In the process of power supply design and debugging, ripple testing is a critical step in assessing power quality. Many engineers set their oscilloscopes to a bandwidth limit of 20MHz and AC coupling mode during ripple testing. This is not a random choice, but rather based on profound technical considerations. This article will delve into the principles and necessity behind these two settings.
What is Power Ripple?
Before discussing testing methods, let’s clarify the concept of power ripple. Power ripple refers to the AC components superimposed on the DC output voltage, typically consisting of two main components: the fundamental frequency and harmonics caused by switching frequency ripple, as well as high-frequency switching noise. For switch-mode power supplies, the typical frequency range of ripple varies from tens of kHz to several MHz.
Why Use a 20MHz Bandwidth Limit?
Filtering Out High-Frequency Noise Interference The primary reason for choosing a 20MHz bandwidth limit is to filter out irrelevant high-frequency noise. In practical testing environments, oscilloscope probes pick up various high-frequency interference signals, including EMI radiation from nearby circuits, noise generated by the probe’s own antenna effect, and RF interference in the testing environment. These high-frequency noises typically range from tens of MHz to GHz, far exceeding the actual frequency components of power ripple. If the bandwidth is not limited, these high-frequency noises will superimpose on the true ripple signal, leading to inflated measurement results that do not accurately reflect the actual performance of the power supply. By limiting the bandwidth to 20MHz, we can effectively attenuate these irrelevant high-frequency components, obtaining a clearer and more accurate ripple waveform.Matching the Spectral Characteristics of the Measured Signal The switching frequency of most switch-mode power supplies is between tens of kHz and several MHz. Even considering higher harmonics, meaningful frequency components are usually within 20MHz. According to signal processing theory, the measurement bandwidth should be appropriately higher than the highest frequency component of the measured signal, but excessively high bandwidth will only introduce more noise without providing additional useful information. A bandwidth of 20MHz is a reasonable balance for the vast majority of power ripple tests, as it can fully capture the main frequency components of the ripple while effectively suppressing high-frequency interference.Industry Standard Requirements Many power testing standards and specifications explicitly require the use of a 20MHz bandwidth limit for ripple testing. For example, Intel’s power design guidelines and various industrial power standards recommend this setting. This standardization ensures the comparability and consistency of results under different testing conditions. Adhering to these standards not only helps achieve accurate measurement results but also facilitates comparison and validation against industry norms.
Why Use AC Coupling?
Eliminating the Impact of DC Offset The power output typically contains a large DC component (such as 3.3V, 5V, 12V, etc.) and a relatively small AC ripple component (usually in the mV range). If DC coupling is used, the entire signal will be sent to the oscilloscope. This means that both several volts of DC voltage and several millivolts of ripple must be displayed simultaneously, which brings several issues. The first is the loss of vertical resolution. To display the full DC level on the screen, a larger vertical scale (e.g., 1V/div) must be used, making the millivolt-level ripple nearly indistinguishable. Secondly, the dynamic range limitation of the oscilloscope’s front-end amplifier comes into play. When a large DC signal is input, the amplifier may approach saturation, affecting its ability to linearly amplify small signals.Improving Measurement Accuracy and Sensitivity Using AC coupling can block the DC component, allowing only the AC ripple signal to pass through. This enables the use of a smaller vertical scale (e.g., 10mV/div or smaller), fully utilizing the oscilloscope’s vertical resolution to observe ripple details. For example, for a 12V output with a 20mV ripple, using DC coupling might require a setting of 2V/div, where the ripple occupies only a small portion of the screen; whereas with AC coupling, it can be set to 5mV/div, clearly displaying the ripple signal on the screen.Avoiding the Impact of DC Drift During long-term testing, the DC output of the power supply may experience slight drift, which, although slow, can affect ripple measurements. AC coupling, with its high-pass filtering characteristics, can automatically compensate for this slow DC variation, keeping the measurement focused on the AC ripple component.
Practical Testing Considerations
Although a 20MHz bandwidth and AC coupling are standard settings, several points need to be considered in practical applications: **The choice of probe and connection method is crucial**. A short ground connection should be used, preferably with a spring ground clip, to reduce noise and ringing introduced by the probe. The probe’s bandwidth should also exceed 20MHz to ensure that no additional attenuation or phase distortion is introduced when limiting the bandwidth. **The low-frequency cutoff point of AC coupling needs to be considered**. Most oscilloscopes have a low-frequency cutoff point for AC coupling around 10Hz, which is not an issue for power supplies with switching frequencies above kHz. However, if lower frequency phenomena, such as load transient response, need to be observed, DC coupling with appropriate bias adjustment may be required. **Certain special cases may require adjustment of settings**. For example, for high-frequency switch-mode power supplies operating at MHz levels, it may be necessary to slightly increase the bandwidth limit; for situations requiring simultaneous observation of DC level changes, DC coupling may be needed. Engineers should flexibly adjust based on specific testing needs and the characteristics of the power supply being tested.
Conclusion
Setting a 20MHz bandwidth limit and AC coupling in power ripple testing is based on the best practices derived from signal characteristics, measurement principles, and practical experience. A 20MHz bandwidth can effectively filter out high-frequency noise while preserving the integrity of the ripple signal, and AC coupling allows us to observe mV-level ripple details with the highest precision. Understanding the principles behind these settings not only aids in obtaining accurate test results but also helps engineers make appropriate adjustments when facing different testing scenarios. Correct testing methods are the prerequisite for obtaining reliable data, and reliable data is the foundation for optimizing power supply design. Mastering these testing techniques will help you better evaluate and improve power supply designs, ensuring the quality and reliability of the final product.