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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Ripple current at output capacitors
I was reading about the selection of output capacitors at the output of buck converters and found this Link
The accepted answer states that before I select an output capacitor, we must fix the desired ripple current and then choose the inductor based on the ripple current.
- How to choose and fix the ripple current? Should we just assume the ripple current to be 20 to 40% of Iout max current as mentioned in the TI App Note and proceed with those calculation steps.
I am not sure I get this. I always thought that the selection of output capacitor was based on ripple voltage.
For example, If I have an IC Whose Vcc ranges from 0.8V to 1.2V and typical Vcc is 1V. My Buck will produce 1V with a ripple that must not exceed the Min and Max Vcc of the IC. So, my ripple should be less than 200mV. So, here, obviously our ripple voltage is very important. If the ripple voltage is very high, then our 1V IC may get damaged. But we also calculate ripple current.
- How is the ripple current more important that the ripple voltage. What happens if we have high ripple current and low ripple current?
So, I think we would select a capacitor based on our calculated ripple voltage (To be less than 200mV).
- So, is our estimate of Inductor ripple current same as the output capacitor ripple current ?
I tried other app notes also. But I couldn’t find a formula that mentions the calculation of output capacitor ripple current. And can someone tell me how output ripple voltage and output capacitor ripple current are related and which is calculated first and based on what parameter.
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What is ripple current in capacitors
first of all I have to say that I am quite ignorant in electronics.
Then please excuse me if this question has been already asked a number of times.
I searched the forums but I did not find a straight answer.
I see in the capacitors’ specs the Ripple Current figure.
What is that ?
Moreover, can this figure be taken as a measure of the quality/performance of a cap ?
I mean, I see remarkable differences from brand to brand for same values of uFs and working Voltage.
My interest is driven by a very simple DIY project of linear PSU to use with a power amp (transformer+diodes bridge+filter caps).
And a last question: if I keep the same transformer and replace the two main filter caps with other bigger and better ones what kind of improvements could I expect in the final sound?
Better bass maybe (a very important point to me) ?
Will these improvements be substantial ?
Thank you very much indeed.
Kind regards,
Hi Beppe,
I see you are still trying to learn.
Ripple current is the current flowing into and out of the capacitor. It can be from the charging pulses coming from the rectifier or it could be the current demand pulses from the output stage.
As the current flows in and out it passes through the ESR and causes heating.
Basically the lower the ESR the higher the ripple rating. Similarly the bigger the surface area of the cap the more heat it can dissipate. Both these features cost money to manufacture, so one tends to find that good ripple rating costs more and due the the way it is achieved it is a rough indicator of cap quality. Notice I said rough.
Careless selection of smoothing caps can lead to overheating and very short life due to high ripple during operating conditions.
A cheap way to improve the ripple rating is multiple parallel lower value caps. It’s about lower ESR and more surface area.
| Originally posted by AndrewT Hi Beppe, 1) I see you are still trying to learn. 2) Ripple current is the current flowing into and out of the capacitor. It can be from the charging pulses coming from the rectifier or it could be the current demand pulses from the output stage. As the current flows in and out it passes through the ESR and causes heating. Basically the lower the ESR the higher the ripple rating. Similarly the bigger the surface area of the cap the more heat it can dissipate. Both these features cost money to manufacture, so one tends to find that good ripple rating costs more and due the the way it is achieved it is a rough indicator of cap quality. Notice I said rough. Careless selection of smoothing caps can lead to overheating and very short life due to high ripple during operating conditions. A cheap way to improve the ripple rating is multiple parallel lower value caps. It’s about lower ESR and more surface area. |
Hello dear Mr. Andrew,
thank you very much for your kind and valuable advice, as always.
1) It is quite clear that my comprehension ability is limited.
I try always to «trivialize» topics in the hope to understand that a little better.
2) Thank you for your kind explanation.
To be more specific I have some huge Sikorel caps (15.000uF/100V) at hand that I would like to use to replace the existing ones in an amp (it has two 7.200uF/50V old Mallorys),
The dimensions of the Sikorels is going to make the replacement not the easiest mod to perform.
I wonder if this mod is advisable.
Would I get any sonic improvements all other things kept the same ? in which way ?
Thank you so much again.
Kind regards,
| Originally posted by AndrewT Hi Beppe, I see you are still trying to learn. Ripple current is the current flowing into and out of the capacitor. It can be from the charging pulses coming from the rectifier or it could be the current demand pulses from the output stage. . . |
I thought about your words a lot before putting another maybe trivial question.
If filter caps with a low current ripple are used in a power amp power supply could they act as a bottle-neck for the current that should reach the output devices of the amp?
In this case a replacement of all the filter caps with others with better current ripple figure should be always extremely beneficial for the overall performance of the amp.
Much more, for instance, than a transformer replacement/upgrade keeping the original caps.
Am I misunderstanding badly the all thing?
Thank you so much as always.
Kind regards,
Hi Beppe,
nothing is ever too trivial, but you will know from some of my postings that I do not suffer lazy fools gladly.
| filter caps with a low current ripple are used in a power amp power supply could they act as a bottle-neck for the current that should reach the output devices of the amp |
| replacement of all the filter caps with others with better current ripple figure should be always extremely beneficial for the overall performance of the amp |
| Much more, for instance, than a transformer replacement/upgrade keeping the original caps |
| Originally posted by AndrewT Hi Beppe, nothing is ever too trivial, but you will know from some of my postings that I do not suffer lazy fools gladly. |
yes. But think about the two sides of ripple in the main smoothing bank. Which is worse? the ripple from the charging pulses or the ripple from the output load current demands? Improving the ripple rating may (or will?) have a direct effect on the capacitor reliability. that is likely to be overstating the improvement potential. Take out always extremely and you could be nearer the truth. again overstating the case.
I believe it depends on how the designer has skimped on the specification of both the transformer AND the smoothing capacitors to save both cost and space.
Improvement may come from either upgrade but equally the result from the upgrade could be worse than the original if the designer has voiced the COMBINATION of PSU and amplifier to sound just right (to his ears and those of his intended market).
Thank you sincerely again for the kind and thoruogh advice.
As always it is a much more complex issue than how it looks to an uneducated eye.
I should just stop to play with low level equipment hoping to upgrading them to an acceptable level and take a more serious approach to the problem.
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