms, not infinite. Obviously, there is a leakage point between the 4-pin through-hole or copper foil and the 5-pin grounding point. At this point, apply a 30V DC voltage between the four pins and the ground. The displayed current value is several tens of milliamps and gradually decreases. When it drops to a few milliamps, cut off the power and measure the resistance value of the four pins to ground, which becomes over 60 kiloohms. Restore the chip and 4-pin components, and troubleshoot the power on issue.
Two examples:
Difficulty in starting vibration. After the chip and main circuit are powered on together, the output of+15V is measured to be 11V and 24V is measured to be 19V, both of which are low. The voltage stabilization sampling is+15V, indicating that it has not reached the starting point of voltage stabilization. The oscilloscope measures the duty cycle of the 6-pin pulse, which is relatively small. Analyze the reason, the output pulse duty cycle is small, which is related to the status of pin 1 and pin 3. 1. Connect pin 2 to feedback optocoupler pins 3 and 4, and check if there are any issues with the voltage regulator circuit. There is an 18V voltage regulator diode connected between pin 3 and pin 7. After removal, the fault was resolved.
The output voltage is low. If it is not due to insufficient self power supply, it may be caused by voltage regulation misoperation or overcurrent. This example is the latter.
Three examples:
Switching power supply, chip power on and main circuit power on, working normally. After removing the chip power supply, it works normally. Check for no load short circuit and no temperature rise in the switch tube. The reason for insufficient self power supply can be ruled out. Even if the power on excitation ability is insufficient, there is no change even after reducing the starting resistance. It is still a problem with the power supply or excitation of pins 5 and 7. Obviously, it contains internal circuits for pins 5 and 7 of the chip.
