KEMET Electronics Corporation is a leading global supplier of electronic components. We offer our customers the broadest selection of capacitor technologies in the industry, along with an expanding range of electromechanical devices, electromagnetic compatibility solutions and supercapacitors. Our vision is to be the preferred supplier of electronic component solutions for customers demanding the highest standards of quality, delivery and service.
As drivers begin to look to electric cars for the future, engineers have to consider various ways to replenish the battery systems. Considering it only takes a few minutes to replenish the current internal combustion engine, many EV systems are looking for better, faster ways to get back on the road.
Currently many EV systems use what is called an On-Board Charger to replenish the battery. This kind of system is preferred, as it takes in readily available AC power from the grid and convert it into DC power for the battery. Considerations about it's size, weight, energy efficiency, and cost all need to be taken into consideration when designing the On-Board Charger (OBC). There are four stages in a OBC; EMI Filter/Input Stage, Power Factor Correction Stage, DC-DC Conversion Stage, and Output Filtering Stage. Throughout this article we will discuss the various stages and what components are in each stage.
Shown above is the architecture for a basic OBC that is supplied by 3-Phase AC power. Similar versions of this architecture would be used in a single phase application. Using this 3 phase architecture allows for a rapid charging period and higher efficiencies.
The first stage of the OBC is the EMI filter. During this stage AC power is filtered to remove any unwanted noise from the typical AC sine wave that is expected. This noise is caused by a wide variety of different things including electric motors, cellular networks, transformers, and other variables traveling throughout the grid. In order to protect the other OBC stages, the EMI filter limits voltage and current spikes that can be harmful. Most commonly these filters are made up of X & Y Safety Capacitors, Common Mode Chokes, & AC Harmonic Filter Capacitors. Many household devices contain EMI filters internally similar to this design (typically single-phase variations) in order to protect the components and the user. Shown in the figure below is the waveform before and after the EMI Filter. This can reduce the amount of filtering needed in further stages.
Power Factor Correction
The second stage of the OBC is the Power Factor Correction Stage. Power Factor is a calculated ratio comparing the power input to the device to the power output from the device. This is an important value in terms of efficiency, however it also can cause issues to the grid, components, and user if not done properly.
Poor power factor can be caused by two things, displacement or distortion. In each case, the voltage and current values must be adjusted in order to maximize the operating power of the circuit. This can be done actively or passively.
Displacement occurs when the circuits voltage and current sine waves are out of phase of one another. Another way to look at this would be to look at the power equation P=IV. If the peak voltage value is not at the peak current value in time, then the circuit is not achieving the maximum power it could. The graphs below demonstrate the differences in peak power when voltage and current are out of phase.
Distortion on the other hand is described as changes in the waveforms original shape. This is often caused by non-linear circuits, rectifiers, etc.
In order to correct both of these losses, a Power Factor Correction stage must be implemented. Both active and passive methods can be used. In the case of the shown circuit, switches convert the incoming waveform into a DC waveform. In order to smooth the switching behaviors of the device, both capacitors and inductors are used in conjunction of one another to buffer the abrupt switching pattern. Shown below is an example waveform outputted by the switching performance of the PFC stage (waveform shown in red). To create a smoother signal, passive components are used to create the black waveform. Once completed this stage creates a ripple voltage that can be filtered out in a later, more refined stage.
DC to DC Converter
In the DC-DC Converter stage, input DC power comes in from the Power Factor Correction Stage and then is stepped up or down depending on the battery system that will be implemented. In the case of most EVs, this voltage is stepped into the range of 400-1000 VDC. The step up/down voltage occurs at the transformer in the center of the circuit. The surrounding active devices work in conjunction to perform more power factor corrections caused by the transformer.
In the fourth and final stage of the OBC, an output filter is placed on the end of the stages in order to finely filter out any remaining harmonics. This is down with a simple RLC circuit in a passive context. Then the newly adjusted power charges the battery of the device, in our case a battery in an electric vehicle. Specific components are used to handle the extreme power conditions that occur in an electric vehicle output filter. Special power resistors, high voltage capacitors, and high power inductors are designed to be able to not only withstand the electrical characteristics, but the physical characteristics required in vehicle operation. Vibration resistance, temperature and humidity requirements, and space constraints are just a few of the characteristics engineers use to select components on an OBC.
Uniquely, YAGEO Group offers the entire portfolio of passive components required in designing an on-board charger. Listed below are products designed specifically for each stage of the OBC. For more information about YAGEO Group's Power conversion components visit: https://www.kemet.com/en/us/applications/power-conversion.html
The KEMET Application Intelligence Center (KAIC) is our brand new, innovative application lab located at KEMET Headquarters in Fort Lauderdale, Florida. I sat down with William Mak, Senior Applications Engineer, to talk about KAIC – what it is, what it does, and what kind of innovations we can expect to find there.
What is KAIC?
Simply put, the KEMET Application Intelligence Center (KAIC) is a place to study applications, especially applications that pertain to KEMET parts. The other part of the lab is also to create educational content for engineers, or even beginners. The lab has three main audiences: people who are just getting into electronics, budding engineers, and also seasoned engineers who are incorporating detailed concepts. KAIC is the place to explore application design with KEMET.
What makes KAIC different from what else is out there?
While existing information focuses mainly on the part itself, KAIC focuses on how the part performs within a system. We are exploring how the parts coexist in ways that designing engineers would use them. If an engineer wants to research a certain type of design, like wireless power for example, that person could go to KAIC to explore options. And even if we didn’t have content on your specific area, we are going put out as much information as possible to help you with your wireless design.
How do you choose which designs to make at KAIC?
I try to choose designs that are popular and relevant in the industry right now. I get a lot of suggestions from the KEMET business groups, which is cool because they have their eyes on the latest parts we’ve developed. But for the most part, I like including designs I think are cool, and things that not many people know about. Something really cool right now are sensors and actuators. For example, the Magnetic/Sensor/Actuator group out of Tokyo made a sensor I have here in the lab. It’s my job to show engineers how to best use it. More than just part information, I provide guidelines, recommendations, and pro tips about these parts. I’m also working with existing data sheets to improve them to make it easier for design engineers.
This pyroelectric sensor could be used in motion sensing, like automatic lights, for example.
How does KAIC make KEMET Easy to Design In?
I work with different parts and KEMET data sheets. I try out the parts within different types of applications. I record my findings to help engineers know exactly how the part will perform within an application. Because of this, I am able to help engineers select the right part for their type of design. This saves the engineer time that may have been spent testing a particular part in a particular application. The KEMET pyroelectric sensor only needs a microcontroller with an ADC port and you can use this sensor right away. At KAIC, we can provide useful information or tips that could make things even easier to design.
What is KAIC’s larger purpose?
I just want electronic design to be something that a person at every level can understand. Whether you’re a seasoned engineer, just starting out, or even a student/hobbyist, we want KAIC to be your go-to place for application information. Our location at the KEMET Global Headquarters symbolizes how important design is to us. For one, it reinforces our global focus. Additionally, it keeps engineers, our customers, at the front of mind to the leadership of KEMET. They are what it’s all about!
Why do you get to have such a cool job?
Well, I have been an electronic designer in different fields for the past eight years. I’ve designed surveillance equipment, cell phones, and networking equipment. Most of my professional life is in application design. I’m an electrical engineer and I really enjoy it. In addition, I’m open minded about design. Learning new things is exciting to me, and I want to share that with the design community.
Why is now an important time to have KAIC?
We are living in a time where we can share information across the globe easily and instantly. At this moment, if you have a computer and the inclination, you can learn. My team and I want KEMET to be a big part of that. We want to provide high quality, verified content about electronics design that can engineers can rely on. And of course, we want engineers to choose KEMET. We hope that by providing quality designs, we’ll make their job easier.
How is KAIC staying in tune with the future of technology?
KEMET is data driven. We look closely at market study. We keep open communication with the business groups and incorporate the trends that they see. We not only identify trends as they happen, but we also have analysts that are in tune with the market and can suggest good projects for KAIC based upon their market predictions.
The KAIC Mission: Exploration. Optimization. Innovation.
The lab will focus on how KEMET products can synergize with current customer applications, future technologies, and educational multimedia. Projects will range from exploring new applications, optimizing designs, or innovating with new solutions.
Custom schematic and PCB design
1GHz mixed signal oscilloscope
3 GHz bandwidth spectrum analyzer
1.5 GHz ENA network analyzer
Power analyzer and DC loads
Full soldering rework station
Current Application Focus
Switched Tank Converters (STC)
How can people become involved with KAIC?
At this time, KAIC is still very new, so the best way for our early adopters to be involved is to send us ideas! They can also sign up for the Engineering Center Newsletter to learn the latest project releases from KAIC. If you want to see what KAIC looks like, check it out here. Be sure to check back often and follow KEMET on social media #KAIC.
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.
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.
Preface: Thanks to Francois Jeanneau, CEO and Sri Peruvemba, VP of Marketing from Novasentis for contributing the source information in this post.
There isn’t any technology that is safe from the march of time and progress. Very little of the early days of mobile phones and mobile devices remains today. Even the venerable and effective 3.5mm audio jack is going to meet an ignominious end. Unlike the audio jack (which I am very sad to see go), the vibrator in a cell phone is holding strong. To be honest, it is an effective technology, an off-center weight at the end of a small DC motor. Its longevity has been a testament to its simplicity. The eccentric rotating mass (ERM), as those of us who are prone to fits of pedantry call it, remains limited in its functionality and features. It is time that someone came up with something that advances the science of haptic feedback and also maintains the core functionality of the technology. Novasentis has done just that by creating an effective piezoelectric polymer haptic device.
Move Aside ERM, Make Way for EMP
No, not electromagnetic pulse (EMP), sadly. Novasentis has created a new class of haptic actuators, using Electro-Mechanical Polymer (EMP) technology, that is extremely light, paper-thin (150 microns), and provide a wide variety of effective haptic outputs from low frequency (a few Hz) to high frequency (KHz). Novasentis has partnered with KEMET to manufacture its actuators in high volume to support demand from leading consumer electronics OEMs.
Piezoelectric Polymers (Warning: Science!)
In an unpowered state, the molecular structure of the EMP film is randomly aligned. Upon the application of a bias voltage, the molecules align in one direction and expand, creating a piezoelectric effect. EMP actuators are created when this material is bonded to a rigid substrate. When powered, the uni-directional expansion of the material causes the substrate to vibrate, thus creating haptic feedback at low frequencies and audio feedback at higher frequencies. EMP products have a unique blend of strain and modulus, making them well suited for a multitude of applications.
What does KEMET have to do with all this, you ask? Well, it is simple, if you look at the structure of an SMT film capacitor it is just a stack of polymer film layers with electrodes going to alternate ends. Novasentis owns the engineering of the piezoelectric actuators and the relevant expertise and KEMET has the manufacturing know-how.
Advanced Haptic Response: More Than Just a Simple Buzz
The Novasentis EMP (man, that sounds cool) actuators act as a haptic skin for devices, capable of providing both localized (personalized) and meaningful haptic feedback. ERMs fall well short of meeting both of these features.
The wide bandwidth of the devices coupled with some science on the physiology of touch and sensation allows for very innovative haptic response rather than the simple on/off, buzz or no buzz notifications associated with ERMs. Consider the following:
Improved wearable notifications: EMP actuators enable hundreds of different haptic alerts to be felt directly on the skin rendering innovative wearable devices more intuitive and effective than ever before. Imagine the myriad of possibilities with devices that help you learn, determine who’s calling, or help you navigate your way through a crowd—all in real-time, potentially unrecognizable to anyone but the user, and without the need for displays.
Restore the sense of touch: Novasentis EMP technology is making history by providing a sense of touch in the user interface of many consumer electronic devices. In gaming controllers, the haptic sensation enhances the experience to a whole other level, and with EMP actuators being very fast, there is no lag between the visual and haptic sensations. In case of AR/VR applications, the visual and audio already exist, the heightened sense of touch delivered via EMP actuators through a variety of outputs allows the user to distinguish between different objects by touching them and feeling the difference.
By controlling the wave shape, duty cycle, amplitude, and frequency of vibration, sensations that are completely different from just a simple vibration can be created. More on that to come as there is a great deal of science associated with this too.
We Are Living in the Future
An off-center rotating mass. What’s cool about that? Haptic technology has been dominated by the ERM for almost two decades now. With a couple of unsuccessful deviations, the ERM has yet to be unseated. The combination of hard work of KEMET and Novasentis (and a little bit of luck) is bringing haptic technology into the 21st century.
It is 2018 and you’re probably reading this because you’re wondering when you’re gonna get your order of MLCCs. Either that or you’ve used some four-letter word to describe the lead times at your favorite distributor. Yes, the MLCCs industry is experiencing quite the capacity crunch. The last time it was like this we were back in the.COM days of ’99-2000. Manufacturers are putting in capacity, but that will take some time to come up. I know that’s cold comfort for the engineers who are dealing with a line down situations, and the supply chain managers who are having to beg, borrow and steal parts. It is times like this that make engineers explore new options and alternative techniques without having to do a massive redesign. Our polymer electrolytic capacitors are one type of alternative that, given certain conditions, can help. Going to KO-CAP isn’t always trivial, but if certain things align, they can be of great relief.
A Primer On KO-CAPs
Let’s all get to the same baseline. KO-CAP is KEMET’s tantalum-based polymer electrolytic capacitor. Like any other tantalum capacitor, it is a slug of sintered tantalum powder that has a tantalum pentoxide layer grown on it, with a layer of conductive polymer acting as the cathode. This conductive polymer gives the capacitor much lower ESR than “traditional” tantalum capacitors. That’s all I have to say about that. If you want to know more about KO-CAP, click here.
MLCC to KO-CAP
I am not delusional, I was (am) an engineer too and selling me on tantalum when I’m looking for MLCCs isn’t exactly the way to my heart. But, as engineers, solving problems is what we do and that means doing things we haven’t previously considered. As with anything else in engineering, making the decision to go to KO-CAP from MLCC is just a matter of managing tradeoffs. There is a slew of things that must be considered when making that decision. The critical design parameters that must be considered when attempting to make the change are; capacitance, voltage, ESR, frequency, leakage current, size, and qualifications. The flow chart below serves as a guide to making that decision when considering each of those critical design parameters. We are going to look at it from the standpoint that you’re trying your best to get as close as possible to the characteristics of your ceramic capacitor.
KO-CAPs tend to have more capacitance than a similarly sized ceramic capacitor. They don’t come in values smaller than 680nF. So, if your total capacitance is less than that, KO-CAP is not a suitable option. When it comes to capacitance, it is usually a very strong value proposition to replace a bank of MLCCs with one or two KO-CAPs.
In KO-CAP or any tantalum-based capacitor for that matter, the dielectric layer is very thin. A typical value is about 20nm. Having such a thin dielectric gives you a large amount of capacitance, but it also has the effect of limiting your voltage. A “high voltage” KO-CAP would be anything more than 35V. In general, if your operating voltage is more than 50V, KO-CAP is not a suitable option.
Ceramic capacitors, in a general sense, have lower ESR than an equivalent KO-CAP counterpart. That is not to say that there aren’t some very low ESR KO-CAPs. Some even go as low as 8mΩ, but a typical cutoff of 10mΩ is adequate. If you need ESR less than that, then KO-CAPs may not be a suitable option.
What you have to watch out for when considering frequency characteristics of KO-CAP is the self-resonant frequency. You generally want to operate capacitors below this point. It isn’t always the case, but if your switching frequency goes beyond 1MHz, then you may be approaching the limits.
KO-CAPs are polar devices, as such, they can not take reverse bias voltage. If the capacitor is placed in a location in which reverse bias is possible or needs to be tolerated, KO-CAPs are not suitable.
Alright, we have our guidelines, so what do we do now? What does this all mean? Let’s take a look at an example using a TI TSP54560B-Q1, that’s a buck for automotive applications.
I am an engineer and I have this circuit designed and everything is great. Until my sourcing person tells me that they can’t find some of the MLCCs I need. After berating them for a little while, I decide it is time to get creative and find a solution. Using the replacement guidelines I come to the conclusions below.
C1, C2, C3, and C10 are my input side capacitors. They are 2.2uF 50V 1206 X7Rs. There isn’t a drop-in replacement for the ceramics, but I can take the total 8.8uF capacitance and replace the 4 ceramics with one 10uF 35V KO-CAP. It is more than the original capacitance we need, but it is still within the required range for this regulator. ESR, Leakage, and frequency are not of concern on the input side, as long as the input isn’t direct battery voltage. Our simulation tool, K-SIM, show their side by side comparison.
Yes, I hear your concern, “what’s this going to cost me?” Well, let’s do the math. The total cost of the 4 MLCCs is $4.16. Replacing those 4 with one KO-CAP save about a bit over a dollar.
On the output side we have C6, C7, C9, and C11; 22uF 10V X7R 1206s. In this case, we are lucky, there is a drop-in replacement of that in KO-CAP. It is 6.3V but more than the output voltage range. In this case the KO-CAP ESR is higher than the ceramic equivalents, but still within the design spec. The switching frequency of the circuit is 300kHz and the SRF of my replacement is around 1MHz, so I am good there.Money-wise, we have a similar story on the output side. That MLCC is nearly $2! Yeah, I know it is a very expensive cap, but you can’t even get it anyway, so it is a moot point. The KO-CAP replacement is just over a dollar. Saving you a whopping 45%.
Other Capacitors and Considerations
C4, C5, and C8 are other caps that support the functionality of the device. There are suitable candidates for substitution because of both their physical size and capacitance value. Caps of this type are not experiencing quite the same capacity crunch. I didn’t mention leakage current very much because, in truth, that is only a concern in systems that have fixed non-rechargeable batteries.
MLCC Substitution Conclusions
Finding a drop-in replacement is doable but it isn’t the same level of the value proposition as replacing a bank of capacitors with a much lower quantity of KO-CAPs. Sometime a substitution won’t be feasible, but during times of capacity crunches such as this, finding solutions through other avenues could make you the hero your project deserves. Remember to contact your local KEMET FAE or head over to our Ask an FAE page if you’re considering options but need some help. Click here to explore other parts and solutions KEMET offers.
When I was working as a Field Application Engineer, about once a month I'd get a phone call from a panicked technician working on an assembly line. The call would always go the same. First the technician would explain that during board test, they were finding problems with their X7R and X5R capacitors being out of tolerance. In some cases the cap value would be measuring too high and sometimes it would measure too low. Depending on where the measurement fell, I could tell the issue. Without checking, I was pretty sure it wasn't that we shipped them the wrong capacitor value. At KEMET we measure each ceramic capacitor's value at least twice. Once to determine the tolerance and once right before it goes on the reel. This assures us we didn't make a gross error.
All capacitors are sold with a capacitance value within a tolerance band. For ceramic capacitors a ±10% tolerance is extremely common. When we, as a manufacturer, set out to build a part our recipe is designed for the nominal, however, that's not always the end result. So every one of the 32 billions capacitors we build per year, have to be measured to know which bin they fall into.
When a ceramic capacitor measures outside of this band, there are two very likely reasons as to why.
When ceramics measure high
If the capacitor measures higher than the expected tolerance band, it is likely related to the aging rate of the ceramic material. Right after exiting the reflow oven, ceramics like X7R and X5R will shift up in capacitance and slowly decay over time. This is called the "Aging Rate." The actual percentage of change varies by dielectric type and by manufacturer. Note that C0G (also known as NPO capacitors do not have this effect.) The first 1,000 hours out of the oven, you can expect an X7R and X5R to exhibit a noticeable shift.
When ceramics measure low
When a ceramic measures lower than tolerance, generally, the mistake is made with the measurement equipment. Understanding how the capacitance meter works is important because if it doesn't compensate for the ceramic's voltage coefficient, it won't measure the capacitance correctly.
Sometimes when looking on a datasheet for Ceramic Capacitors you might see references to the terms"PME" and "BME"? What does these mean and why should you pay attention? These acronyms are describing the metals used for the electrode plates.
BME stands for "Base Metal Electrode" while PME stands for "Precious Metal Electrode." This means PME typically uses metals like Silver-Palladium (AgPd). BME, on the other hand, will use less expensive metals like Nickel or Copper. Nickel is the most common while Copper may be used in specialized capacitors like KEMET's CBR Series which are RF Ceramic Capacitors. Copper has similar conductivity to Silver-Palladium without the extra cost.
Cost isn't the only reason to give consideration to PME or BME. Another reason refers to RoHS Compliance. In order to manufacture a PME ceramic with Silver-Palladium, the dielectric material will have Lead(Pb) added to it. BME ceramics, on the other hand, do not. All PME capacitors, that I know of, are inherently not RoHS compliant. While all BME, at least from KEMET, are RoHS compliant.
For example, KEMET's line of CommercialC0G, X7R, X5R, Y5V, and Z5U ceramic capacitors are all BME and RoHS compliant.
Even though we produce on the order of 1,000 capacitors per second, 100% of KEMET's capacitors are tested for parameters like Leakage Current (or Insulation Resistance), ESR, DF, and Capacitance before being shipped. Typically we'll use an Agilent E4980 (HP4284) or similar meter for our measurements. However, not everyone has access to that kind of equipment on a day to day basis. Don't worry though because John Fattaruso, professor and former analog engineer, provides a relatively simple circuit to measure either capacitance or inductance of a component in this EDN article.
(Image/Schematic hosted by edn.com)
The idea behind the circuit is to measure the time constant of a component when driven with a square wave. Dr Fattaruso's write-up fully describes how the circuit works. One of the trade-offs is that you aren't going to be "reading" the value of the component directly. So you'll need to do some calibration with a potentiometer before you make your first measurements.
Keep in mind when building a circuit like this, you want to use C0G Ceramic Capacitors, where possible. Their stability with voltage and temperature make them ideal for both reference values and for stable measurements.
(Note I have not built and tested this circuit myself, yet.)