In the field of power electronics, switching power supplies are widely used due to their high efficiency and small size. However, debugging a switching power supply is a complex and meticulous task. Before debugging, it is necessary to first check whether all components of the switching power supply are correctly installed, whether the solder joints are secure, and whether there are any short circuits or open circuits. In addition, necessary debugging tools and instruments, such as multimeters and oscilloscopes, should be prepared, and these tools should be ensured to be in good working order.
(1) Input voltage adjustment: Adjust the input voltage to the specified range according to the design requirements of the switching power supply, and observe the working status of the power supply.
(2) Output voltage adjustment: Adjust the potentiometer or other related components inside the switching power supply to make the output voltage reach the predetermined value. During this process, the stability of the output voltage needs to be closely monitored.
(3) Load adjustment: Connect different loads to the output terminal and observe the changes in the output voltage and current of the switching power supply to ensure that it can work normally under various load conditions.
In the research and development and production of electronic devices, the debugging of switching power supplies is a crucial step. However, some problems often arise during this process. Today, we'll review 10 of the most common problems encountered during switching power supply debugging, hoping to provide you with some helpful information.
Problem 1: Unstable output voltage
Unstable output voltage is one of the most common problems encountered during switching power supply commissioning. This can be caused by factors such as feedback loop failure, input voltage fluctuations, excessive load changes, or parameter drift of internal components. Solving this problem requires checking the components in the feedback loop for proper functioning, optimizing the input filter circuit, and designing a suitable load match.
Question 2: Output ripple is too large
Excessive output ripple can affect the normal operation of electronic equipment. This problem may be caused by insufficient filter capacitor capacity, inappropriate inductor parameters, improper switching frequency settings, or unreasonable PCB layout. It can be resolved by increasing the filter capacitor capacity, optimizing inductor parameters, adjusting the switching frequency, and improving PCB layout.
Question 3: Low power efficiency
Inefficient power supplies lead to energy waste and heat generation. This can be caused by excessive switching losses, high losses in magnetic components, high losses in the drive circuit, or an inappropriate control strategy. Solutions include using switching transistors with low on-resistance, optimizing the design of magnetic components, improving the efficiency of the drive circuit, and refining the control algorithm.
Question 4: Electromagnetic interference (EMI) exceeds the standard
Excessive EMI can interfere with surrounding electronic equipment, affecting its normal operation. High-frequency switching in switching power supplies is a major source of EMI. EMI can be reduced through proper PCB layout, adding EMI filters, optimizing transformer design, and employing soft-switching technology.
Question 5: Overcurrent protection failure
Overcurrent protection failure can damage the power supply and load. This may be due to a faulty overcurrent detection circuit, an improperly set protection threshold, or a slow response time in the protection circuit. It is necessary to check the components of the overcurrent detection circuit, set the protection threshold appropriately, and improve the response speed of the protection circuit.
Question 6: Overheating problem
Overheating can affect the reliability and lifespan of a power supply. Causes of overheating may include poor heat dissipation of power devices, excessive operating current, and high ambient temperature. It can be resolved by increasing the heatsink area, improving airflow, reducing the operating current, or enhancing ventilation.
Question 7: Difficulty in starting up
Difficulty starting the power supply may be caused by a faulty startup circuit, excessively long charging time of the input capacitor, a malfunctioning soft-start circuit, or an abnormal operation of the control chip. It is necessary to check the components of the startup circuit, optimize the input capacitor parameters, check the soft-start circuit, and replace the faulty control chip.
Question 8: Abnormal no-load output voltage
An abnormal output voltage under no-load conditions may be caused by factors such as malfunction of the feedback loop under no-load conditions, leakage in the transformer secondary winding, or parasitic capacitance at the output terminal. The solution involves checking the operation of the feedback loop under no-load conditions, repairing transformer leakage, and reducing parasitic capacitance at the output terminal.
Question 9: Poor load regulation
Poor load regulation can cause significant fluctuations in output voltage with changes in load. This may be due to insufficient gain in the feedback loop, inadequate output filter capacitor capacity, or excessive internal resistance of the power supply. It can be resolved by increasing the gain of the feedback loop, increasing the output filter capacitor capacity, and reducing the internal resistance of the power supply.
Question 10: Short-circuit protection issue
A short-circuit protection failure may prevent timely protection of the power supply and load in the event of a short circuit at the power output. This could be due to a faulty short-circuit detection circuit, an improperly set protection action threshold, or a delayed response from the protection circuit. It is necessary to check the short-circuit detection circuit, properly set the protection action threshold, and improve the response speed of the protection circuit. Next, we will briefly analyze the difficulties in debugging switching power supplies to provide valuable reference for relevant personnel. When starting up under high or low voltage input, including light load, heavy load, capacitive load, and environments such as output short circuits, dynamic loads, and high temperatures, the current in the transformer and switching transistors may exhibit non-linear growth. Under this phenomenon, the current peak becomes difficult to predict and control, potentially leading to current overstress and subsequently causing overvoltage damage to the switching transistors.
Situations that can easily lead to transformer saturation include: excessive transformer inductance, insufficient turns, transformer saturation current point below the IC's maximum current limit, and lack of soft-start functionality. To address these issues, measures such as lowering the IC's current limit and enhancing soft-start can be taken to ensure that the current through the transformer rises slowly. Under the worst conditions, such as maximum input voltage, maximum load, maximum ambient temperature, and during power-on or short-circuit testing, the maximum value of Vds should not exceed 90% of its rated specifications. To reduce Vds, one can try reducing the plateau voltage, which typically involves adjusting the transformer's primary-to-secondary turns ratio. Additionally, Vds can be reduced by decreasing peak voltage, which can be achieved by reducing transformer leakage inductance and adjusting the snubber circuit (e.g., using a TVS diode, a slower diode, or inserting a damping resistor).
Overheating of an IC may be caused by excessive losses in the internal MOSFETs, poor heat dissipation, or excessively high ambient air temperature. To address this, the distance between the transformer windings can be increased to reduce interlayer capacitance, or heat dissipation can be improved, such as by increasing the copper foil area and applying more solder. Simultaneously, ensure the IC is located in a well-ventilated area, away from other high-temperature components.
When the power supply is under no-load or light-load conditions, it may fail to start, manifesting as the Vcc voltage fluctuating repeatedly between the startup and shutdown voltages. This is usually due to the induced voltage in the Vcc winding being too low under no-load or light-load conditions. To resolve this issue, you can try increasing the sensitivity of the startup circuit or adjusting relevant parameters.
Unable to apply heavy load after startup. Cause and solution: Vcc is too high under heavy load. Under heavy load, the induced voltage in the Vcc winding increases, causing Vcc to become too high and triggering the IC's overvoltage protection, resulting in no output. Measures must be taken to prevent further voltage increases and damage to the IC.
Internal rate limiting was triggered
a. The rate limit is set too low.
Under heavy loads or capacitive loads, if the current limiting point is set too low, the MOSFET current will be limited, resulting in insufficient output. To solve this problem, the current limiting pin resistor should be increased to raise the current limiting point.
b. The current rise slope is too large.
An excessively steep current rise may trigger internal current limiting protection. To avoid this, the inductance can be increased while maintaining an unsaturated transformer. High standby input power: Under no-load or light-load conditions, insufficient Vcc leads to excessive input power and output ripple. Cause: Insufficient Vcc causes frequent IC startups, with the high voltage charging the Vcc capacitor, causing losses in the startup circuit. If there is a resistor between the startup pin and the high voltage, the power dissipation will increase; therefore, a startup resistor with sufficient power rating must be selected. Additionally, the power IC not entering Burst Mode or an excessively high Burst frequency may also cause problems. Solution: Adjust feedback parameters to reduce the feedback speed. Excessive short-circuit power: When the output is short-circuited, the input power and Vds are excessively high. When the output is short-circuited, there are many repetitive pulses and a large peak current of the switching transistor, leading to excessive input power. Excessive switching current stores too much energy in the leakage inductance, causing Vds to rise when turned off. Furthermore, a short circuit may trigger OCP or internal current limiting protection, causing the switching transistor to stop working. Take appropriate measures based on the specific situation, such as triggering OCP for protection or adjusting the internal current limiting settings. Output bounce phenomenon under no-load and light-load conditions: Phenomenon description: When the input voltage is turned off under no-load or light-load conditions, the output voltage (e.g., 5V) appears. Cause analysis: After the input voltage is turned off, the 5V output begins to drop, and Vcc also decreases accordingly. Once Vcc drops below the IC's shutdown voltage, the IC will stop working. However, under no-load or light-load conditions, the voltage drop rate of the large capacitor in the power supply is slower, and it can still provide sufficient current to the high-voltage start-up pin, thereby triggering the IC to restart, causing the 5V output to reappear, forming a bounce.
