Exploring the advantages of X7S MLCCs in high-precision electronics
X7R and X7S are commonly used MLCCs in the industrial and automotive markets. While X7R is the more popular choice, it is important to consider both the TCC and DC-bias when selecting an MLCC.
The most common MLCCs (multi-layer ceramic capacitors) in the industrial and automotive markets come with a TCC (temperature co-efficient of capacitance) known widely as X7R, where the ‘X’ stands for -55°C, the ‘7’ for +125°C, and the ‘R’ for ±15% tolerance over this temperature range.
| TCC | Temperature range | Tolerance |
|---|---|---|
| X7R | -55~125°C | ±15% |
| X7S | -55~125°C | ±22% |
With its narrower capacitance-change tolerance band of ±15%, many design engineers will simply select the X7R product over X7S. However, when choosing an MLCC, it is important to consider not only TCC but also DC-bias, namely the capacitance change at the applied DC voltage. Samsung Electro-Mechanics Co (SEMCO) focuses on MLCC solutions that provide strong performance in both TCC and DC-bias parameters. And while some X7R products offer better DC-bias than X7S, the opposite is also true, with certain X7S MLCCs providing a distinct advantage.
So how come more companies are not specifying X7S MLCCs? The answer, it appears, is nothing more than a lack of awareness.
In modern automotive and industrial electronics, higher integration and increasing functionality are common trends, both of which tend to demand more and more capacitance. Here, an MLCC offering a TCC with X7S characteristics possibly provides higher effective capacitance than an MLCC that has a TCC with X7R attributes (at the same nominal capacitance rating), because TCC is just one factor that affects capacitance. Another major influencer is DC bias.
DC bias is the change in capacitance when a DC voltage is applied across the terminals. MLCCs need to deal with DC voltage biasing and are therefore also susceptible to this phenomenon. The capacitance value will drop as DC voltage rises, which can negatively affect many types of circuit.
Moving Forward
Any electronic or electronic component engineers involved in the automotive, industrial, or other markets will gain rewards from taking a closer look at X7S for their high-capacitance MLCC requirements. The results set out by SEMCO demonstrate that X7S MLCCs can sometimes offer a notably better DC bias than like-for-like X7R counterparts. Even when taking TCC into consideration, X7S MLCCs often show themselves as the best overall performers.
There are many ongoing developments in MLCC design as suppliers look to meet the increasingly high demands of engineers in the modern electronics industry. Research into new materials and the effects of parameters such as TCC and DC bias mean that correct MLCC selection hinges very much on the specific application. X7R and X7S MLCCs can provide the optimal solution depending on the project’s objectives. For designers wishing to avoid handing the advantage to competitors, the overriding message is do not simply continue with historical MLCC choices for new projects, but consider all options. The gains available might provide a rewarding surprise.
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