DC bias - how to minimize capacitance loss
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DC bias - how to minimize capacitance loss

Ceramic capacitors are used in a broad range of applications in today’s electronic circuits. The popularity of these components is mainly due to their small sizes, low impedance and impressive ability to store charge. The performance characteristics of passive components vary depending on a wide range of factors. One of the factors that greatly influence the properties of a multilayer ceramic capacitor (MLCC) is the characteristics of the material used. The DC bias phenomenon affects ceramic capacitors based on barium titanate (BaTiO3) and results in a reduction in the capacitance of a component when a DC voltage is applied. Figure 1 shows how the capacitance of two ceramic components is affected when the rated DC voltage is applied.


DC bias characteristics of Class II ceramic capacitors


The capacitance of some ceramic capacitors decreases when DC voltage is applied across their terminals. Since electrolytic capacitors and tantalum capacitors do not exhibit DC bias, it is common for circuit designers who are new to MLCCs not to consider the phenomenon. Failing to consider the effect can result in poor performance of a circuit. To ensure that the performance of a circuit is not impacted by this phenomenon, it is important to factor the effect at the circuit design stage. There are many techniques of overcoming the effects of DC bias, and we’ll explore some of them in this article.

Barium titanate is used as the dielectric material in most of today’s Class II ceramic capacitors. As compared to other dielectric materials, BaTiO3 offers a very high dielectric constant, typically over 7000. The crystal structure of this material changes depending on temperature, and the DC bias phenomenon is attributed to these structural changes.

At temperatures above 130oC (Curie Point), the shape of a crystal of barium titanate changes into a cube. The barium ions form the vertices of the cube while the oxygen ions are at the center of the cube’s faces. The titanium ion is at the center of the cube.

As the temperature drops below 130oC, the structure of a crystal of barium titanate has a tetragonal shape, also commonly known as perovskite shape. This tetragonal crystal structure is formed when one axis shrinks while another stretches. This transition results in one axis being more positive than the other.

A crystal of BaTiO3 maintains the cubic shape for all temperatures below 130oC and in the range of ambient temperature. At temperatures between ~10 and -70oC, the structure of the crystal goes through a transition from tetragonal to orthorhombic. Below ~-75oC, the shape changes to rhombohedral. The variation in dielectric constant with changes in temperature is as shown in Figure 2.


The asymmetry of the barium titanate’s crystal structure causes polarization of the dielectric material. The dipoles are aligned randomly throughout the crystal structure of the material. Since this polarization is independent of external forces, it is commonly known as spontaneous polarization. This spontaneous polarization can be reversed by an electric field, and this property is commonly known as ferroelectricity. When reverse spontaneous polarization occurs under no voltage stress, a higher capacitance is achieved.

When a DC voltage is applied across the terminals of a capacitor, the reverse spontaneous polarization is inhibited. Application of an external electric field causes the dipoles to be aligned parallel to the direction of the electric field. This means that the polarization is fixed in a certain direction. As the voltage is increased from zero to the rated voltage of a capacitor, the number of dipoles that are aligned in the same direction as the electric field increases. This reduces the capacitance to a value lower than the nominal capacitance.

The percentage reduction in capacitance of a component when a DC voltage is applied varies depending on the type of ceramic capacitors. For Y5V capacitors, the capacitance can drop by up to 80% when the rated voltage is applied. In the case of X7R capacitors, the drop in capacitance is usually between 15 and 20%. It is important to note that this effect is not voltage related. Figure 3 shows how application of a DC voltage affects the capacitance of three ceramic capacitors.


Electrolytic capacitors and tantalum capacitors do not exhibit DC bias characteristic. This is because these capacitors are based on materials with very low dielectric constants. In the case of electrolytic capacitors, a non-ferroelectric material is used. Its dielectric constant is quite low, and this means that electrolytic capacitors are immune to the DC bias effect.


Overcoming DC Bias in ceramic capacitors


Different approaches are employed to overcome DC bias in ceramic capacitors. One of the methods that capacitor manufacturers are using to lower the DC bias effect is by advancing the barium titanate ceramic technology with the aim of making it easier to achieve polarization reversal. It is expected that advancements in ceramic technology will help to lower the DC bias effect and boost the overall performance of ceramic capacitors.

Most circuit engineers use online software tools to account for the effect of DC bias when designing circuits. With such tools, it is possible to study the DC bias curve and understand the behavior of a component at the designing stage. By comparing the performance of different MLCCs, it is easy for a circuit designer to select a component that offers the desired capacitance when DC voltage is applied.

There are various ways of minimizing capacitance loss caused by DC bias. The most common method is using a capacitor with a higher capacitance rating. Another common approach is using two or more capacitors of lower capacitance values. These capacitors are placed in parallel with each other.

The drop in capacitance due to DC bias decreases with an increase in the case size of a component. This means that capacitance loss can be minimized by selecting components with larger case sizes. It is therefore common for circuit designers to select components with larger case sizes. This approach is commonly used in cases where the size of a component is not a critical design factor.


Conclusion


The characteristics of a capacitor are greatly influenced by the properties of the material that it is based on. Class II ceramic dielectrics such as X7R, X5R, and Y5V are based on barium titanate, and the structural and electrical properties of this material are highly dependent on temperature. Applying a DC voltage across the terminals of a Class II capacitor inhibits reverse spontaneous polarization. This results in a reduction in capacitance of a component. This reduction varies from one type of ceramic capacitors to another, and it is necessary for design engineers to factor this effect when selecting components. Unlike ceramic capacitors, tantalum and electrolytic capacitors do not exhibit DC bias characteristics.

 
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