Ripple voltage in capacitor

Ripple Current and its Effects on the Performance of Capacitors

source: Capacitor Faks blog

Capacitors are critical elements in most analog and digital electronic circuits. They are used for a broad array of applications including decoupling, filtering, bypassing, coupling, and so on. Different applications have different performance requirements and demand capacitors with specific characteristics. The power dissipated by a capacitor is a function of ripple current and equivalent series resistance. As such, the ripple current capability is one of the key parameters to consider when selecting a capacitor for a specific application. Other critical parameters include capacitance, voltage rating, equivalent series resistance, and equivalent series inductance.

In most electronic devices, the DC current signal applied to a circuit has an AC portion. This AC portion is referred to as the ripple current. Some capacitors have high ripple current ratings while others have low ripple current ratings. Although there are standards for calculating these ratings, some manufacturers use their own techniques. In capacitors, power loss and internal heating are dependent on ripple current.

The temperature rise depends on ripple current, thermal resistance, and equivalent series resistance. The overall thermal resistance is dependent on thermal resistance between the component and the ambient environment and internal thermal resistance. Thermal resistance varies from one capacitor to another depending on external surface area and internal construction. In most capacitors, the equivalent series resistance is dependent on operating temperature and frequency.

The ripple current degrades a capacitor by raising its internal temperature. The failure rate of capacitors is directly related to the temperature of operation, and operating capacitors at high temperatures shortens their life. As such, ripple current lowers the reliability of capacitors, thereby limiting the overall reliability of electronic devices. For some capacitors, manufacturers recommend voltage deration when they are operated at temperatures above 85C. Since ripple current increases the core temperature of a capacitor, it is a parameter of interest when considering the voltage deration requirements for a given capacitor.

Ripple current for ceramic capacitors
Internal heating within ceramic capacitors is a problem that affects the performance of many electronic circuits. In these capacitors, the maximum ripple current is determined by temperature characteristics of the component. The ripple current of ceramic capacitor varies depending on the temperature of operation. Ceramic capacitors operating at higher temperatures have less ripple current capability compared to those operating at lower temperatures. For this reason, this parameter is usually measured at room temperature. The method of measuring ripple current of these components varies from one manufacturer to another. As such, it is critical to understand the method used by a supplier when analyzing ripple current data for different capacitors.

Exceeding the ripple current rating of a ceramic capacitor can significantly affect its performance. Although heating a capacitor beyond the temperature specified by the manufacture may not cause immediate failure, overheating ceramic capacitors accelerates their failure rate. Compared to small footprint components, physically larger ceramic capacitors have higher tolerances to ripple current. The greater thermal mass and volume of larger capacitors enables them to absorb more energy, and it takes more time before their maximum rated temperature is reached.

The coefficients of thermal resistance for ceramic capacitors of a given chip size can be different. This is due to variation in the number of electrode plates. High capacitance components have more electrode plates compared to low capacitance components of same size. Electrode plates act as heat sinks, and capacitors with a higher number of these plates release heat from their ceramic blocks more easily when compared to components of same size but with fewer plates.

Heating in ceramic capacitors can cause thermal gradients. These thermal gradients can cause cracking. To prevent cracking, the maximum temperature rise in ceramic capacitors is usually limited to 50C. Unlike aluminum and tantalum capacitors, ceramic capacitors are not prone to negative ripple voltage pulse problem. This is because ceramic capacitors are non-polar components.

Ripple current for tantalum capacitors
Within a chip tantalum capacitor, heat is generated by DC leakage current and by the AC signal. This heat is lost to the surroundings through a combination of the following heat transfer methods: conduction, convection, and radiation. The rate at which heat is lost to the surroundings depends mainly on the temperature gradient between the component and the ambient temperature. At a specified frequency, the heat generated by ripple current is equal to the product of the square of the rms value of the current and the ESR of the capacitor, I2R. On the other hand, the leakage current generates heat that is equal to the product of the current and the applied voltage.

Most of today’s high performance circuits operate at high switching speeds, high currents, and low voltages that demand very low ESR capacitors. Capacitor manufacturers have been reducing the equivalent series resistance of tantalum capacitors to meet the evolving requirement of electronic circuits. For low-voltage circuits that operate at high currents such as some modern CPUs, the demand for very low ESRs is even higher. Low equivalent series resistance enables capacitors to withstand high ripple currents. In comparison, capacitors with high ESR ratings dissipate more heat, and are unsuitable for high ripple current environments. Since temperature rise in tantalum capacitors is a function of ESR, ripple current flowing through a capacitor, and thermal resistance, reducing ESR helps to improve the ripple current capability of these components.

Electronic circuits that operate at very high clock speeds have higher current requirements compared to those that operate at lower speeds. Circuits operating at such high speeds expose capacitors to large ripple currents, and very low ESR components are required to minimize power dissipation. Excess power dissipation can raise the internal temperature of tantalum capacitors to unacceptable limits. Exposing tantalum capacitors to high temperatures lowers their reliability and increases their susceptibility to failure.

Ripple current for aluminum electrolytic capacitors
Aluminum electrolytic capacitors are used for a broad spectrum of applications including energy storage, smoothing, and filtering applications. Some applications such as smoothing and filtering load electrolytic capacitors with AC ripple current. This ripple current causes power dissipation and heating, and subjecting electrolytic capacitors to high temperatures shortens their life. In addition, high temperatures affect capacitance, aluminum resistivity, electrolyte conductivity, and leakage current of these electrolytic capacitors.

In many electronic circuits, the capacitor is the component that limits the life of the system. As such, it is important to consider all the factors that can accelerate the failure rate of these components when analyzing the overall reliability of a system. For aluminum electrolytic capacitors, the factors that can accelerate failure include extreme temperatures, reverse bias, extreme frequencies, transients and high voltages.

Temperature rise in aluminum electrolytic capacitors is a function of equivalent series resistance, root mean square value of current flowing through a capacitor, and thermal characteristics of a component. The hot spot temperature, temperature at a given spot within a capacitor, is the key factor that determines the operational life of an aluminum electrolytic capacitor. The hot spot temperature is a function of the ambient temperature, thermal resistance, and power loss due to AC current. Inside an aluminum electrolytic capacitor, temperature rise and power loss have a linear relationship. Power loss in electrolytic capacitors is mainly due to voltage changes across the dielectric, leakage current losses, and ohmic resistance losses.

When selecting an electrolytic capacitor for power electronics applications, it is important to select components that are optimized to withstand high ripple currents. Such capacitors are specially designed to operate under severe conditions. The most common way of enhancing the ripple current capability of electrolytic capacitors is by minimizing the equivalent series resistance.

The equivalent series resistance of electrolytic capacitors decreases with an increase in the number of electrodes tabs. Increasing laser-welded tabs enhances ripple current capability, thus reducing internal heating and lengthening the life of a capacitor. In addition, using multiple laser-welded tabs helps to improve vibration and shock resistance of aluminum electrolytic capacitors.

Ripple current for film capacitors
In power electronic circuits, film capacitors are used for a wide range of applications including DC-link and DC output filtering applications. Polypropylene is widely used in the construction of film capacitors. This dielectric material offers low dissipation factor, and has good performance over a wide range of temperatures and frequencies. As compared to aluminum electrolytic capacitors, film capacitors have higher ripple current capacity and voltage capability. The ripple current capacity of these capacitors is about three times that of aluminum electrolytic capacitors. In addition, film capacitors have high tolerance to shock and vibrations.

Film capacitors, compared to conventional electrolytic capacitors, have lower equivalent series resistance. This characteristic enables these capacitors to tolerate higher ripple currents. Furthermore, the ESR of polymer film capacitors is relatively constant over a wide range of temperatures. As in other capacitors, ripple current causes power dissipation in film capacitors. This power dissipation raises the internal temperature of film capacitors, thus reducing their life. The operational life of metallized polymer film capacitors is greatly determined by the core temperature.

Conclusion
The ripple current capability of a capacitor is one of the key parameters to consider when selecting a capacitor for a given application. The AC ripple current causes power dissipation and heating in capacitors. In most capacitors, temperature rise is a function of ripple current and equivalent series resistance. Using capacitors with very low ESRs helps to minimize power dissipation and enhance the capacity of the circuit to withstand high ripple currents. The operational life of most types of capacitors is greatly determined by internal temperature, hence the need to minimize the heat generated by ripple current.

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Choosing a capacitor? Ripple current capability matters as much as Farads

by Derick Stephens, KEMET Corporation

Selecting capacitors for decoupling and filtering in power circuits may seem like a basic chore for electronics designers. Getting it right, however, can critically influence reliability and longevity, but is complicated by the fact that parameters tend to change with factors such as the temperature and operating frequency. Proper attention should be paid to capacitor selection, taking advantage of the technical resources now more widely available online to simplify and accelerate the process.

Capacitor ripple current capability

In power-conversion circuits, such as AC/DC power supplies, DC/DC converters, and even DC links, capacitive filters are needed to counter fluctuations that cause instability. Success is usually manifested as a lack of noise present in the DC power output and free of disturbances transferred into nearby circuitry.

The fluctuations in question are superimposed on the ideal, stable waveforms. Interference can arise from a variety of sources. One common source of noise is the rectification of AC; the resultant DC output from a rectifier usually has some amount of the source AC content superimposed on it. Switching regulators of all types create a certain amount of ripple when performing its primary function. Good designs usually try to mitigate this ripple as much as possible, but it can’t be completely eliminated. As a general principle, the capacitors are placed in the circuit to absorb and discharge the energy associated with these fluctuations on a continuous basis, and so minimize the peaks and troughs.

As a result of this action, the capacitor continuously passes a varying current. This current is called ripple. Although ripple current is the inevitable result of the capacitor performing its required task, it causes undesirable I2R heating as it passes through the Equivalent Series Resistance (ESR) that is associated with any capacitor. If the I2R effects exceed the capacitor’s ability to dissipate heat, its temperature can rise and hence adversely affect reliability. At the least, the component lifetime may be affected according to the Arrhenius Law, which states that lifetime is reduced by half for every 10°C increase in operating temperature. More extreme heating, exceeding the specified maximum temperature, can destroy the capacitor by causing drying or boiling of liquid electrolyte, cracking of ceramic capacitors, or ignition. A heatsink could be used to limit the temperature rise, if the application space and weight constraints allow. On the other hand, calculating the ripple current and understanding the properties of suitable capacitors can help to achieve the most space-efficient and cost-effective solution.

The capacitor datasheet indicates a ripple current rating that broadly describes the maximum ripple the device can withstand. This can be used as a guide, with the understanding that it is evaluated under controlled conditions. These are defined in standards such as EIA-809 or EIA/IS-535-BAAE, although there is some ambiguity in these documents. To help engineers understand the issues surrounding ripple current, KEMET has published an article, Ripple Current Confusion, in its online technical library (ec.kemet.com), which describes these standards and their applicability in detail. Discrepancies in the measurement of ripple current capability prevent easy direct comparisons between the ripple current capabilities of different manufacturers’ capacitors. Datasheet figures are useful, however, for comparing products from the same manufacturer.

Calculating ripple voltage and current

To choose the right capacitor for the input filter of a switching regulator, for example, the capacitance needed to achieve a desired voltage ripple can be calculated, if the operating conditions of the regulator are known. When the capacitance is calculated, a candidate component can be identified, and the ripple current determined from the known ESR. This ripple current must be within the capacitor’s ripple current handling capability, if the device is to be suitable for use. This is where selection can become difficult, because both ESR and capacitance are known to vary with temperature, operating frequency, and the applied DC bias.

The capacitance can be calculated using the equation (from TI Application Report SLTA055)

Where CMIN = minimum capacitance required

IOUT = output current

dc = duty cycle (usually calculated as dc=Vout/(Vin*Eff))

fSW = switching frequency

VP(max) = peak-to-peak ripple voltage

Assuming, for example, a regulator with 12v input; 5v output; 2amp output; 85% efficiency; 400kHz switching, and an allowable input ripple voltage of 65mV:

Note that the chosen device must provide this value of capacitance at the regulator operating frequency of 400kHz.

The rms value of the peak-to-peak ripple voltage can be calculated from the equation:

The ripple current in the capacitor can then be calculated by applying Ohm’s law, if the capacitor’s ESR is known.

A note of caution

At this point, the variability of capacitor properties, according to operating conditions, must be considered. Most engineers understand the temperature stability issues of class II/III dielectrics. Fewer understand the magnitude of capacitance loss due to the operating frequency and the applied voltage.

Recall that 19.22µF, as calculated earlier, is the capacitance required at the regulator’s operating frequency of 400kHz. The ESR must also be known at this frequency, to calculate the ripple current.

If a capacitor with nominal capacitance of 22µF and voltage rating of 16V is chosen, as the nearest standard value above 19.22µF, the actual capacitance of this device is 5.951µF at 400kHz, as shown in figure 1, and the ESR is 3.328mΩ. The resulting ripple voltage and current can be calculated as 210mVp-p/74.23mVrms, and 22.3A respectively. These are significantly greater than the target ripple voltage and maximum allowable ripple current for the capacitor.

Figure 1. capacitance loss with frequency.

The value of simulation

Every manufacturer of Class ii components will advocate simulating the component behavior allowing for application voltage, temperature and frequency. KEMET’s K-SIM online electrical parameter simulator lets engineers assess capacitor performance under a variety of operating conditions. it is available in the KEMET Engineering Center, alongside the ripple-voltage calculator mentioned earlier and other tools and support information including technical notes and application guides.

Using K-SIM, engineers can quickly analyze one or multiple capacitors that may be suitable for the application they are working on. Among the various features, K-SIM can display impedance and ESR, or capacitance and voltage versus operating frequency, and also predict temperature rise depending on ripple current and frequency. An on-screen cursor helps ensure accurate measurement. K-Sim also allows capacitor S parameters to be evaluated, and SPICE models and STEP files obtained for components of interest.

With the aid of this tool, a 47µF X5R capacitor was identified, with the same case size and voltage rating as the 22µF/16V device selected earlier. The capacitance value is 19.9µF at 400kHz under the applied DC bias, and thus restricts the peak-to-peak ripple voltage to 63mV. Hence Vrms = 22.27mV. This capacitor’s ESR is 3.246mΩ at 400kHz, suggesting the ripple current is 6.86A, which is below the maximum for the device.

Conclusion

The issue of ripple current can be challenging to analyze and to predict accurately under expected circuit-operating conditions. When left unchecked the heating caused by ripple currents can adversely affect the life of the capacitor. Nevertheless, proper assessment of the ripple voltage and current is vital, to ensure a power circuit like a switching regulator will deliver the required performance over its intended lifetime. Online tools and information provide valuable help to calculate the capacitance needed and accelerate component selection.

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Comments

ripple current of 1 component with 47uf capacitance is 110mA, however for the other component with same capacitance value has a 115mA, also they have 25V of rated voltage and with 20% tolerance. Does it mean that these 2 components is the equivalent or similar?. they only differ in ripple current.

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