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Measuring Ripple in Power Supplies
Ripple voltage is a crucial parameter in power supply performance, especially when you need stable voltage to power sensitive electronic devices. Understanding ripple and how to measure it can help keep your power supplies reliable and functional. Oscilloscopes and spectrum analyzers can help you measure ripple, while the proper probe measurement techniques can ensure an accurate read. This article also explores ways to reduce ripple voltage.
Understanding Power Supply Ripple
Power supply ripples are small fluctuations in a power supply's output voltage. They often arise when an alternating current (AC) is not entirely suppressed within the direct current (DC) signal. These fluctuations are measured as an AC voltage superimposed on the DC output. Ripple can significantly impact the performance of electronic components, leading to signal distortion, higher noise levels and lower efficiency.
The quality of DC power supplies varies. While some applications are sensitive to ripple, others are not. In addition, power supplies' capacitors can lose the ability to filter ripples effectively as they age, resulting in noisy power. Analog circuits or digital systems with high-speed operations and excessive ripple can malfunction or degrade performance.
For instance, operational amplifiers may experience offset errors due to ripple, while microcontrollers can reset or behave unpredictably if the voltage fluctuates beyond acceptable limits. As a result, it's essential to understand and know how to measure ripple to keep your electronic devices functioning reliably.
What Is Ripple Factor?
The ripple factor (RF) measures the amount of ripple in a load's current or voltage. It offers a standardized method for expressing ripple features and allows you to compare different power supplies easily. It is the ratio of the root mean square (RMS) value of the alternating current (AC) component to the average value of the direct current (DC) component. This ratio is usually expressed as a percentage. The lower the ripple factor, the better the power supply's performance and stability.
Ripple Voltage Formula
You can calculate the ripple factor using the formula:
- RF = Vac / Vdc
Where Vac is the AC component of the voltage, and Vdc is the average DC output in volts.
Vac is the effective or RMS value of the AC component of the total load voltage and is calculated using this formula:
- Vac = √(VL2 - Vdc2)
Where VL is the total load voltage.
Substituting the first equation into the second, the RF can be expressed as:
- [√(VL2 - Vdc2)] / Vdc
- = √[(VL2 / Vdc2) - (Vdc2 / Vdc2)]
- = √[(VL / Vdc)2 - 1]
The ratio VL / Vdc is also called the form factor (FF), meaning we can simplify to:
- RF = √(FF2 - 1)
The formula represents the amount of ripple voltage in a power supply relative to its average DC output voltage.
How to Measure Power Supply Ripple
Various tools and techniques can help you accurately measure power supply ripple and ensure the functionality and reliability of your electronic devices.
Tools and Equipment
Oscilloscopes and spectrum analyzers can help measure power supply ripples:
- Oscilloscope: Oscilloscopes can detect ripples from most power supplies. The tool's AC coupling will reject the supply's DC and expose any ripple features. This way, you can visualize the output voltage's waveform, seeing the DC level and AC fluctuations superimposed on it. To accurately capture the ripple features, you'll need to select the right settings, including the appropriate time base and voltage scale. Connect an oscilloscope probe across the power supply's output terminals. Ensure proper grounding, and use AC coupling mode to set the oscilloscope to display both AC and DC components. The straightforward technique lets you visualize the ripple voltage directly at its source.
- Spectrum analyzers: Spectrum analyzers are also valuable tools for measuring ripple. They analyze the frequency spectrum of signals, revealing the specific frequencies contributing to ripple. This tool is especially useful when diagnosing issues related to electromagnetic interference (EMI) or other high-frequency noise sources affecting power supply stability.
Probe Measurement Techniques
When using an oscilloscope, the loop created by the probe's return path can cause an inductance, amplifying internal noise and picking up external noise. Probes often come equipped with an alligator-style ground clip. While simple to connect, they can lead to large ground loops that obscure your measurements. Instead, use the following methods to achieve a small ground loop:
- Tip and barrel: This strategy removes the ground cover and probe clip, leaving the probe's tip and barrel exposed. Applying the probe's tip to the output voltage will angle the barrel so that it contacts the ground at a point closer to the tip. Ideally, the probe should be placed as close to the output capacitor as possible.
- Paperclip: The paperclip method incorporates a coil of wire with a short lead in the barrel rather than just a tip and barrel. The probe's tweezer-like tip allows for more flexible probe placement while maintaining a small loop area.
While these are not the only methods for getting a good signal, regardless of your chosen technique, it's important to make efforts to keep the ground loop as small as possible.
You might also use a differential probe to yield more accurate results when common-mode noise interferes with measurements. This tool lets you measure the voltage differences between two points without introducing ground loops or noise from other circuit elements, making it especially useful when measuring high-voltage or sensitive signals.
Figure 1 is an example of an output voltage ripple waveform, taken for a hysteretic converter that develops an auxiliary 12V DC rail from 120V DC featured in one of A TDI’s high power products. This waveform was measured with the scope input set to 20 MHz bandwidth. The reader can verify a 0.07 figure for ripple factor. The shape of the ripple waveform reflects the process of filtering a switched current to second order (using a L-C filter network). Note that there is a small remnant of switching (called a switching artefact), a voltage spike that is coupled with the ripple which occurs at the same phase in each ripple cycle.
Figure 2 – an example of a “paper clip” terminated probe that was used to extract the measurement shown in figure 1. Note that the probe has an effective bandwidth of 500 MHz which indicates that its impact on the overall measurement bandwidth set by the oscilloscope (20 MHz) is negligible. The loop area of the paperclip ground and the probe tip is approximately 10mm2 which is much smaller than that of a long ground lead probe.
Techniques for Ripple Reduction
You can try various filtering techniques while implementing design best practices to reduce power supply ripple and ensure the performance and reliability of your devices.
Filtering Techniques
Capacitors and inductors can help you reduce ripple. Capacitors store charge during periods of high demand and release it when needed, smoothing out fluctuations in voltage. Inductors resist changes in current flow, helping maintain a steady current through varying load conditions.
Resistor-capacitor (RC) filters are often used for low-pass filtering applications to attenuate high-frequency noise while letting DC signals through. Similarly, inductor-capacitor (IC) filters are effective for more substantial reductions in both AC ripple and noise by creating resonant circuits that further stabilize output voltages.
Design Considerations
Designing a proper printed circuit board (PCB) layout can help you minimize ripple effects. You'll need to keep power traces short and wide, reducing resistance and induction while ensuring adequate grounding. This can mitigate noise pickup from nearby components.
Additionally, to improve overall performance, choose high-quality components designed specifically for low-ripple applications. Capacitors with low equivalent series resistance (ESR) can enhance the ability to filter ripples effectively. Experts like Astrodyne TDI can advise you on designing a PCB layout while directing you to products that filter ripples to ensure the performance of your electronic devices.
Get Support From Astrodyne TDI
Ripple voltage is a significant aspect of power supply performance, impacting the stability and functionality of electronic devices. Knowing how to measure and reduce ripple can help you keep devices running reliably. For more detailed insights into the proper filtering techniques and design practices to reduce ripple, consider reaching out to the experts at Astrodyne TDI.
We can help you with comprehensive solutions, from product design to manufacturing. Choose from various EMI filters and power supplies to help you reach your design goals. We also provide customizable power supplies and solutions to match your specific application.
Let us worry about the power so you don't have to. Contact us today, and we can find a solution for you.