At the same time, with the development of many high-tech, switching power supply technology is constantly innovating. The design of the switching power supply should be based on safety and reliability. Under the condition that various indicators meet the requirements of normal use, in order to make the power supply work safely and reliably under sudden fault conditions, this paper will analyze the structure of the switching power supply in detail. , principles and faults to provide maintenance skills for your reference.
Switching power supply structure
The switching power supply is mainly composed of four parts: a control circuit, a detection circuit, and an auxiliary power supply.
(1) Main circuit
Inrush current limiting: Limits the inrush current on the input side of the power supply.
Input filter: Its function is to filter the clutter in the grid and to prevent the clutter generated by the machine from feeding back to the grid.
Rectification and filtering: directly rectify the grid AC power to a smoother DC.
Inverter: The rectified DC power is changed to high frequency AC, which is the core part of the high frequency switching power supply.
Output rectification and filtering: Provides a stable and reliable DC power supply according to the load requirements.
(2) Control circuit
On the one hand, sampling from the output, comparing with the set value, then controlling the inverter, changing its pulse width or pulse frequency to stabilize the output, on the other hand, according to the data provided by the test circuit, the protection circuit provides identification and provides The control circuit performs various protection measures on the power supply.
(3) Detection circuit
Provides various parameters and various meter data in operation in the protection circuit.
(4) Auxiliary power supply
Software (remote) startup of the power supply, power supply for protection circuits and control circuits (chips such as PWM).
Introduction to the principle of switching power supply
The block diagram of the switching power supply is shown in the figure below.
1. At the moment of power-on, after the bulb flashes, it gradually goes out, then the power supply is normal from input to rectification and filtering, and the fault should be in the back circuit. Otherwise the power fuse or the input filter inductor is open.
2. If the rectifier filter circuit is normal, detect whether there is 310V voltage across the switch tube. If not, the sample resistor R0 or the transformer primary open circuit.
3. If the voltage of the switch tube is normal, check whether the switch circuit of the switch tube has a voltage of several volts to ten volts. If not, the start resistor and the drive circuit are detected.
4. If the drive has voltage and the switch is normal, the self-excited winding is faulty or the feedback circuit is faulty.
5. If the bulb is always on, the switch tube breaks down (short circuit) or the rectifier bridge breaks down (short circuit).
6. If the lamp is periodically turned off, the load has a short-circuit fault, which can focus on detecting the load.
7. If the switch tube is replaced multiple times, the peak voltage elimination circuit and the load are detected to have an open circuit fault.
8. After the above maintenance steps and detecting that the load voltage is basically normal, the switch K can be closed, and if the output is normal during the re-detection, the switching power supply has been repaired.
Note: The negative feedback circuit of the switching power supply and the secondary of the transformer must not be opened, otherwise it will damage other parts of the circuit.
Switching power supply common fault
Burn or fry
2. No output, normal fuse
3. There is output voltage, but the output voltage is too high
4. Switching power supply load has short circuit fault
5. Output voltage terminal rectifier diode, filter capacitor failure
Switching power supply maintenance tips
1. For the fault of burning or bombing, check the large filter capacitor on 300V, the diodes of the rectifier bridge and the switch tube. The problem of anti-interference circuit will also lead to burn-in and blackout. It should be noted that the burnout will generally burn the current sense resistor and the power control chip due to the breakdown of the switch tube. The negative temperature coefficient thermistor is also easily burned out together with the insurance.
2. For the case of no output, the fuse is normal, indicating that the switching power supply is not working or has entered the protection state. Firstly, it is necessary to measure whether the starting pin of the power control chip has a starting voltage. If there is no starting voltage or the starting voltage is too low, it is necessary to check whether the starting resistor and the external component of the starting pin are leaking. At this time, if the power control chip is normal, the above check is performed. You can quickly find the fault. If there is a starting voltage, then whether the output of the control chip has a high or low transition at the instant of power-on, if there is no jump, it means that the control chip is bad, the peripheral oscillator circuit component or the protection circuit has a problem, and the control can be replaced first. Chip, then check the external components; if there is a jump, the switch is generally bad or damaged.
3. For faults with output voltage, but the output voltage is too high, the fault is generally derived from the regulated sampling and voltage regulation control circuit. In the DC output, sampling resistor, error sampling amplifier such as TL431, optocoupler, power control chip and other circuits together form a closed control loop, any problem will lead to an increase in output voltage.
4. Switching power supply load has short-circuit fault (especially short circuit of DC/DC converter or poor performance). At this time, all loads of the switching power supply circuit should be disconnected to distinguish whether the switching power supply circuit or the load circuit is faulty. If the voltage output of the load circuit is disconnected, it means that the load is too heavy; or it is still abnormal, indicating that the switching power supply circuit is faulty.
5. For the case where the output voltage terminal rectifier diode and filter capacitor are invalid, it can be judged by the substitution method.
In short, the troubleshooting of switching power supply faults is easy, but as long as you grasp the core things, that is, fully familiar with the basic structure and characteristics of the switching power supply, it can quickly eliminate the switching power supply failure, and achieve twice the result with half the effort.
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