If the output voltage of the switching power supply is low, it could be due to a variety of reasons. Here are some possible causes and corresponding repair methods:
1. Input Voltage Issue: Cause: Unstable or below-rated input voltage. Repair Method: Check the input voltage and ensure it is within the equipment's specified range. If the input voltage is unstable, a voltage regulator or voltage adjuster may be necessary.
2. Switching element failure: Cause: The switching element (usually a transistor or MOSFET) is damaged. Repair method: Use an oscilloscope to check the waveform of the switching element for any abnormalities. If a fault is found, replace the damaged switching element.
3. Output inductor problem: Cause: The output inductor is damaged or short-circuited. Repair method: Use a multimeter to check the resistance and short circuit of the output inductor. If there is a problem, replace the damaged inductor.
4. Filter capacitor problem: Cause: Faulty output filter capacitor. Repair method: Use a capacitance meter to check the capacitance value of the output capacitor. If the capacitance value is lower than the specified value, the capacitor may need to be replaced.
5. Feedback Circuit Problem: Cause: A problem with the feedback component (such as an optocoupler or resistor) in the control loop. Repair Method: Use an oscilloscope to check the feedback signal. If the signal is unstable or problematic, it may be necessary to check and replace the components in the feedback circuit.
6. PWM Control Issue: Cause: Damaged or misconfigured PWM control chip. Repair Method: Use an oscilloscope to check the PWM control signal. If the signal is abnormal, it may be necessary to replace the PWM control chip or reconfigure the control parameters.
7. Load Issue: Cause: The connected load exceeds the power supply's rated capacity. Repair Method: Check if the connected load exceeds the power supply's rated capacity. If so, a higher capacity power supply is required.
Before performing any repairs, ensure safe operation and strictly follow the relevant equipment's repair manual or the manufacturer's recommendations. If you are unfamiliar with repairing switching power supplies, it is recommended that you have a professional technician perform the repairs.
1. Power supply failure. Due to prolonged use or other reasons, the power supply may experience failures such as no output voltage or low output voltage.
2. Output capacitor failure. If the capacitor at the output of the switching power supply fails, it will cause the output voltage to drop.
3. Improper power supply output voltage adjustment. Improper adjustment of the power supply output voltage may cause the output voltage to be lower than the standard value.
II. Methods to solve low voltage in switching power supplies
1. Replace the power supply. If the power supply itself is found to be faulty, then replacing the power supply is probably the most direct solution.
2. Check the output capacitor. If the output capacitor is faulty, consider replacing it to restore the output voltage.
3. Adjust the power supply output voltage. If the output voltage of the switching power supply is lower than the standard value, you can use the voltage regulation function built into the switching power supply or adjust it by changing the power supply feedback circuit.
At the same time, please note the following points:
1. When replacing the power supply, be sure to choose the appropriate model. Do not replace or disassemble the power supply at will.
2. When checking the output capacitor, pay attention to the capacitor polarity and avoid reverse connection.
3. When adjusting the power supply output voltage, you should follow the instructions in the power supply operation manual to avoid other malfunctions caused by improper operation.
The above methods are applicable to various types of switching power supplies. Different models and brands of power supplies may vary, and the specific solution needs to be determined based on the actual situation.
Causes and maintenance methods for low output voltage of switching power supply
1. Reasons for low voltage output from switching power supply:
(1) The 220V AC voltage input and the rectifier filter circuit provide insufficient operating voltage to the switching transistor, which exceeds the operating range of the pulse width adjustment circuit.
(2) Overcurrent in the load circuit causes the load on the switching power supply to increase and the output voltage to decrease.
(3) Switch malfunction: After the online scanning circuit starts working, the switching power supply immediately enters standby mode. This fault applies to computers without a backup power supply. The CPU power comes from the same power source and is not supplied by an auxiliary power supply.
(4) The terminals of the on/off interface circuit are in a state between startup and standby due to a fault, which causes the output voltage of the switching power supply to be lower than the normal value and higher than the standby value.
(5) Due to the fault, the end of the protection circuit enters the conduction state, causing the power supply to enter the weak vibration state, which in turn causes the output voltage of the switching power supply to drop.
(6) The diodes, filter capacitors and current limiting resistors in the rectifier output circuit are damaged, resulting in low output voltage.
(7) A fault in the pulse width modulation circuit prevents it from responding correctly to changes in the output voltage of the switching power supply, and the direction of the base voltage of the switching transistor is incorrectly adjusted, resulting in a low output voltage of the switching power supply.
(8) Changes in the value of the positive feedback resistor in the positive feedback circuit, reduced performance of the freewheeling diode, or failure of the constant current source can lead to insufficient positive feedback, resulting in a longer oscillation period, a lower oscillation frequency, and a reduction in the output voltage of the switching power supply.
(9) The excitation switching power supply has a low output voltage because it does not receive the line flyback pulse, and therefore is in a low frequency state.
2. Methods and steps for determining the fault
As can be seen from the above analysis, the cause of low voltage involves all parts of the switching power supply itself and all circuits related to the switching power supply, and the scope of the fault should be reduced during maintenance.
(1) First, measure the voltage at the C terminal of the switch and confirm that the power supply is normal.
(2) Determine the fault based on the voltage of each output terminal of the switching power supply.
Some switching power supplies have normal output voltages, while others have lower than normal levels. If the fault lies in the rectifier output circuit where the output voltage is low, the current-limiting resistor, rectifier diode, and filter capacitor in the circuit should be checked and replaced. If the current-limiting resistor becomes hot, it indicates an overcurrent load, and the load has already been checked.
Each output of the switching power supply is low. This indicates that the load and rectifier output circuits are normal, and the fault may be caused by the positive feedback circuit, pulse width modulation, on/standby circuit, and protection circuit of the switching power supply.
Some output voltages exhibit a larger rate of decrease, while others show a smaller rate of decrease. Measurements indicate that a larger output voltage drop is caused by a circuit fault. In this case, the load can be disconnected. If the circuit is disconnected, a dummy load should be connected. After disconnecting the load, measure the output voltage of the switching power supply again. If it returns to normal, it can be inferred that the open-circuit load had an overcurrent. If the problem persists, the fault lies in the rectifier and filter circuit.
3. Disconnect the main load and connect the light bulb to determine if the load is faulty.
On some machines that experience screen flickering and voltage instability after loading, it can be difficult to determine whether the problem is a power supply failure or a load failure. You can use the "borrowing method" to load this power supply onto another computer of the same size and with the same B+ voltage. Infer from this.
4. Standby startup, positive feedback, soft start, and negative feedback circuits.
The transistors at the end of the backup control circuit are deactivated one by one to remove various protection circuits. Open and observe whether the fault has been eliminated to gradually narrow down the fault range.
Note: Do not disconnect circuits with voltage regulation functions (such as couplings). When disconnecting protection circuits, please be careful and take precautions to prevent voltage rise.
5. Repair the pulse width modulation circuit using alternative methods. Replace the original sampling circuit with a homemade sampling circuit to determine the scope of the fault.
(1) After replacement, the voltage returned to normal, indicating that the fault was in the sampling circuit and the coupler circuit.
(2) If the voltage remains low, disconnect the B+ connection point of the original sampling circuit. If the voltage remains low, check the B+ filter capacitor. Once confirmed to be working properly, troubleshooting can be done in the hot base section. First, check if the soft-start circuit is shunting the B terminal of the switch. If it still doesn't work, check the positive and negative feedback circuits.
The method for checking the negative feedback of the hot plate is similar to that for checking high voltage. The method involves forcing the B+ output to a high level (note: changing the operating point will not cause B+ to go too high, which could amplify the fault).
In short, in power supply maintenance, you can use reverse thinking when the voltage is unstable: reduce the voltage when it is high, increase the voltage when it is low, and manually change the operating point voltage if necessary. Maintenance personnel must be sensitive in order to locate the fault.
The principle of switching power supplies and their advantages and disadvantages.
A switching power supply mainly consists of an input mains filter, an input rectifier filter, an inverter, an output rectifier filter, a control circuit, and a protection circuit.
Their functions are:
1. Input power grid filter: Removes power grid interference, such as from motor starting, electrical switching, lightning strikes, etc. It can also prevent high-frequency noise generated by switching power supplies from spreading into the power grid.
2. Input rectifier filter: Rectifies and filters the input voltage from the power grid to provide DC voltage for the converter.
3. Inverter: It is a key component of the switching power supply. It converts DC voltage into high-frequency AC voltage and isolates the output section from the input power grid.
4. Output rectifier filter: Rectifies and filters the high-frequency AC voltage output by the frequency converter to obtain the required DC voltage, while preventing high-frequency noise from interfering with the load.
5. Control Circuit: Detects the output DC voltage, compares it with a reference voltage, and then amplifies it. The pulse width of the oscillator is modulated to control the converter to maintain a stable output voltage.
6. Protection Circuit: When an overvoltage or overcurrent short circuit occurs in the switching power supply, the protection circuit will stop the switching power supply to protect the load and the power supply itself.
A switching power supply first rectifies the alternating current (AC) into direct current (DC), then converts the DC back to AC, and finally outputs the required DC after rectification. In this way, the linear power supply and transformer in the voltage feedback circuit are eliminated. The inverter circuit in a switching power supply is fully digitally regulated, which also allows for very high adjustment accuracy.
The main advantages of switching power supplies are: small size, light weight (only 20-30% of the size and weight of linear power supplies), high efficiency (typically 60-70%, while linear power supplies are only 30-40%), strong anti-interference capability, wide output voltage range, and modularity.
The main disadvantage of switching power supplies is that the high-frequency voltage generated in the inverter circuit can cause interference to surrounding equipment. Therefore, good shielding and grounding are required.
After AC rectification, DC power can be obtained. However, due to variations in AC voltage and load current, the rectified DC voltage typically exhibits a 20% to 40% voltage fluctuation. To obtain a stable DC voltage, a voltage regulator circuit must be used to regulate the voltage. Based on different implementation methods, regulated power supplies can be classified into three types: linear regulated power supplies, phase-controlled regulated power supplies, and switching regulated power supplies. Among these, switching power supplies represent the trend towards low-carbon, environmentally friendly, and advanced power supplies.
A common low-voltage DC switching power supply directly rectifies 220V AC power to approximately 300V DC power through an EMI filter, then uses circuitry to manipulate switching transistors to switch and cut off high-speed channels. This is then converted to high-frequency AC power to supply a transformer for voltage conversion. One or more sets of desired voltages are generated and then rectified to allow the desired voltage to flow through the DC current. The reason for converting to high-frequency AC is that the efficiency of high-frequency AC in the transformer circuit is much higher than 50Hz, allowing the main transformer to be made very small and using a magnetic core. However, this is not the case. It gets extremely hot during operation. Furthermore, at high frequencies, the energy stored in the filter, as well as the capacitance and inductance of the filter, are much smaller than at 50Hz, and the cost is significantly lower. Without changing 50Hz to a higher frequency, a switching power supply would be meaningless! The switching transformer is not magical; it's just a regular magnetic core transformer! This is a switching power supply.
Switching power supplies are implemented using electronic technology. Their main steps are: rectifying to DC power, converting the required voltage (mainly voltage regulation) into AC power, and then rectifying it back into DC voltage output.
Switching power supplies are particularly small in size because they lack bulky transformers and heat sinks. Furthermore, their internal components are entirely electronic, resulting in high efficiency and low heat generation. Despite their drawback of significant electromagnetic interference (EMI), they are currently sold in Europe, the United States, Japan, and China. However, no shortcuts have been taken in switching power supplies; EMI filters and shielding technologies already exist.
Switching power supplies can be broadly classified into two types: isolated and non-isolated. Isolated power supplies must have a switching transformer, while non-isolated power supplies may or may not necessarily have one.
In short, the working principle of a switching power supply is:
1. The AC power input is rectified and filtered into DC;
2. The switch is operated via high-frequency PWM (Pulse Width Modulation) or PFM (Pulse Frequency Modulation), and this DC current is added to the primary winding of the switching transformer;
3. The secondary frequency of the switching transformer generates a high-frequency voltage, which is supplied to the load through rectification and filtering.
4. The output section feeds back to the control circuit through a positive feedback circuit to control the duty cycle of the PWM and achieve stable output.
When AC power is input, it is usually filtered by inductors and capacitors to remove interference to the power grid and to control power supply. At the same power, a higher switching frequency allows for a smaller switching transformer, but also places higher demands on the switching transistors. The secondary winding of the switching transformer can have multiple windings or windings with multiple taps to achieve the desired output. Typically, some protection circuitry should be added, such as no-load, short-circuit, and other protection functions; otherwise, the switching power supply may burn out.
The reason why a bridge rectifier is not needed on the secondary side of a switching power supply
The reason why switching power supplies do not use bridge rectification on the secondary side is due to their inherent characteristics. Switching power supplies generally come in two types: forward-biased and flyback-biased. The secondary diodes are primarily used to rectify the cutoff waveform, removing reverse peak voltages. Both of these types do not require an input voltage waveform because switching power supplies have a PWM chip.
