As high-tech innovations continue to advance, switching power supply technology is constantly evolving. The design of these power supplies must prioritize safety and reliability. Under normal operating conditions, ensuring the power supply functions safely and reliably even under unexpected faults is crucial. This article provides a detailed analysis of the structure, working 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
- Inrush current limiting: Prevents excessive current from flowing into the system when power is first applied.
- Input filter: Reduces noise from the grid and prevents internal noise from returning to the grid.
- Rectification and filtering: Converts AC input into a smoother DC voltage.
- Inverter: Converts the DC voltage into high-frequency AC, which is the core part 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 samples the output voltage, compares it with a set value, and adjusts the inverter’s pulse width or frequency to maintain stability. It also works with the protection circuit to detect faults and implement necessary safeguards.
(3) Detection Circuit
This circuit monitors various operational parameters and provides data for the protection and control circuits.
(4) Auxiliary Power Supply
Supplies power to the control and protection circuits, including components like PWM chips, and supports remote or software-based startup.
Principle of Switching Power Supply
The block diagram of a switching power supply is shown below. When the power is turned on, if the indicator light flashes and then goes out, the power supply may be working normally. If the fuse is intact but there's no output, the fault likely lies in the back-end circuit. If the rectifier filter is normal, check for 310V across the switch tube. If not, there may be an issue with the sampling resistor or the primary winding of the transformer. If the switch tube voltage is normal, check the drive circuit for several volts to ten volts. If not, the start resistor or driver circuit may be faulty. If the drive has voltage and the switch is functioning properly, the self-excited winding or feedback circuit might be at fault. If the bulb remains on, the switch tube or rectifier bridge could be shorted. If the bulb turns off periodically, there may be a short-circuit in the load. Repeated switch tube failures often indicate issues with the peak voltage suppression circuit or the load. After following these steps and confirming the load voltage is normal, closing the switch K should restore normal operation. Note: Never open the negative feedback circuit or the secondary winding of the transformer, as this can damage other parts of the circuit.
Common Faults in Switching Power Supplies
- Burned or exploded components
- No output despite a good fuse
- Output voltage is too high
- Load short circuit
- Failed rectifier diode or filter capacitor at the output terminal
Maintenance Tips for Switching Power Supplies
1. For burning or exploding faults, check the large filter capacitor on the 300V line, the rectifier bridge diodes, and the switch transistor. Issues in the anti-interference circuit can also cause such problems. Be aware that a failed switch transistor can burn out the current sensing resistor and the power control chip. The NTC thermistor is also prone to failure along with the fuse.
2. If there is no output but the fuse is intact, the power supply may be in a protection state or not working. First, check if the starting pin of the power control chip has a proper starting voltage. If not, inspect the start resistor and external components for leakage. If the chip is functioning correctly, you can quickly identify the fault. If there is a starting voltage, check whether the control chip outputs a high-to-low transition at power-on. If not, the chip or its peripheral oscillator or protection circuit may be faulty. Replace the chip first, then check the external components. If the transition occurs, the switch transistor is likely damaged.
3. If the output voltage is too high, the issue is typically in the feedback sampling or voltage regulation circuit. These circuits include resistors, the TL431 error amplifier, optocoupler, and the power control chip, all forming a closed-loop control system. Any fault in these components can lead to elevated output voltage.
4. For load short circuits—especially in DC/DC converters—disconnect all loads to determine if the issue is in the power supply or the load itself. If the load voltage is still abnormal after disconnection, the power supply circuit is likely at fault.
5. To check for failed rectifier diodes or capacitors, use the substitution method for accurate diagnosis.
In summary, while troubleshooting switching power supplies can seem complex, mastering the core concepts—such as understanding their basic structure and characteristics—can greatly speed up the repair process. With the right knowledge and approach, you can achieve efficient and effective repairs, saving both time and effort.
Watch Screen Protector,Watch Film Protector,Smart Watch Screen Protector,Hd Watch Screen Protector
Shenzhen Ruidian Technology CO., Ltd , https://www.wisonen.com