Kemet Electronics, a global brand associated with capacitors, has been demonstrating its expertise, not only with capacitors and components but also in consulting for engineering applications and technologies with its customers. Now in its 32nd season, the Kemet Institute of Technology (KIT) recently completed a series of technology workshops throughout South Africa, including Johannesburg, Durban and Cape Town.
In collaboration with Arrow Altech Distribution, each KIT session was filled with eager-to-learn electrical engineers. The experience and design disciplines of the attendees was varied, but each learned something new about selecting capacitors for their designs. Each KIT seminar is solely focused on providing the often missed technical detail on passive components.
The following examples illustrate the type of information shared during these seminars.
Considerations for SMPS MLCC selection
Taking a switched-mode power supply (SMPS) as an example, some years ago it would have been common practice to use a small case tantalum capacitor for the output filtering function. Today most designers prefer to use multi-layer ceramic capacitors (MLCCs), without understanding all of the trade-offs involved.
When attending a KIT seminar, engineers learn the reason some ceramic capacitors sing; the danger of flex cracking; the different ways a ceramic capacitor can lose capacitance; and specific considerations for ESR in SMPS designs.
Ultimately any of the options could work, but there are several factors to consider for a robust and efficient design.
Singing ceramics
Ceramic capacitors designated as X5R or X7R are known as Class-II dielectrics. The dielectric material is made of barium titanate, the crystal structure of which gives the ceramic material piezoelectric, or microphonic, characteristics. When external stresses are applied to the dielectric material, the titanium molecule oscillates back and forth.
One example of an external stress is an electrical signal. Electrical energy can mechanically distort the dielectric; this distortion, or movement, creates a characteristic singing or buzzing noise that some designers experience when using ceramic capacitors in their design. Some engineers mention after a KIT session that they heard the noise in the past, but were not aware the MLCC was the source.
Alternatively, electrical noise can be generated from mechanical stress applied to the capacitor’s dielectric. This effect is particularly troublesome when using an X5R or X7R in the filter of a data acquisition system.
Flex cracking
With large case MLCCs (case sizes 0603 and larger) the most common failure is due to flex cracks. When a printed circuit board is flexed after a capacitor is mounted, the mechanical stress applied to the MLCC’s fixed terminations causes microscopic cracks to form inside of the capacitor.
Often, these cracks may not show an immediate failure, but over time moisture can ingress through these cracks, which often leads to a voltage breakdown between the dielectric layers. In the KIT seminars, it is discussed how to avoid (and detect) flex cracks.
Losing capacitance
When ordering capacitors, and other passive components, the rated value usually has some tolerance range. Most engineers know about this tolerance and choose their components for values to be inside of this range. So the most obvious way a ceramic capacitor can ‘lose’ capacitance is through its tolerance value.
However, other factors come into effect as well.
Temperature
All ceramic capacitors have a 3-letter temperature coefficient. X5R and X7R, for example, identify the temperature range and loss across a specified temperature range. The ‘X’ means -55°C, the ‘5’ means 85°C, and the ‘7’ means 125°C. The ‘R’ indicates a change of ±15% (hint: the change is rarely positive).
In other words, from -55°C to 125°C, an X7R can drop as much as 15% from the capacitance found at room temperature.
Ageing
Over time, the crystalline structure of the barium titanate changes shape. As the form changes, capacitance begins to drop. The starting point for this ‘ageing rate’ is when the capacitor experiences temperatures at or above its Curie point. For X5R and X7R capacitors, this point is between 130°C and 150°C. Every decade-hour later (1 hour, 10 hours, 100 hours, etc) some percentage of capacitance is lost.
DC bias effect
Everyone understands that MLCCs can be operated safely at full rated voltage. Still it is often observed that engineers employ a 50% derating in safety critical designs for maximum reliability. This derating offers another benefit that many engineers may not realise.
The capacitance of a Class II dielectric is susceptible to applied voltage. When DC voltage is applied to an X5R or X7R, the effective capacitance drops due to the ferroelectric properties of the barium titanate. This effect becomes stronger with thinner dielectric layers.
The DC bias effect of Class-II dielectrics is a critical design factor that is covered in detail during a KIT.
Low ESR considerations
Consider the compensation loop of the SMPS. Putting many MLCCs in parallel may lead to a potential issue. Most SMPS designs require a minimum ESR value to maintain the error amplifier stability. If the ESR of the output filter capacitor is too low, then the compensation loop can become unstable. Since every capacitor placed in parallel lowers the total effective ESR, the ESR of the capacitor network needs careful planning.
K-SIM: simulating ceramics
During the KIT seminars, Kemet’s experts discuss the above issues and demonstrate their effects using K-SIM, an online simulation tool that allows engineers to simulate capacitor performance with their application voltage, temperature and switching frequencies. K-SIM can even simulate the overall impedance of the network containing multiple capacitors. Evaluating real-world SMPS examples have led engineers to ask “is there an alternative to ceramics?”
Introducing polymer tantalum capacitors
As most engineers learn during a KIT seminar, there is a simple alternative: polymer tantalum capacitors. These capacitors offer very small case size for space-critical designs and high-capacitance, low-ESR options for high-current designs.
A polymer capacitor offers several benefits compared with high-capacitance MLCCs. First, they do not suffer from the DC bias effect; they have virtually no capacitance loss due to ageing; their capacitance is (relatively) stable with temperature; there are no piezoelectric effects; and they do not suffer from flex cracking issues. Additionally, they have high capacitance in a small package while offering ceramic-like ESRs down to single-digit milliohms.
Often designers will wish to compare the cost of an MLCC and a polymer capacitor. A single MLCC versus a polymer is not a fair trade, but taking into account the capacitance losses of Class II ceramics and the high-capacitance options in polymers, when one considers the final solution a polymer design can work out as a viable solution.
Conclusion
Capacitors are never the first component selected for a design. However, engineers can avoid potential issues during purchasing and prototype phases with a bit of insight early on. Kemet’s KIT seminars provide these insights, such as the design trade-offs with ceramic capacitors and their alternatives.
For more information contact Stuart Hanford, Arrow Altech Distribution, +27 (0)11 923 9600, [email protected], www.arrow.altech.co.za
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