Troubleshooting Skills and Frequent Problems of Switching Power Supply

Currently, switching power supply technology has been fully integrated into the design of various electronic products. However, with the expansion of usage, switching power supplies are beginning to face various problems, including damage. In this case, there is a need for a technology that can repair switching power supplies, which is called switching power supply maintenance technology. Strictly speaking, the maintenance of switching power supplies belongs to the category of chip level maintenance, which plays a very important role in the field of industrial maintenance. This article will introduce some maintenance techniques for switching power supplies, and provide answers to common failures.
Maintenance Skills
The maintenance of a switching power supply can be carried out in two ways, one is to observe when the power is off, and the other is to power on for testing. In the event of a power outage, the cause of damage to the switching power supply can be determined by "looking, smelling, asking, and measuring.".
Power failure detection
Look: Open the housing of the power supply, check whether the fuse is blown, and then observe the internal condition of the power supply. If there is a scorch or component fracture on the PCB board of the power supply, it is necessary to focus on checking the components and related circuit components here.
Smell: Smell if there is a sticky smell inside the power supply and check if there are scorched components.
Q: Ask about the history of the power supply damage and whether there have been any illegal operations on the power supply.
Measurement: Before powering on, use a multimeter to measure the voltage across the high-voltage capacitor. If the fault is caused by a switch power supply that does not vibrate or an open circuit in the switch tube, in most cases, the voltage at both ends of the high voltage filter capacitor is not discharged, and this voltage is over 300 volts. Be careful. Use a multimeter to measure the forward and reverse resistance at both ends of the AC power line and the charging condition of the capacitor. The resistance value should not be too low, otherwise there may be a short circuit inside the power supply. The capacitor shall be able to charge and discharge. Disconnect the load and measure the resistance to ground at each group of output terminals. Under normal conditions, the meter needle should have a capacitor charge and discharge swing. The last indication should be the resistance value of the discharge resistance of the circuit.
Power on detection
After powering on, observe the power supply to determine whether there is any damage, such as burnt fuses or smoke from components. If necessary, timely cut off the power supply for maintenance. Measure whether there is a 300V output at both ends of the high-voltage filter capacitor. If not, focus on checking the rectifier diode and filter capacitor. Measure whether the secondary coil of the high-frequency transformer has output. If not, focus on checking whether the switch tube is damaged, whether it vibrates, and whether the protection circuit operates. If yes, focus on checking the rectifier diodes, filter capacitors, and three-way regulator tubes at each output side.
If the power supply is stopped once it is started, it is in a protected state. You can directly measure the voltage of the PWM chip protection input pin. If the voltage exceeds the specified value, it indicates that the power supply is in a protected state, and you should focus on checking the cause of the protection.
Common faults
Fuse blown
If a fuse blows, it is highly likely that there is a problem with the wiring inside the power supply. As the power supply operates at high voltage and high current, fluctuations and surges in the grid voltage can cause an instantaneous increase in the current in the power supply and cause the fuse to blow. The key points should be to check the rectifier diode, high-voltage filter electrolytic capacitor, inverter power switch tube, etc. at the power input terminal, and check whether these components have breakdown, open circuit, damage, etc. If a fuse is indeed blown, you should first check the various components on the circuit board to see if their appearance has been burnt or if electrolyte has overflowed.
If none of the previous situations occur, it is necessary to use a multimeter to measure the switch tube and check for any breakdown of the switch tube. It is important to note that when a component is damaged, it should not be started directly after replacement, as it is likely that the replaced component will be damaged due to other high-voltage components still having faults. It is necessary to thoroughly inspect and measure all high-voltage components of the above circuit before thoroughly eliminating the fuse blown fault.
No DC voltage output or unstable voltage output
Under load conditions, if the fuse is intact and the DC voltage at all levels is not output. This situation is mainly caused by the following reasons: open circuit and short circuit phenomena in the power supply, overvoltage and overcurrent protection circuit failures, auxiliary power supply failures, oscillation circuit not working, excessive load on the power supply, breakdown of the rectifier diode in the high-frequency rectifier filter circuit, filter capacitor leakage, etc. After measuring the secondary components with a multimeter and eliminating high-frequency rectifier diode breakdown and load short circuit conditions, if the output is zero at this time, it can be determined that there is a fault in the control circuit of the power supply. If there is a partial voltage output, it indicates that the front stage circuit is working normally, and the fault is in the high-frequency rectifier filter circuit. The high-frequency filter circuit mainly consists of a rectifier diode and a low-voltage filter capacitor to form a DC voltage output. The breakdown of the rectifier diode will cause the circuit to have no voltage output, and the leakage of the filter capacitor will cause faults such as unstable output voltage. Using a multimeter to statically measure the corresponding component can detect the damaged component.
Poor power load capacity
This phenomenon is relatively common, thinking that it often occurs in old-fashioned or long-time working power supplies. The main reasons are aging of various components, unstable operation of switch tubes, and failure to conduct heat dissipation in a timely manner. Emphasis should be placed on checking whether the voltage regulator diode has heat and leakage, the rectifier diode is damaged, and the high-voltage filter capacitor is damaged. For example, taking the red spot laser spectrometer as an example, after starting up, it is unable to complete the self test and give an alarm, and the main board indicator continues to flash. Upon inspection, there is only about 2.3 volts left in the DC 5V power supply for the spectrometer main board. Remove the load from the 5V DC power supply and power on to measure the 5V DC power supply again. At this time, there is 5V. It is preliminarily judged that the load carrying capacity of this 5V DC power supply is poor. Remove the power supply housing for maintenance. Since there is a DC 5V output when powered on without a load, focus on checking the output rectifier circuit at the secondary coil side, Connect a dummy load to the 5 volt power supply and conduct the measurement. It is found that the voltage between pins 1 and 2 of the three-way regulator 7805 is 5.2 volts, but there is 2.3 volts left between pins 2 and 3.