Fault analysis of switch power supply in automation control supporting equipment

The switching power supply is a frequently used accessory in the automation system control supporting equipment, and its role is very important. Based on this, the troubleshooting of the switching power supply cannot be ignored.

　　Switching power supply failure 1: Fuse blown

　　Generally speaking, a blown fuse indicates that there is a problem with the internal circuit of the power supply. Since the power supply operates at high voltage and high current, the fluctuation and surge of the grid voltage will cause the current in the power supply to increase instantly and cause the fuse to blow. The focus should be on checking the rectifier diodes, high-voltage filter electrolytic capacitors, inverter power switch tubes, etc. at the input end of the power supply to check whether these components are broken down, open circuited, damaged, etc. If the fuse is indeed blown, you should first check the various components on the circuit board to see if the appearance of these components is burnt or if there is any electrolyte overflow. If the above situation is not found, use a multimeter to measure whether the switch tube is broken down and short-circuited. It is important to note that when a component is found to be damaged, it is not allowed to be replaced and turned on directly. This is likely to damage the replaced component due to the failure of other high-voltage components. All high-voltage components of the above circuits must be fully inspected and measured before the fuse is completely eliminated.

　　Switching power supply fault 2: no DC voltage output or unstable voltage output

　　If the fuse is intact, there is no output of DC voltage at each level under load. This situation is mainly caused by the following reasons: open circuit and short circuit in the power supply, failure of overvoltage and overcurrent protection circuit, failure of auxiliary power supply, failure of oscillation circuit, overload of power supply, breakdown of rectifier diode in high-frequency rectifier filter circuit, leakage of filter capacitor, etc. After measuring the secondary components with a multimeter and excluding the breakdown of high-frequency rectifier diode and load short circuit, if the output is zero at this time, it can be confirmed that there is a fault in the control circuit of the power supply. If there is partial voltage output, it means that the previous 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. The damaged components can be checked by statically measuring the corresponding components with a multimeter.

　　Switching power supply failure three: poor power load capacity

　　Poor power load capacity is a common fault, which usually occurs in old power supplies or those with long working time. The main reasons are aging of components, unstable operation of switch tubes, and failure to dissipate heat in time. It is important to check whether the voltage regulator diode is heated and leaking, whether the rectifier diode is damaged, and whether the high-voltage filter capacitor is damaged.

　　Switching power supply maintenance tips

　　The maintenance of the switching power supply can be divided into two steps:

　　In case of power outage, "look, smell, ask, and measure" -

　　Look: Open the power supply casing and check whether the fuse is blown, then observe the internal situation of the power supply. If there are burnt areas or broken components on the PCB board of the power supply, you should focus on checking the components there and related circuit components. Asset Management

　　Smell: Smell the inside of the power supply to see if there is a burnt smell and check if there are any burnt components.

　　Q: Please tell me how the power supply was damaged and whether there was any illegal operation 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 the switching power supply not vibrating or the switch tube being open, in most cases, the voltage across the high-voltage filter capacitor is not discharged. This voltage is more than 300 volts, so be careful. Use a multimeter to measure the forward and reverse resistance at both ends of the AC power line and the charging of the capacitor. The resistance value should not be too low, otherwise there may be a short circuit inside the power supply. The capacitor should be able to charge and discharge. Disconnect the load and measure the resistance to ground of each group of output terminals respectively. Under normal circumstances, the needle should swing when the capacitor is charged and discharged, and the final indication should be the resistance value of the discharge resistor of this circuit.

　　Power-on detection

　　●After powering on, check whether the power supply has burned out fuses or individual components are smoking. If so, cut off the power supply immediately for maintenance.

　　●Measure whether there is a 300-volt output at both ends of the high-voltage filter capacitor. If not, focus on checking the rectifier diode, filter capacitor, etc.

　　●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 is oscillating, whether the protection circuit is working, etc. If yes, focus on checking the rectifier diodes, filter capacitors, three-way voltage regulator tubes, etc. on each output side.

　　●If the power supply starts and then stops, it is in the protection state. You can directly measure the voltage of the PWM chip protection input pin. If the voltage exceeds the specified value, it means that the power supply is in the protection state. You should focus on checking the cause of the protection.