Ripple is a phenomenon caused by voltage fluctuations in a DC power supply. Since a DC power supply is generally formed by rectifying and regulating an AC power supply, it inevitably contains some AC components in the DC power supply. This AC component superimposed on the DC power supply is called ripple.
The definition of ripple is complex. Its form generally includes harmonics resembling a sine wave with frequencies higher than the power frequency (50Hz in China), and narrow pulse waves. The requirements for ripple vary depending on the application. For capacitors, regardless of the type of ripple, as long as it's not too large, it generally won't affect the capacitor's quality. However, for power supplies used in industrial control computers or audio equipment, narrow pulses lack sufficient energy to drive the speaker cone or telephone handset, causing noise. Therefore, the requirements for such narrow pulses can be relaxed. For sine wave-like ripple signals within the audio range, although their amplitude is not very high, their energy can cause buzzing noise in speakers or handsets. Therefore, certain requirements should be placed on this type of ripple. In some control applications, narrow pulses of a certain height can interfere with digital or logic control components, reducing the reliability of equipment operation. Therefore, the amplitude of such narrow pulses should be limited. Sine wave-like ripples, generally, have low amplitude and do not significantly interfere with control components. Ripple can be represented by effective value or peak value, and can be expressed as an absolute or relative quantity. For example, if a power supply is operating in a regulated state with an output of 100V/5A, and the measured effective value of the ripple is 10mV, this 10mV is the absolute quantity of the ripple. The relative quantity, i.e., the ripple factor = ripple voltage / output voltage = 10mV / 100V = 0.01%, which is equal to one ten-thousandth.
This article explains what ripple current is in power supply circuits, what causes it, and how to use smoothing capacitors to reduce or eliminate ripple current.
What is ripple in a power supply circuit?
In all AC-to-DC power supplies, the DC output is obtained by rectifying the AC input power and filtering it through a smoothing capacitor.
Although the process cleans the alternating current to almost pure direct current, a small amount of unwanted residual alternating current will always remain in the direct current content. This unwanted interference in the direct current is called ripple current or ripple voltage.
The unwanted alternating current remaining in the DC is mainly due to insufficient filtering or suppression of the rectified DC, or sometimes due to other complex phenomena, such as feedback signals from inductive or capacitive loads associated with the power supply, or possibly from high-frequency signal processing units.
The aforementioned residual ripple factor (γ) is technically defined as the ratio of the root mean square (RMS) amplitude of the actual ripple voltage to the absolute amount introduced into the DC output line of the power supply, usually expressed as a percentage.
Ripple factor
Another alternative way to represent the ripple factor is through the peak-to-peak voltage value. This method appears to be easier to express and measure using an oscilloscope and can be easily evaluated using available formulas.
Before understanding the formula for assessing DC ripple content, it is essential to understand the process of converting alternating current (AC) to direct current (DC) using rectifier diodes and capacitors.
Typically, a bridge rectifier consisting of four diodes is used to convert alternating current (AC) into full-wave direct current (DC).
However, even after rectification, the resulting DC current may still have significant ripple due to the large peak-to-peak voltage (valley) in the DC current. This is because the rectifier's function is limited to converting the negative cycle of AC to a positive cycle.
The persistent deep valley between each rectification half-cycle introduces the maximum ripple, which can only be solved by adding a filter capacitor at the output of the bridge rectifier.
The large peak-to-peak voltage between the valley and peak periods is smoothed or compensated using a filter capacitor or a smoothing capacitor at the output of the bridge rectifier.
Working principle of filter capacitor
This type of smoothing capacitor is also called an energy storage capacitor because it functions similarly to a liquid reservoir and stores energy during the peak period of the rectified voltage.
The filter capacitor stores peak voltage and current during the peak rectification cycle, while the load also receives peak power during these cycles. However, at the falling edge or trough of these cycles, the capacitor immediately kicks the stored energy back to the load, ensuring compensation for the load and allowing the load to receive fairly consistent DC current, unlike the actual ripple without the capacitor.
The cycle continues as the capacitor charges and discharges in the process, attempting to minimize the difference in the actual peak-to-peak ripple component of the connected load.
Smoothing efficiency depends on load current
The smoothing efficiency of a capacitor is highly dependent on the load current, as this increases the capacitor's smoothing capability proportionally. This is why larger smoothing capacitors are needed in a power supply for larger loads.
