MLCC dVdt Ratings

dV/dt Ratings for Multi-layer Ceramic Capacitors

Historically the dV/dt rating of Multi-Layer Ceramic Capacitors (MLCCs) is not a parameter that has generally been requested by customers or provided by manufacturers. In recent years, however, as the maximum capacitance values of MLCCs have increased in any given case size, many customers have looked to MLCCs as replacements for Metallized Film Capacitors (MFCs), which have suffered from supply difficulties and shortages. The dV/dt rating of MFCs is very important to this capacitor technology and is quoted by all manufacturers. Buyers of MFCs, being conscious of their dV/dt rating, also request this data when looking for an MLCC replacement. Firstly we need to understand why the dV/dt rating is so critical for MFCs

The basic construction of MFCs is described as follows.

To begin plain dielectric film is placed in a vacuum and a thin layer of metal alloy is deposited on it. The metalized film is wound into a cylinder so that a metalized edge extends on each end of the wound capacitor section. Both ends are then sprayed with small droplets of molten metal that adhere along, and make contact with, the metallization on the film. The wire lead is welded to this end spray. The end spray process does not result in a uniform connection to the metallization and there is some damage and connection loss at the edge of the film from the heat contained in the metal droplets. During application, the capacitor current must be shared by all the minute connection points of the metal end spray to the metallization of the film. Since the actual contacts between the metal end spray and the metallization are relatively few and tiny, and the contact is made to a very thin layer of metallization, the current density at each contact is enormous. If the peak current rises past a certain point the metallization vaporizes adjacent to the connections with the highest current density. Since these connection points are now gone, the remaining connection points must carry more current. After a finite number of pulses, the capacitor fails and becomes an open circuit.

Therefore when MFCs are used in pulse applications the overriding concern is the value of the peak current during a voltage transition. Because it is very difficult to measure fast current pulses directly, the maximum pulse current is usually specified by dV/dt of voltage waveforms across the capacitor.
I = C * dV/dt.

It is also worth noting that metalized polyester film capacitors have higher dV/dt ratings than comparable metalized polypropylene because the film does not sustain as much damage by the hot metal end spray or the heat generated at lead welding.

In contrast, we should also look at the construction of MLCCs. These are manufactured by interleaving parallel layers of dielectric and metal to form multi-layer construction. Thus the effective internal electrode contact area to either the external lead (radial leaded) or nickel barrier termination (SMD) is orders of magnitude greater than the equivalent MFC. This results in much lower current densities during voltage transitions. Therefore an MLCC can withstand much higher dV/dt values than the equivalent MFC. Failure of an MLCC due to high dV/dt values is not generally recognized as a normal failure mechanism and hence dV/dt ratings are not normally quoted by manufacturers of MLCCs. It has been demonstrated in snubber applications that MLCCs are capable of withstanding dV/dt ratings of greater than 8KV/µSec.

Although we have shown that MLCCs are not susceptible to high dV/dt ratings in the same way as MFC we also need to consider the effects of the power rating (or temperature rise)of the capacitor. Any continuously changing voltage condition will result in a current flowing through the capacitor.

Irms = Vrms/Xc with the power being dissipated as I2 * R (where R is the ESR at the voltage frequency). If the power rating of the capacitor is exceeded it will lead to excessive self-heating and ultimately failure of the capacitor. This is applicable to both MLCCs and MFCs.

When using an MLCC as a replacement for an MFC it is usually recommended that NP0/C0G dielectric is used. This dielectric material is similar to MFCs in terms of Temperature and Voltage Coefficients.

In summary, when replacing a Metalized Film Capacitor with an (NP0/C0G) Multi-Layer Capacitor the dV/dt rating should not be of concern however careful attention should be paid to the maximum power rating of each type of capacitor technology.