With the rapid advancement of high-tech industries, switching power supply technology has seen continuous innovation and improvement. The design of a switching power supply must prioritize safety and reliability. Under normal operating conditions, ensuring that all performance indicators meet specifications is essential. However, it is equally important to guarantee that the power supply can operate safely and reliably even in the event of unexpected faults. This article provides a detailed analysis of the structure, principles, and common faults of switching power supplies, offering practical maintenance tips for reference.
Switching Power Supply Structure
A typical switching power supply consists of four main components: the main circuit, control circuit, detection circuit, and auxiliary power supply.
(1) Main Circuit
The main circuit includes several key stages:
- Inrush Current Limiting: This component limits the initial surge of current when the power supply is turned on, protecting the internal components from damage.
- Input Filter: It filters out noise from the grid and prevents the power supply’s own noise from being fed back into the grid.
- Rectification and Filtering: Converts AC input from the grid into a smoother DC voltage.
- Inverter: Converts the DC voltage into high-frequency AC, which is the core function of a switching power supply.
- Output Rectification and Filtering: Provides a stable and reliable DC output based on the load requirements.
(2) Control Circuit
The control circuit plays a crucial role in maintaining stable output. It samples the output voltage, compares it with a reference value, and adjusts the inverter’s pulse width or frequency accordingly. Additionally, it uses data from the detection circuit to trigger protection mechanisms, such as overcurrent, overvoltage, and thermal shutdown, ensuring safe operation under abnormal conditions.
(3) Detection Circuit
This circuit monitors various operational parameters and provides real-time data for the protection system, enabling timely responses to potential issues.
(4) Auxiliary Power Supply
The auxiliary power supply provides power to the control circuits and chips, such as PWM controllers, and supports remote or software-based startup of the main power supply.
Principle of Switching Power Supply
The block diagram of a switching power supply is shown in the figure below. Understanding its working principle is essential for effective troubleshooting and maintenance.
When the power is first turned on, a bulb may flash briefly before going out, indicating that the power supply is functioning normally from the input stage through rectification and filtering. If a fault occurs, it likely lies in the downstream circuit. A blown fuse or an open input filter inductor could also be the cause.
If the rectifier filter circuit is working properly, check if there is 310V across the switch transistor. If not, the issue may be with the sampling resistor R0 or an open primary winding of the transformer.
If the switch transistor voltage is normal, check if the drive circuit provides several volts to tens of volts. If not, inspect the start resistor and driver circuit.
If the drive circuit is working but the switch is still not functioning, the self-oscillating winding or feedback circuit may be faulty.
If the bulb remains on, the switch transistor or rectifier bridge may have failed due to a short circuit.
If the bulb turns off periodically, the problem is likely due to a short-circuit in the load. Focus your testing on the connected devices.
If the switch transistor needs frequent replacement, the peak voltage suppression circuit or the load may have an open circuit fault.
After completing the above checks and confirming that the load voltage is normal, you can close the switch K. If the output returns to normal, the power supply has been successfully repaired.
Note: Do not open the negative feedback circuit or the secondary winding of the transformer, as this can cause damage to other parts of the circuit.
Common Faults in Switching Power Supplies
- Burned or exploded components
- No output voltage despite a good fuse
- Output voltage is too high
- Short circuit in the load
- Failure of rectifier diodes or filter capacitors at the output terminal
Maintenance Tips for Switching Power Supplies
- For burning or explosion faults: Check the large filter capacitor at 300V, the rectifier bridge diodes, and the switch transistor. Issues with the anti-interference circuit can also lead to similar problems. Note that a burned-out switch transistor often damages the current-sensing resistor and power control chip. The NTC thermistor may also be affected.
- For no output but a good fuse: This indicates the power supply may not be working or has entered a protection mode. First, check if the start pin of the power control chip has a proper starting voltage. If not, inspect the start resistor and external components. If the chip is normal, the fault can usually be found quickly. If there is a starting voltage but no transition in the control chip’s output, the chip itself or the oscillator and protection circuits may be faulty. Replace the chip first, then test the external components. If the output transitions, the switch transistor is likely damaged.
- For high output voltage: This is typically caused by a failure in the sampling and voltage regulation circuit. In the DC output, the sampling resistor, error amplifier (e.g., TL431), optocoupler, and power control chip together form a closed-loop control system. Any fault in these components can lead to an increase in output voltage.
- For load short circuits: Especially in DC/DC converters, a short circuit in the load can cause serious issues. Disconnect all loads to determine whether the fault is in the power supply or the load circuit. If the load is disconnected and the output is still abnormal, the power supply itself is likely faulty.
- For faulty rectifier diodes or capacitors: These can be identified using the substitution method.
In summary, while troubleshooting switching power supplies can seem challenging, having a solid understanding of their basic structure and characteristics can greatly simplify the process. With the right knowledge and approach, you can quickly identify and resolve issues, achieving efficient and effective repairs.
Phone Case,Eco Phone Case,Cases For Phone,Popular Phone Case
Shenzhen Ruidian Technology CO., Ltd , https://www.wisonen.com