All capacitors have a voltage rating that is primarily for safety and which will largely determine the application usage of a capacitor. Ceramic capacitors can be a bit of a different beast and their voltage ratings have multiple meanings. First of all, the usage of ceramic capacitors has largely been driven by the need for miniaturization in high-density, high-speed, and high-frequency electronics.These systems can require small capacitances and small packages, which is where ceramic dielectrics are quite useful.
Ceramic capacitors have another problem related to their voltage ratings, which is a decrease in the voltage as a function of an applied DC bias. This is sometimes called the DC bias effect and it is usually discussed in the context of EMC. This effect on the capacitance of a ceramic capacitor under DC bias is what often drives the use of ceramics with very high voltage ratings in DC systems, even when the DC bias will just be a few volts.
If you find yourself in a situation where you have to use ceramics that will be greatly affected by the DC bias effect, here's how to spot what the problems may be and how to solve them.
The DC Bias Effect in Ceramics
When we typically think of parasitic or functional defects and capacitors, we're more worried about ESR and ESL values. Both are important in high-speed design, filter design for RF signals, and even some cases of precision measurement. High DC bias creates an additional problem in ceramic capacitors, where the capacitance of the component is lower when the DC bias is higher.
Typically, DC bias greater than roughly 5-10% of the rated voltage for a ceramic capacitor will start to create a noticeable decrease in the component’s capacitance value. The exact value depends on several factors, notably the following:
The ceramic material used as the capacitor dielectric
Package size: smaller packages have higher internal electric field
What may be surprising is that the capacitance change is relatively constant across voltage ratings as long as the component packages are the same. The example below shows this lack of dependence on voltage rating by comparing three parts up to their rated voltage limits.
The change in capacitance is independent of the capacitor’s voltage rating. (Source: Murata SimSurfing)
In summary, ceramic capacitors may have effective capacitance that is too low given the voltage across a capacitor. This is particularly acute in power electronics, which could operate above the voltage ranges shown above. The potential problems this creates are large switching voltage ripple and low phase margin.
Simulations With DC Bias Effect
Because this is a material specific effect and not a circuit-based effect, this reduction in capacitance with applied DC voltage is not something that can be predicted from a SPICE simulation. Some capacitor manufacturers have made the effort to make data available, and some offer an empirical simulation tool that will calculate the capacitance reduction with input voltage. These simulators are based on measured data gathered from their materials, they are not based on SPICE circuit models.
The example below shows Murata’s SimSurfing application, which allows you to examine capacitance derating during applied DC bias with select capacitor part numbers.
Derating determination with SimSurfing. (Source: Murata SimSurfing)
The materials simulation information could be used to build a custom SPICE model which determines the capacitance directly. For example, the voltage in a given time step determines the capacitance, and that capacitance could be used to calculate the change in voltage on the capacitor on the next time step. This is not a typical approach, and instead most designers will just oversize their capacitors to compensate for an estimated drop in capacitance with the applied DC bias.
Anytime you need to simulate the effects of DC bias with custom capacitor models, use the complete set of circuit simulation features in PSpice from Cadence. PSpice users can access a powerful SPICE simulator as well as specialty design capabilities like model creation, graphing and analysis tools, and much more.