Fault diagnosis and troubleshooting methods for switching power supplies

In the era of industrial automation, switching power supplies have been used in all industries. Their precision circuit boards and strict requirements for current power supply make switching power supply circuit board maintenance a common fault equipment that is difficult to repair in the PCB maintenance industry. We have been engaged in power supply maintenance for more than 10 years. Before repairing the switching power supply, we must understand the working principle of the switching power supply. The power supply first rectifies the high-voltage AC through a full-bridge diode to become a high-voltage fluctuating DC, and then becomes a relatively smooth high-voltage DC after capacitor filtering. At this time, the control circuit controls the high-power switch tube to send the high-voltage DC to the primary of the high-frequency transformer in batches according to a certain high-frequency frequency. Then, the stepped-down high-frequency low-voltage AC output from the secondary coil is converted into a low-voltage and high-current DC that can make the load work through rectification and filtering. Among them, the control circuit is an indispensable part. It can effectively monitor the voltage value at the output end and send a signal to the power switch tube to control the amplitude of the voltage adjustment up and down. In a switching power supply, the input part has the highest failure rate because it operates under high voltage and high current conditions. Secondly, the rectifier diode, protection diode, and high-power switching transistor in the output DC part are more susceptible to damage, followed by the feedback and protection part of the pulse width modulator.

1. In case of power outage, “look, smell, ask, and palpate”

First, before the switching power supply is powered 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 is open, in most cases, the voltage across the high-voltage filter capacitor is not discharged. This voltage is more than 300 volts. If you accidentally touch it, it will definitely leave you with an unforgettable memory!

Since the maintenance of the power supply requires contact with 220V high voltage electricity, the human body will be in danger of life once it contacts a voltage above 36V. Therefore, if possible, try to check whether there is an obvious short circuit or component damage in the power-off state. First, open the power supply shell and check whether the fuse is blown, and then observe the internal situation of the power supply. If the component on the PCB board of the power supply is broken, you should focus on checking this component. Generally speaking, this is the main cause of the failure; smell whether there is a burnt smell inside the power supply, check whether there are burnt components; ask about the damage of the power supply and whether the power supply has been operated illegally. This is necessary for repairing any equipment. After the preliminary inspection, the power supply should be tested more deeply.

Use a multimeter to measure the forward and reverse resistance at both ends of the AC power line and the charging status of the capacitor. If the resistance value is too low, it means that there is a short circuit inside the power supply. Under normal circumstances, its resistance value should be able to reach more than 100 kilo-ohms; the capacitor should be able to charge and discharge. If it is damaged, it will be manifested as low resistance at both ends of the AC power line, which is a short circuit state, otherwise it may be a switch tube breakdown. Then check the DC output part to disconnect the load and measure the ground resistance of each group of output ends respectively. Under normal circumstances, the needle should have capacitor charging and discharging swings, and the final indication should be the resistance value of the discharge resistor of this circuit. Otherwise, it is mostly caused by the reverse breakdown of the rectifier diode.

2. Power-on detection

After passing the above tests, you can perform a power-on test. This is the key point, which requires certain experience, electronic basics and maintenance skills. Generally speaking, you should focus on checking the input end of the power supply, the switching transistor, the power supply protection circuit, and the output voltage and current of the power supply. If the power supply starts and stops, the power supply 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. Due to contact with high voltage, it is recommended that friends who do not have electronic basics need to be careful in operation.

3. Common faults

1. The fuse is 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 components on the circuit board to see if the surface 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.

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 or short circuit in the power supply, failure of overvoltage or overcurrent protection circuit, 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 the control circuit of the power supply is faulty.

3. 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 the switch tube, 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.

Power supply maintenance example 1: After the power supply of a certain device is powered on, there is no voltage output, and the power supply emits a "squeaking" sound. This is a typical feature of power supply overload or no load. First, carefully check each component, focusing on the rectifier diode, switch tube, etc. After careful inspection, it was found that the surface of a rectifier diode (1N4007) was burned black, and the circuit board was also burned black. Find a diode of the same model to replace it, and then use a multimeter to measure it. It is indeed broken. After connecting the power supply, the fan does not rotate, and the "squeaking" sound still exists. Use a multimeter to measure the +12V output is only +0.2V, and +5V is only 0.1V. This shows that the power supply starts self-protection when the component is broken down. Measure the primary and secondary switch tubes and find that one of the primary switch tubes is damaged. Replace it with a switch tube of the same model and the fault is eliminated. The above inspection took a detour. Before powering on, you should measure whether the switch tube is damaged.

Power supply maintenance example 2: There is no "squeaking" sound, and a fuse burns when a fuse is installed. As the fuses are constantly blown, the search range is narrowed. There are three possible situations: rectifier bridge breakdown, large electrolytic capacitor breakdown, and primary switch tube breakdown. The rectifier bridge of the power supply is generally four discrete rectifier diodes, or four diodes are solidified together. It is normal to remove the rectifier bridge. The large electrolytic capacitor is also normal after removal and testing. Pay attention to the positive and negative poles when soldering it back. The last possibility is the switch tube. This power supply has only one high-power switch tube in the primary. After removing it, it is indeed broken. Find a switch tube of the same model to replace it, and the problem is solved.

In fact, it is not difficult to repair a power supply. Generally, power supply failures can be summarized as: fuse blown, rectifier diode damaged, filter capacitor open circuit or breakdown, switch tube breakdown, power supply self-protection, etc. In addition, the circuit of the switching power supply is relatively simple, and the types of failures are relatively few, so it is easy to determine the fault location. Therefore, as long as you have enough basic electronic knowledge, read more related technical articles, do more hands-on work, and pay attention to the accumulation of maintenance experience, power supply failure repair is still easy to solve.

Any chip-level maintenance work cannot be separated from the accumulation of experience and flexible maintenance ideas. When encountering a fault that cannot be eliminated or the root cause of the fault cannot be found, it is often necessary to calm down or simply take a short break, think from multiple angles and troubleshoot, which often has unexpected results.

When we are engaged in circuit board maintenance work, we must be more careful and prudent when encountering power supply maintenance, especially plateau power supply circuit board maintenance, so as to avoid more serious consequences of equipment burning.