Three Examples of Switching Power Supply Faults of Inverter
An example:
The 2844 chip is powered on separately, there is no pulse at pin 6, and the voltages at pins 1, 2, and 3 are all normal. The voltage of pin 4 is 1.8V, and there is no sawtooth wave pulse at pin 4 measured by the oscilloscope; the former repairer replaced four chips in a row. Measure the resistance of 4 feet to the ground, it is obviously too small, and other circuits are more than ten thousand ohms. The machine is about 3 thousand ohms. Remove the chip and the peripheral components of the chip, make the 4 pins floating, and measure the resistance value to the ground as 9 kohms, which is not infinite. Obviously, the 4-pin via hole or the copper foil has a leakage point to the 5-pin ground point. At this time, try to apply a 30V DC voltage between the 4 pins and the ground, and the displayed current value is tens of milliamperes and is getting smaller and smaller. When the power is off when it is as small as a few milliamperes, then measure the resistance of the 4 pins to the ground, and it becomes 60 kohms. above. Restore the chip and 4-pin components, and eliminate the power-on fault.
Two examples:
Difficult to vibrate. After the chip and the main circuit are powered on together, the measured output of +15V after power supply is 11V, and the output of 24V is 19V, both of which are low. The oscilloscope measures the duty cycle of the 6-pin pulse, which is small. Analyzing the reason, the output pulse duty cycle is small, which is related to the state of pin 1 and pin 3. Pins 1 and 2 are connected to pins 3 and 4 of the feedback optocoupler, and there is no problem in checking the voltage regulator circuit. There is an 18V Zener diode connected between pin 3 and pin 7. After removing it, the fault is eliminated.
If the output voltage is low, if it is not insufficient self-power supply, it is the cause of voltage regulation miscontrol or overcurrent. This example is the latter.
Three examples:
Switching power supply, chip power-on and main circuit power-on, work normally, and work normally after removing the chip power supply. Check that there is no load short circuit, and the switch tube has no temperature rise. The cause of insufficient self-power supply can be ruled out. That is, the power-on excitation ability is insufficient, and there is no change after reducing the starting resistance. Still 5, 7 pin power supply or excitation problem. Obviously, it contains the internal circuit of pins 5 and 7 of the chip.
After changing the chip it works fine.
