Three examples of switch mode power supply faults in frequency converters
Example:
The 2844 chip is powered on separately and there is no pulse on pin 6. The voltage on pins 1, 2, and 3 is measured to be normal. The voltage on pin 4 is 1.8V, and the oscilloscope measures that there is no sawtooth wave pulse on pin 4; The former repairman replaced four chips in a row. Measure the ground resistance of pin 4, which is significantly lower than the other circuits, with a resistance of tens of thousands of ohms. This machine is around 3000 ohms. Remove the chip and its peripheral components, suspend the four pins, and measure the ground resistance value 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 time, between the four pins, try applying a 30V DC voltage, and the displayed current value is tens of milliamperes and gradually decreasing. When it is as low as a few milliamperes, cut off the power and measure the resistance of the four pins to the ground again, which becomes over 60 kiloohms. Restore the chip and 4-pin components, troubleshoot power on issues.
Two examples:
Difficulty in starting vibration. After the chip and the main circuit are powered on together, the output of the power supply is measured to be 11V at 15V and 19V at 24V, both of which are low. The voltage stabilization sampling is+15V, indicating that it has not reached the voltage stabilization starting point. 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 states of pin 1 and pin 3. 1. Connect pins 2 to feedback optocoupler pins 3 and 4, and check that there is no problem with the voltage stabilizing circuit. There is an 18V voltage regulator diode connected between pins 3 and 7. After removal, the fault was resolved.
The output voltage is low, and if it is not due to insufficient self power supply, it is due to voltage regulation or overcurrent. This example is for 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 will be no change after reducing the starting resistance. It is still a power supply or excitation issue with pins 5 and 7. Obviously, it contains internal circuits of pins 5 and 7 of the chip.
