At present, switching power supply has gradually entered our daily life and production. It has replaced the traditional "linear power supply" which is both cumbersome and low in efficiency, with the advantages of energy saving, environmental protection and high cost performance. accept. This article focuses on common faults, precautions, and repair techniques for switching power supplies.
A. Switching power supply common faults
1, the fuse is blown
In general, a blown fuse indicates a problem with the internal wiring of the power supply. Due to the high voltage and high current of the power supply, the fluctuations and surges of the power grid will cause the current in the power supply to increase instantaneously and the fuse will be blown. The key points should be to check the rectifier diode of the power input terminal, high-voltage filter electrolytic capacitor, inverter power switch tube, etc., check whether the component has breakdown, open circuit, damage, etc. If the fuse is really blown, you should first check the components on the board to see if the appearance of these components is burnt, and if there is any electrolyte overflow. If the above situation is not found, use a multimeter to measure the switch tube for short circuit. . Special attention should be paid to: Do ​​not directly detect the damage of a component when it is damaged. It is very likely that the other high-voltage components will still be damaged and the replacement components will be damaged. It is necessary to comprehensively apply all the high-voltage components of the above circuit. After the measurement is checked, the fuse blowout can be completely eliminated. ,
2, no DC voltage output or voltage output is unstable
If the fuse is intact, there is no output of DC voltage at all levels under load. This situation is mainly caused by the following reasons: open circuit, short circuit phenomenon in the power supply, overvoltage, overcurrent protection circuit failure, auxiliary power failure, oscillating circuit not working, power supply overload, rectifier diode in high frequency rectification and filtering circuit It is broken down, the filter capacitor is leaking, and so on. After measuring the secondary component with a multimeter and eliminating the breakdown of the high-frequency rectifier diode and the short-circuit of the load, if the output is zero at this time, it is sure that the control circuit of the power supply has failed. If some of the voltage output indicates that the pre-stage circuit is working properly, the fault is in the high-frequency rectification and filtering 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 causes the circuit to have no voltage output, and the leakage of the filter capacitor may cause an unstable output voltage. Statically measure the corresponding component with a multimeter to check the damaged component.
3, poor power supply capacity
Poor power load capacity is a common fault, which usually occurs in old-fashioned or long-duration power supplies. The main reason is that the components are aging, the switching tube is unstable, and there is no heat dissipation in time. It is important to check whether the Zener diode is hot or leaky, the rectifier diode is damaged, and the high voltage filter capacitor is damaged.
B. Switching power supply precautions
1, should pay attention to when selecting switching power supply
1) Select the appropriate input voltage specifications.
2) Choose the right power. In order to increase the life of the power supply, a model with a maximum output power rating of 30% can be selected.
3) Consider the load characteristics. If the load is a motor, a light bulb or a capacitive load, the current is large when the power is turned on. A suitable power supply should be used to avoid overload. If the load is a motor, consider voltage backflow at shutdown.
4) In addition, it is necessary to consider the working environment temperature of the power supply, and whether there is additional auxiliary heat dissipation equipment. In the case of excessively high temperature, the power supply needs to be derated. The derating curve of the ring temperature to the output power.
5) Select each function according to the application:
Protection functions: over voltage protection (OVP), over temperature protection (OTP), overload protection (OLP), etc.
Application functions: signal function (normal power supply, power failure), remote control function, telemetry function, parallel function, etc.
Special features: power factor correction (PFC), uninterruptible power (UPS)
6) Select the required safety and electromagnetic compatibility (EMC) certification.
2, the precautions for using switching power supply
1) Before using the power supply, first determine whether the input and output voltage specifications match the nominal value of the power supply used;
2) Before powering on, check whether the input and output leads are connected correctly to avoid damage to the user equipment;
3) Check whether the installation is firm, whether the mounting screws are in contact with the power board device, and measure the insulation resistance of the housing and the input and output to avoid electric shock;
4) To ensure the safety of use and reduce interference, please ensure that the grounding terminal is reliably grounded;
5) Multi-output power supply is generally divided into main and auxiliary outputs. The main output characteristic is better than the auxiliary output. In general, the output current is large as the main output. In order to ensure the output load regulation rate and output dynamics and other indicators, it is generally required to have at least 10% load per channel. If the auxiliary road is not used, the main road must be added with a proper dummy load;
6) Please note: frequent power switch will affect its life;
7) The working environment and the degree of loading will also affect its life.
C. Maintenance skills of switching power supply
1. When repairing the switching power supply, first use a multimeter to check whether the power components are short-circuited. The switching power supply housing is such as a power rectifier bridge, a switching tube, a high-frequency high-power rectifier; whether the high-power resistor that suppresses the inrush current is blown. Then check whether the resistance of each output voltage port is abnormal. If the above components are damaged, they need to be replaced.
2. After the first step is completed, it will not work normally after the power is turned on. Then, the power factor module (PFC) and the pulse width modulation component (PWM) should be detected. Check the relevant information and familiarize with the functions of each pin of the PFC and PWM modules. The prerequisites for the normal operation of the module.
3. Then, for a power supply with a PFC circuit, it is necessary to measure whether the voltage across the filter capacitor is about 380 VDC. If there is a voltage of about 380 VDC, the PFC module works normally, and then the working state of the PWM component is detected, and the power input terminal VC is measured. The reference voltage output terminal VR starts to control whether the voltage of the Vstart/Vcontrol terminal is normal. The 220VAC/220VAC isolation transformer is used to supply power to the switching power supply. The oscilloscope is used to observe whether the waveform of the PWM module CT to ground is a linear sawtooth wave or triangle, such as TL494. The CT end is a sawtooth wave, and the FA5310 has a CT wave at the CT end. Whether the waveform of the output terminal V0 is an ordered narrow pulse signal. Switching power supply maintenance methods and techniques
4, in the switching power supply maintenance practice, there are many switching power supply using UC38 & TImes; & TImes; series 8-pin PWM components, switching power adapter most power supply can not work because the power starting resistor is damaged, or the chip performance is degraded. When there is no VC after R is disconnected, the PWM component cannot work, and the same resistance as the original power resistance needs to be replaced. When the PWM component startup current increases, the R value can be reduced until the PWM component can operate normally. When repairing a GE DR power supply, the PWM module is UC3843, and no other abnormalities are detected. After a 220K resistor is connected to R (220K), the PWM component works and the output voltage is normal. Sometimes due to peripheral circuit failure, the VR terminal 5V voltage is 0V, the PWM component does not work. When the Kodak 8900 camera power supply is repaired, this situation is encountered. The external circuit connected to the VR terminal is disconnected, and the VR changes from 0V. 5V, PWM components work normally, and the output voltage is normal. Switching power supply maintenance methods and techniques
5. When there is no voltage of 380VDC on the filter capacitor, it means that the PFC circuit does not work normally. The key detection pin of the PFC module is the power input pin VC, the start pin Vstart/control, CT and RT pin and V0 pin. When repairing a Fuji 3000 camera, test that there is no 380VDC voltage on the filter capacitor on the board. VC, Vstart/control, CT and RT waveforms and V0 waveforms are normal. The measured field effect power switch tube G has no V0 waveform. Since FA5331 (PFC) is a patch component, the machine will appear between the V0 terminal and the board after a long time. Solder, V0 signal is not sent to the FET of the FET. Solder the V0 end to the solder joint on the board, and measure the filter capacitor with a 380 VDC voltage with a multimeter. When the Vstart/control terminal is low and the PFC is not working, the relevant circuit whose terminal is connected to the periphery is detected.
In short, the switching power supply circuit is easy to have, the power is large and small, and the output voltage is various. As long as you grasp the core things, that is, fully familiar with the basic structure of the switching power supply and the characteristics of the PFC and PWM modules, the basic conditions for their operation, according to the above steps and methods, more hands-on maintenance of the switching power supply, you can quickly eliminate the switch Power failure, achieve twice the result with half the effort.
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