The following figure shows the actual output power ripple signal and voltage. In addition to the signals we want, there are also some ripples that engineers don't want.
The evaluation of ripple is very important, but because many engineers do not have a thorough understanding of ripple, there are usually some inaccurate evaluations of problems. Or because the test method is wrong, the measurement result is not the ripple value; Or the connection mode is wrong, and some non ripple factors are introduced, resulting in large ripple, etc. There is a big gap between the ripple test results obtained by different test methods. Different test devices are connected, and the measurement results are also quite different. This paper mainly introduces the influence of different connection modes on ripple measurement results.
General engineers usually use the traditional test connection method as shown in the figure below:
It can be seen that in the traditional test method, the test loop is very large, and the ripple voltage of switching power supply can not be obtained accurately. The reason is that if the line is too long, a large ring antenna will be formed to attract the nearby noise, resulting in large ripple and inaccurate test results.
Therefore, it is necessary to improve such test methods and reduce the antenna loop, as shown in the figure below:
From the figure, we can see the difference between the new measurement method and the traditional measurement method. It can be clearly seen that the difference is obvious. One ground wire is particularly long and the other ground wire is particularly short; One test loop is large and the other is small:
The test loop of this method is small. Of course, the noise absorbed from the surrounding will be much smaller than the former, but how much impact will this small ground wire have in practical application? The following is an illustration of the test results, as shown in the following figure:
From the figure, we can see the difference between the results obtained by using the two measurement methods. When the test bandwidth is relatively small, such as using 20MHz bandwidth for testing, there may be no big difference, but when we gradually widen the test bandwidth, at 500MHz, we can see that the ripple of the results measured by the two methods is very different. It shows that the large ring antenna effect has a great influence on the power supply ripple measurement.
Seeing this, all engineers should understand that during measurement, whether it is power signal, low-frequency signal or high-frequency signal, they should pay attention to their own test method and the connection of the measuring device, otherwise they will get the wrong result accidentally and can't find the problem. Finding the right way is very important.
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