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 check these indicators from its technical manual. However, when we test in the laboratory or in actual 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;

2. The output ripple noise exceeds the specification;

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

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



1. The output voltage is low or high

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

1) Accidentally touching the adjustable resistor:

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

Solution: You can rotate the adjustable resistor according to the direction on the label to appropriately increase the output voltage to meet actual needs;







2) The 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;







2. The output ripple noise exceeds the specification.

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

1) The ground loop introduces high-frequency signal interference, and the oscilloscope bandwidth setting is 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 correct testing methods. Inappropriate testing methods will cause test distortion, lead to misjudgment of results and other problems. Usually during ripple and noise testing we will limit the bandwidth, which mainly depends on the operating frequency range of the module under test. Take the power module as an example. Usually the operating frequency of the power module is below 500KHz, and the frequency of switching noise is also mostly below 5MHz. Therefore, during the test process, it is recommended to limit the bandwidth to 20MHz, so as to ensure that the tested is the ripple noise generated by the real power supply. At the same time, in order to avoid interference on the ground line of the oscilloscope's power supply during testing, it is recommended to cut off the ground pins on the oscilloscope's power line.



Conventional testing methods are parallel line testing method and dependent testing method. The parallel line testing method is as follows:







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

The test method is as follows:








Compared with the conventional wire-splitting test, the lean-to test method mainly reduces the area of the ground loop to avoid coupling interference in the ground loop during the test, which affects the test results. The main method is to remove the ground clamp as shown in the picture and directly use the copper ring 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 N line of the power module/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 that customers connect the isolated power module N line/input ground-output ground together. If the actual application requires the negative pole to be grounded and the requirements for EMC and ripple noise are not high or there are no requirements for isolation voltage, the ground can be shared. use;

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

Solution:

a. The customer system layout keeps the power module away from high-power equipment;

b. Add filter peripheral circuit to the front end of the power module;

c. The output leads of the power module should be shortened to reduce noise coupling. If necessary, a metal shell can be used to shield external interference;