The following are some of the lessons learned by engineers during the maintenance process. For your reference, I hope you can help.
Several maintenance steps of the switching power supply are as follows:
1. Check whether the rectifier circuit D1 - D4 breaks down or opens, whether the capacitor of the filter circuit is damaged, whether the balance resistors R1, R2 are normal, and whether the step-down resistor R3 is blown or the resistance value increases failure (test under power off conditions).
2. Check if there is a breakdown short circuit in the bee junction and ce junction of the switch and measure whether there is a short circuit in each winding of the switch transformer to determine the quality of the switch tube and the switch transformer (test under power off conditions).
3. Inspect the rectifying filter components of the secondary output windings, focusing on whether the filter capacitor is bulged or damaged to eliminate the possibility of a secondary circuit short circuit.
4. Check whether the absorption circuit D5, R11, C9 is normal (test under power off conditions).
5. Under the condition that the above components are normal, we can remove the switching power supply board from the inverter and perform the power-on test alone. Use a voltage regulator to slowly adjust the rated voltage of the switching power supply. At this point, you should be able to hear the hum of the transformer. If you do not hear a start-up sound, use a multimeter to check the positive and negative levels of the UC3844. Is there a DC voltage of about 12V to 16V?
6. After confirming that the voltage of the power supply terminal of UC3844 is normal, you can use the oscilloscope to check whether the PWM wave output from the UC3844's 6-pin to the trigger terminal of the switch tube (according to different circuit designs, the frequency of the PWM wave is generally between 20KHZ and 100KHZ). .
7. If there is no PWM wave output, replace the timing element C5, R8, C6 or UC3844. After the elimination of the above several steps, the switching power supply should be able to work normally. In the inverter, there are many types of switching power supply, but the basic principles are the same. For example, each PWM management chip has a power supply terminal, RC network for timing elements, ports for outputting PWM waves, etc., as long as we understand their work. Principle, according to a certain method steps can eliminate the fault.
The following engineers enumerated the problems and solutions encountered in actual maintenance for your reference.
[Case 1]: Inverter (Failure phenomenon: no display on power-up) It was found that the main circuit, charging resistor, and main circuit contactor of the power supply are all normal. Therefore, it is determined that the switching power supply board is faulty. Follow the above repair procedure to measure the switching power supply board. During the first step of measurement, it was found that the 330KΩ/2W step-down resistor between the DC bus 560V and the PWM modulation chip was damaged, and the nominal 330KΩ/2W resistor was actually measured over 2MΩ, so the PWM modulation chip was not The power to start, so I can not start working. In order to be cautious, we have also inspected key components such as switches, transformers, rectifier diodes, and filter capacitors. After confirming that there are no problems, we will test the power supply. OK! The switching power supply starts to vibrate, and the output voltage of each group is normal. After the inverter is installed in the inverter, the test is normal. This inverter repair is completed (Note: Maintenance personnel in the maintenance, we must form a habit: Do not rush to replace the test machine after discovering bad components, we must put the power, easy to bad components are measured, determine no problem Retry the machine so that it is both safe and safe).
[Case 2]: Inverter (Failure phenomenon: No display on power-up) It was found that the main circuit, charging resistor, and main circuit contactor of the power supply are all normal, and the fault is determined on the power board. Follow the repair procedure to measure the switching power supply board. The first step is to pass the measurement. In the second step, it is found that the switch tube ce junction breaks down, remove it, and then detect the transformer, rectifier diode, filter capacitor, and other key devices. After confirming that there are no problems, the power-on test will be performed and the outputs will be output. The voltage is normal, the installation test is normal, and the fault is removed.
[Case 3]: Inverter (Failure phenomenon: No display on power-up) The main circuit, charging resistor and main circuit contactor of the power supply have been found to be normal after the detection, and the fault is determined on the power board. Follow the repair procedure to measure the switching power supply board. The first step is passed, the second step passes, the third step passes, and the fourth step passes. Then the power supply board is powered on. The power supply end of the PWM modulation chip has a voltage of about 12.5 V to ground, indicating that the power supply is normal. . Looking at the PWM output of the chip with an oscilloscope, it was found that there was no PWM modulation waveform. After replacing the PWM modulation chip, the power-on test is normal and the fault is eliminated.
[Case 4]: Inverter (Failure phenomenon: No display on power supply) After repeated detection of the switch, the main circuit, charging resistor and main circuit contactor are normal. The fault is determined on the power board. Follow the repair procedure to measure the switching power supply board. The first step was measured. The second step found that the switch was broken down. The third step passed the measurement. The fourth step passed the measurement. The new switch was replaced, and the power board was individually powered on. The tube was again burned. After removing the switch tube, the tube is not installed, and the power test is performed. The voltage of the power supply end of the PWM modulation chip is about 12V to the ground, and it is also normal. With the oscilloscope to see the chip's PWM output, found that the PWM wave is only about 5-6 KHZ, after removing the power to remove the measurement of the timing component, found that the timing resistor resistance becomes larger, replace the timing resistor, switch the power after the normal, no longer Burn power tube, troubleshooting. Case 5: Lenze inverter (failure phenomenon: no display on power supply) The fire-retardant switch tube measures the switching power supply board according to the maintenance procedure. The first step was measured. During the second step, the ce junction of the switch was found to be punctured. The third, fourth, fifth, sixth, and seventh steps were measured. Install a new switch tube power test, as the regulator voltage increases, you can hear the buzzing sound, which is a little loud, after the voltage is adjusted to the rated voltage, the measured output voltage is lower than the normal value, less than 2 minutes, suddenly smelled a burning smell, the fuse was broken, quickly turned off and found that the switch is very hot, the measurement found that it has broken down. Remove the switch tube electrification test, measure the voltage of the PWM modulation chip's power supply to ground to about 12V, use the oscilloscope to see the PWM output of the chip, and find that there is a PWM wave output and the frequency is about 30KHZ, which is also normal. Therefore, it was suspected that the quality of the switching tube that had just been replaced was not good. It was replaced by another one and the test was powered on. As a result, the tube was burned again. After the power failure, the components of the absorption circuit were inadvertently encountered. They were found to be hot, but they were normal when measured. Ah, then again, it's normal. Simply dismantle the absorption circuit first, and put on a tube to test electricity. It is found that the hum of the transformer is small, and the output voltage of each group is also normal. After running the switch for 20 minutes, the switch has not been burned again. After the power is turned off, the switch tube is slightly hot, and it is in a normal heating state. Therefore, the fault is absorbed in the circuit, and the absorption circuit element is replaced. The fault is eliminated.
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