Thread Content
117 Power supply ripple: Power supply ripple is the most straightforward indicator of a power supply’s performance. DC regulated power supplies are generally created by converting alternating current into direct current through processes such as rectification and voltage regulation; as a result, there is inevitably some alternating current component present in the DC output. This alternating current component that is superimposed on the DC voltage is what is referred to as ripple. Setting up the oscilloscope: 1. First, select the appropriate setting for the probe. If the voltage is high or a high bandwidth is required, the X10 setting can be used; under normal circumstances, it is recommended to use the X1 setting to avoid unnecessary noise attenuation that could affect the measurement of ripple. At the same time, remember to set the attenuation ratio of the oscilloscope channel to X1 as well. https://pic4.zhimg.com/80/v2-d22fb734fab2710ffda8d5fcf394f1f3_720w.jpg 2. Ripple is an AC component; therefore, the \"channel coupling\" mode should be set to AC coupling in order to prevent the input of DC signals. Additionally, the adjustable range of the vertical scale on an oscilloscope is limited; therefore, when the DC signal is too strong, it may be impossible to see the ripple. By selecting AC coupling, only the AC ripple signal is displayed, making it easier to observe the waveform. https://pic2.zhimg.com/80/v2-a151b7bdcbcd00e2bb44d45961e08bb5_720w.jpg3. Use the bandwidth limitation feature; generally, the ripple frequency of output from switching power supplies ranges from 0 to 20 MHz. Noise caused by high-frequency synchronous switching noise and signal reflection, etc., lies in the 0–1 GHz range. Therefore, it is recommended to use a 20MHz bandwidth limit to filter out unnecessary high-frequency noise. https://pic4.zhimg.com/80/v2-21db530357eae61a227e3801f130073b_720w.jpg4. To avoid interference with the signal caused by electromagnetic radiation and similar factors, the grounding wire of the oscilloscope probe should be as short as possible; usually, the probe’s built-in grounding spring is used for grounding. https://pic2.zhimg.com/80/v2-189e40927469bf503c3c1cd240d469f1_720w.jpg5. The FFT function can be used to perform a frequency-domain analysis of the ripple waveform, allowing for an accurate determination of the noise at each frequency point as well as the magnitude of the ripples caused by switching. https://pic4.zhimg.com/80/v2-d42cc439bf9aacfd0f141f44b56f0267_720w.jpg In summary: use setting X1 for the probe, AC coupling for channel coupling, a 20M low-pass bandwidth limit, and use a ground spring pin to keep the ground wire as short as possible. After adjusting the waveform in both the vertical and horizontal directions to appropriate positions based on the measurement results, enable the measurement options of the oscilloscope and select peak-to-peak to directly obtain the peak-to-peak value of the ripple. So what is a suitable peak-to-peak value for the ripple? Standard digital I/O: The tolerance for power supply ripple noise is relatively high; values around 100 mV are not a problem. For relay outputs and optocoupler outputs, ripple noise up to 100 mV can be tolerated. Power supplies for industrial communication ports, such as those used in RS-232, RS-485, CAN, and other bus-based systems, handle digital signals; RS-485 and CAN transmit in differential form, so they are less sensitive to power supply ripple noise. In these cases, it’s sufficient to keep the ripple noise at around 75 mV. Low-speed, low-precision data acquisition systems: Since these systems have modest requirements regarding precision and speed, a ripple noise level of 50 mV is usually sufficient to meet their needs. Power supplies for low-voltage CPUs, such as those supplying CPUs with voltages of 1.2V or 0.8V, are quite sensitive to power supply ripple noise; high levels of ripple noise can interfere with the proper functioning of the CPU and even damage it. It’s generally necessary to keep the ripple noise below 30 mV. High-speed, high-precision data acquisition systems: These systems have high demands both in terms of precision and speed, and they are very sensitive to power supply ripple noise. In addition to requiring low ripple noise, it’s also necessary to use operational amplifiers with high precision and high common-mode rejection ratios. In such cases, the ripple noise should generally be kept below 10 mV.