Switching power supply maintenance methods and techniques

Central topics:

• Explore the maintenance methods and techniques of switching power supply

Solution:

• Fully familiar with the basic structure of switching power supply
• Utilize the characteristics of PFC and PWM modules

The switching power supply is material-saving, power-saving and highly efficient, and has basically replaced the traditional power supply of transformer transformation, diode rectification and transistor voltage stabilization. Since the switching power supply circuit is complex, there are many protection circuits and it is difficult to maintain. In order to quickly eliminate the fault of the switching power supply, it is very important for maintenance personnel to master the basic composition and working principle of the switching power supply. At present, all the switching power supplies of most electronic instruments are mainly composed of electromagnetic interference filters, surge control circuits, rectifier filter circuits, switching transformers, switching elements, pulse width modulation components and other components. Many switching power supplies have added power factor correction circuits between the rectifier circuit and the filter circuit to improve the power factor of the power supply, so that the current waveform of the power supply is at the same frequency and phase as the voltage wave, and the harmonic components in the current are eliminated as much as possible. The power factor of the switching power supply can reach 99%. The power factor correction circuit is composed of switching devices and power factor correction modules. The

voltage value of most boost power factor correction circuits on their filter capacitors is about 380V DC, and its composition is shown in the box below:

The power supply for Fuji 3000 thermal camera, Kodak 8900 laser camera and Hitachi EUB525 all belong to the above structure. Most of the switching power supplies sold in the domestic electronic market do not have the power factor correction circuit part, and the circuit is relatively simple. The maintenance methods and techniques for switching power supplies are introduced as follows:

1. When repairing the switching power supply, first use a multimeter to check whether each power component is short-circuited, such as the power rectifier bridge stack, switch tube, high-frequency high-power rectifier tube; whether the high-power resistor that suppresses the surge current is burned out. 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 still cannot work normally after the power is turned on. Then check the power factor module (PFC) and pulse width modulation component (PWM), consult relevant information, and be familiar with the function of each foot of the PFC and PWM modules and the necessary conditions for the normal operation of the modules.

3. Then, for the power supply with 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 properly. Then detect the working status of the PWM component, measure whether the voltage of its power input terminal VC, reference voltage output terminal VR, and start control Vstart/Vcontrol terminal is normal, use 220VAC/220VAC isolation transformer to power the switching power supply, and use an oscilloscope to observe whether the waveform of the CT terminal of the PWM module to the ground is a well-linear sawtooth wave or triangle, such as the TL494 CT terminal is a sawtooth wave, and the FA5310 CT terminal is a triangle wave. Whether the waveform of the output terminal V0 is an orderly narrow pulse signal.

4. In the practice of switching power supply maintenance, many switching power supplies use UC38×× series 8-pin PWM components. Most power supplies cannot work because the power supply startup resistor is damaged or the chip performance is degraded. The startup current increases, and the startup circuit is shown in the figure below.

When R is disconnected and there is no VC, the PWM component cannot work and needs to be replaced with a resistor with the same power resistance as the original one. When the starting 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 is UC3843. No other abnormalities were found during the detection. After connecting a 220K resistor to R (220K), the PWM component worked and the output voltage was normal. Sometimes due to a peripheral circuit failure, the 5V voltage at the VR end is 0V and the PWM component does not work. When repairing the Kodak 8900 camera power supply, this situation was encountered. The external circuit connected to the VR end was disconnected, and VR changed from 0V to 5V. The PWM component worked normally and the output voltage was normal.

5. When there is no voltage of about 380VDC on the filter capacitor, it means that the PFC circuit is not working properly. The key detection pins of the PFC module are the power input pin VC, the start pin Vstart/control, the CT and RT pins and the 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 as well as V0 waveform are all normal. There is no V0 waveform when measuring the G pole of the field effect power switch tube. Since FA5331 (PFC) is a patch component, the V0 terminal and the board are poorly soldered after the machine has been used for a long time, and the V0 signal is not sent to the G pole of the field effect tube. Solder the V0 terminal to the solder joint on the board, and use a multimeter to measure the filter capacitor to have a voltage of 380VDC. When the Vstart/control terminal is at a low level, the PFC cannot work either. Then it is necessary to detect the relevant circuits connected to its terminals and the periphery.

In short, the switching power supply circuit is easy or difficult, the power is large or small, and the output voltage is varied. 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, and the basic conditions for their operation, follow the above steps and methods, and do more hands-on maintenance of the switching power supply, you can quickly eliminate the switching power supply failure and achieve twice the result with half the effort.