How to repair common faults in switching power supply?

How to repair common faults in switching power supply?

A switching power supply is a power supply that uses modern power electronics technology to control the time ratio of switching transistors on and off to maintain a stable output voltage. They are widely used in electronic circuits, industry, military, scientific research, communication, medical treatment, and various household appliances.

The primary circuit of the switching power supply is composed of an input electromagnetic interference filter (EMI), a rectification filter circuit, a power conversion circuit, a PWM controller circuit, and an output rectification filter circuit. The auxiliary circuit includes an input overvoltage and under voltage protection circuit, output overvoltage and under voltage protection circuit, output overcurrent protection circuit, output short circuit protection circuit, etc.

1. Maintenance skills

Common switching power supply maintenance can be divided into the following two steps:

1) In a power failure, "look, smell, ask, measure."

Look: Open the case of the power supply, check whether the fuse is blown, and then observe the internal conditions of the power supply. If you find burnt spots or broken components on the PCB board of the power supply, you should focus on checking the components and related circuit components here.

Smell: Smell whether there is a burnt smell inside the power supply, and check whether there are burnt components.

Ask: Ask about the process of power supply damage and whether the power supply has been operated in violation of regulations.

Quantity: Before the power is turned on, use a multimeter to measure the voltage across the high-voltage capacitor. Suppose the failure is caused by the switching power supply not oscillating or the switch tube being open in most cases. In that case, the voltage at both ends of the high-voltage filter capacitor is not released, and the voltage is more than 300 volts, so be careful. Use a multimeter to measure the positive and negative 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 should be able to charge and discharge. Take off the load, and measure the ground resistance of each group of output terminals respectively. When normal, the needle should have a capacitor charge and discharge swing, and the last indication should be the resistance value of the discharge resistor of this circuit.

2) Power-on detection

After power is on, observe whether the power supply has burnt fuses or smoke from individual components, and if so, cut off the power supply in time for maintenance. Measure whether there is a 300V output at both ends of the high-voltage filter capacitor. If not, check the rectifier diode and filter capacitor.

Measure the output of the secondary coil of the high-frequency transformer. If there is no output, focus on checking whether the switch tube is damaged, whether it is oscillating, whether the protection circuit is active, etc. If so, check the rectifier diode, filter capacitor, three-way stabilizer pressure tube, etc.

If the power supply starts and then 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, the power supply is protected. Therefore, you should focus on checking the cause of the protection.

2. Common fault analysis

1) The fuse is blown

Generally, a blown fuse indicates a problem with the internal wiring of the power supply.

a. Short circuit: a short circuit fault occurs on the line side, and the fuse is quickly broken;

b. Overload: The load current exceeds the rated current of the fuse, and the fuse will heat up and blow for a long time (generally 1.1 times the rated current will blow in about 1 hour);

c. Pulse: When the circuit is started, or the power supply is unstable, an instantaneous large current causes the fuse to disconnect;

2) No DC voltage output or unstable voltage output

If the fuse is intact, there is no DC voltage output at all levels under load. This situation is mainly caused by the following reasons: an open circuit and short circuit in the power supply, failure of the overvoltage and overcurrent protection circuit, failure of the auxiliary power supply, failure of the oscillation circuit, and overload of the power supply, the rectifier diode is broken down in the high-frequency rectification filter circuit, and the filter capacitor leaks, etc.

3) Poor power load capacity

The poor load capacity of the power supply is a common fault, which usually occurs in old-fashioned or long-working power supplies. The main reason is that the components are aging, the switching tube is unstable, and the heat dissipation is not performed in time. Therefore, it should check whether the Zener diode generates heat and leakage, whether the rectifier diode and high-voltage filter capacitor are damaged, etc.