Abstract: This article briefly describes the abnormal output problem in the common problems of switching power supply applications, and briefly analyzes the causes of the problem. At the same time, it gives the corresponding verification methods and solutions and preventive methods to reduce the possibility of abnormal output of the power supply in different applications and improve system reliability.




In daily work, engineers and friends often encounter the following problems of abnormal power output:


(1) The nominal power supply is 24V, why not 24V during the test?


(2) Why does the actual measurement of the nominal ripple noise and other parameters of the power supply exceed the nominal value of the manual?


(3) The input voltage is normal. After working for a while, why is the output gone?



In this regard, this article will sort out common output abnormalities, analyze their causes and give corresponding solutions.


Common output anomalies


Power supply is an essential core component of electrical equipment. We usually hope that it is stable, reliable, high-precision, and high-performance. We can get these indicators from its technical manual. However, when we test in the laboratory or in practical applications, we often find that the test parameters do not match the nominal parameters. Usually there are the following situations:


  1. The output voltage is low or high;


  1. The output ripple noise exceeds the specification;


  1. There is no output after working continuously for a period of time;


When we encounter these problems, how to analyze, confirm and eliminate them? Next, we will analyze the problems one by one


  1. The output voltage is low or high


Usually, there are two reasons for the low or high output voltage:


1) Touch the adjustable resistor by mistake:


Judgment method: Use a multimeter to directly measure the output terminal voltage to judge whether it is the standard output voltage;


Solution: Rotate the adjustable resistor according to the direction on the label paper, and increase the output voltage appropriately to meet the actual demand;




2) The line distance between the customer load and the power supply is too long, and the output line loss is too large


Judgment method: test and compare the voltage at the output terminal of the power supply and the voltage at the input terminal of the customer’s load;


Solution: It is recommended to shorten and thicken the wiring between the power supply and the load;


  1. The output ripple noise exceeds the specification


Typically, there are three reasons why the output ripple noise is out of specification:


1) The ground wire loop introduces high-frequency signal interference, and the bandwidth of the oscilloscope is set to be large


Judgment method: check whether the method of testing ripple noise complies with the recommended method and wiring method in the technical manual;


Solution: use the correct test method, inappropriate test method will make the test distortion, lead to misjudgment of the results and so on. Usually, we will limit the bandwidth during the ripple and noise test, which mainly depends on the working frequency range of the module under test. Taking the power module as an example, usually the working frequency of the power module is below 500KHz, and the frequency of switching noise is mostly below 5MHz. Therefore, during the test, it is recommended to limit the bandwidth to 20MHz, so as to ensure that what is tested is the ripple noise generated by the real power supply. At the same time, when testing, in order to avoid interference from the ground wire of the power supply of the oscilloscope, it is recommended to cut off the ground pin on the power wire of the oscilloscope.


The conventional test methods are parallel line test method and relying test method. The parallel line test method is as follows:




Capacitor C1 in the figure is a ceramic capacitor; C2 is an electrolytic capacitor, and the default values of C1 and C2 are 1uF and 10uF respectively;


The test method is as follows:



Compared with the conventional wire drop test, the relying method mainly reduces the area of the ground wire loop to avoid coupling to interference in the ground wire loop during the test, which affects the test results. The main method is to remove the clamp of the ground wire as shown in the figure, and directly use the ring copper on the probe as the ground. This can effectively avoid the impact of the external electromagnetic environment on the test.


2) The N line of the isolated power module or the input ground is short-circuited to the output negative pole


Judgment method: Check whether the power module N line/input ground–output ground is short-circuited, and check whether there is a negative ground point on the back-end load board;



Solution: It is not recommended for customers to connect the N line/input ground–output ground of the isolated power module together. If the actual application requires negative grounding and the requirements for EMC, ripple noise are not high, or there is no requirement for isolation voltage, the common ground can be used;


3) There are high-power and strong interference equipment or components near the power module




  1. The customer’s system layout keeps the power module away from high-power equipment;


  1. The front end of the power module is equipped with a filter peripheral circuit;


  1. The lead wires at the output end of the power module are shortened to reduce noise coupling, and the metal shell can be used to shield external interference if possible;









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