At present, the switching power supply has gradually entered our daily life and production. It has the advantages of energy saving, environmental protection, and high cost performance. It has quickly replaced the conventional linear power supply that is both bulky and inefficient, and has quickly been replaced by people. accept. This article focuses on the common faults, precautions, and maintenance techniques for switching power supplies.

A. Switching power supply failures

1, the fuse blows

In general, a blown fuse indicates a problem with the internal wiring of the power supply. Because the power supply operates under the conditions of high voltage and high current, the fluctuation and surge of the power grid voltage will cause the current in the power supply to increase instantaneously and cause the fuse to blow. Focus should be checked on the power input rectifier diode, high voltage filter electrolytic capacitor, inverter power switch, etc., check whether the breakdown of this component, open circuit, damage and so on. If it is indeed a fuse blow, you should first look at the various components on the circuit board, see whether the appearance of these components are burned, there is no electrolyte overflow, if you do not find the above situation, then use a multimeter to measure the switch with or without breakdown short circuit . Special attention should be paid to the fact that: When a component is found to be damaged, it must not be directly switched on after it is replaced. It is likely that other high-voltage components will be damaged and the replaced component will be damaged. All high-voltage components of the above circuit must be fully implemented. After checking the measurement, the fuse blowout fault can be completely eliminated. ,

2, no DC voltage output or voltage output instability

If the fuse is intact, the DC voltage at each stage has no output 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 supply failure, oscillation circuit does not work, power load is heavy, rectifier diode in high-frequency rectifier filter circuit Breakdown, filter capacitor leakage and so on. After measuring 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, the control circuit of the power supply is sure to be faulty. If some of the voltage outputs indicate that the previous stage circuit is working properly, the fault is in the high-frequency rectifying filter circuit. The high-frequency filter circuit is mainly composed of a rectifier diode and a low-voltage filter capacitor to form a DC voltage output, in which the rectification diode breakdown will cause no voltage output of the circuit, and the filter capacitor leakage will cause the output voltage to be unstable and other faults. Use a multimeter to statically measure the corresponding component to check for damaged components.

3, poor power supply capacity

Poor power load capability is a common fault, and it usually occurs in old-style or long-time power supplies. The main reasons are aging of various components, unstable operation of switch tubes, and lack of timely heat dissipation. Should focus on checking whether the zener diode heat leakage, rectifier diode damage, high-voltage filter capacitor damage.


B. Switching power supply considerations

1, when selecting the switching power supply should pay attention to matters

1) Select the appropriate input voltage specification.

2) Choose the right power. In order to increase the life of the power supply, more than 30% of the output power rating can be selected.

3) Consider load characteristics. If the load is a motor, a light bulb or a capacitive load, the current will be greater at the instant of power-up and the appropriate power supply should be selected to avoid overload. If the load is a motor, voltage back-flow should be considered when shutting down.

) In addition, the operating environment temperature of the power supply must be taken into consideration, and whether there is additional auxiliary cooling device, and the output power at the excessively high ambient temperature needs to be derated. Ring temperature derating curve for output power.

5) Select various functions according to the application requirements:

Protection functions: Over-voltage protection (OVP), over-temperature protection (OTP), over-load protection (OLP), etc.

Application functions: Signal function (normal power supply, power supply failure), remote control function, telemetry function, parallel function, etc.

Special features: PFC, UPS

6) Select the required safety and electromagnetic compatibility (EMC) certification.

2, the use of switching power supply considerations

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 power 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 components, and measure the insulation resistance of the casing and the input and output to avoid electric shock.

) To ensure the safety of use and reduce the interference, please ensure that the grounding terminal is reliably grounded;

5) Multiple output power supply is generally divided into main and auxiliary outputs. The main output characteristic is better than the auxiliary output. Under normal circumstances, the output current is mainly the main output. In order to ensure that the output load adjustment rate and output dynamics and other indicators, generally require at least 10% of the load each road. If the auxiliary road is not used for the main road, the main road must be loaded with an appropriate dummy load;

6) Please note: frequent power switch will affect its life;

7) The working environment and loading degree will also affect its life.


C. Switching Power Supply Maintenance Tips

1, repair switching power supply, first with a multimeter to detect whether the power components breakdown short circuit, switching power supply housing such as rectifier bridge rectifier, switch, high-frequency high-power rectifier; suppression of high-power resistance of the inrush current is blown. Then check whether the output voltage of each output voltage is abnormal, and replace the above parts if they are damaged.

2. After the first step is completed, after the power is turned on, it still cannot work normally, and then the power factor module (PFC) and the pulse width modulation component (PWM) are to be detected. Refer to the related materials and be familiar with the functions of each pin of the PFC and PWM module. The prerequisites for the normal operation of its modules.

3. Then, for a power supply with a PFC circuit, it is necessary to measure whether the voltage across the filter capacitor is about 380VDC. If there is a voltage of about 380VDC, it means that the PFC module is working normally. Then, the working status of the PWM component is measured and the power input terminal VC is measured. Reference voltage output VR, start control Vstart/Vcontrol terminal voltage is normal, use 220VAC/220VAC isolation transformer to power the switching power supply, use an oscilloscope to observe whether the waveform of the PWM module CT to ground is a good linear sawtooth wave or triangle, such as TL9 The CT end is a sawtooth wave, and the FA5310 has a triangular wave at its CT end. Whether the waveform of the output V0 is an orderly narrow pulse signal. Switching power supply maintenance methods and techniques

In switching power supply repair practices, there are many switching power supplies that use the UC38× × series of 8-pin PWM components. Most switching power supply adapters cannot operate because the power supply startup resistor is damaged or the chip performance is degraded. When the R circuit is disconnected without VC, the PWM module cannot work and the resistor with the same power resistance as the original one needs to be replaced. When the startup current of the PWM component increases, the R value can be reduced until the PWM component can work normally. When repairing a GE DR power supply, the PWM module was UC383. No abnormality was detected. After a 220K resistor was connected to R (220K), the PWM module was working and the output voltage was normal. Sometimes because of peripheral circuit breakdown, cause 5V voltage of VR end to be 0V, PWM component does not work too, when repairing Kodak 8900 camera power, meet this situation, disconnect the external circuit connected with VR end, VR changes from 0V 5V, PWM components work normally and the output voltage is normal. Switching power supply maintenance methods and techniques

5. When the filter capacitor has no voltage of about 380 VDC, it means that the PFC circuit is not working properly. The schematic diagram of the switching power supply circuit is the power input pin VC, start pin Vstart/control, CT and RT pin and V0 pin. When repairing a Fuji 3000 camera, there was no 380VDC voltage across the filter capacitor on the test board. VC, Vstart/control, CT and RT waveforms and V0 waveforms are all normal. The measured field-effect power switch G has no V0 waveform. Since the FA5331 (PFC) is a patch element, the machine will use the V0 terminal and the board for a long time. Welding, V0 signal is not sent to the FET G pole. Solder the V0 terminal to the on-board solder joint. Measure the filter capacitor with a multimeter at 380VDC. When the Vstart/control terminal is at a low level, the PFC cannot work, and the related circuits whose terminals and peripherals are connected are detected.

In short, the switching power supply circuit is easy and difficult, the power is varied, and the output voltage is varied. As long as they 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 work, in accordance with the above steps and methods, more hands-on maintenance of the switching power supply, can quickly eliminate the switch Power failure, to achieve a multiplier effect.

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