High Voltage Ceramic Capacitors for Power Electronics


Dr. Henry Laville and Maude Fabre at Exxelia Technologies

This article presents the characteristics and performances of a new range of high voltage ceramic capacitors manufactured using a new ceramic material. This dielectric allows to get under working voltage the same capacitance values than using an X7R material with the advantage of a very low dissipation factor typical for NPO/COG materials (less than 5.10-4). What makes these capacitors to be ideally suited for power applications where heat dissipation may be detrimental for performances and reliability.

Miniaturization is a driving need for future power electronics. This evolution, which is true whatever the application is, implies some modifications linked to a greater difficulty to dissipate the heat generated by the components and the temperature increase of the electrical circuit. Two options can be considered to enable multilayer ceramic capacitors to withstand these new constraints: 

  • To design capacitors able to work at higher temperature with the same reliability, what means a complete change of the design and/or materials of these components.
  • To design alternative components with reduced losses in order to minimize heating. As losses are mainly due to the ceramic dissipation factor, such a choice implies a complete change of the ceramic dielectric.

This second possibility lead up Exxelia Technologies to develop a completely new High Voltage ceramic capacitors range based on a new dielectric material we called “C48”.

Technical Constraints

Two classes of dielectric are mainly used to manufacture ceramic capacitors.

The first class is composed of NPO ceramics. These ceramics are mainly made of titanium dioxide with a low dielectric constant (εr ≤ 100). These ceramics are very stable with only minor changes under stress to (Figure 1):

  • Temperature
  • Voltage
  • Frequency

Figure 1 : Typical capacitance variation under temperature for NPO ceramics

The second class is composed of X7R ceramics. These ceramics are mainly made of barium titanate with a high dielectric constant (1000 ≤ εr ≤ 5000). These ceramics present some variation (Figure 2) under:

  • Temperature
  • Voltage
  • Frequency

Figure 2 : Typical capacitance variation under temperature for X7R ceramics

With the aim of changing the dielectric material used to manufacture our capacitors, it was obviously necessary to use a ceramic whose performances would allow to:

  • Develop ranges with the same capacitance / voltage / volume characteristics than the X7R dielectrics
  • Dissipate less energy than X7R materials, what means selecting a dielectric with a dissipation factor much lower than X7R’s dissipation factor –which is typically for high voltage parts much greater than 50.10-4.

So, we had to find a material which combines most of the dielectric properties of NPO and X7R materials.
Our choice has been a dielectric with an intermediate dielectric constant value (about 500) able to be processed using a greater voltage gradient (ratio of voltage and dielectric thickness) so that its capacitance per volume could be comparable with the capacitance per volume of an X7R material.

Dielectric Performances and Comments

The main characteristics of the selected material which combines most of the advantages of NPO and X7R materials are summarized in Table 1.

Table 1: Main characteristics of “C48” material

The dielectric constant of this ceramic, smaller than the dielectric constant of classical X7R materials, enable to manufacture about half the capacitance of X7R ranges when measured under standardized measurement conditions (Figure 3), what, at a first glance, appears, of course, to be a limitation.

Figure 3: Comparison of capacitance ranges in the same size package for NPO, C48X and X7R

But this dielectric is very stable under voltage. The loss of capacitance versus dc voltage is only a couple of % (Figure 4) when it’s about 60% or more for classical X7R (2R1) ranges.

Figure 4: capacitance change of C48 versus dc voltage

So, when looking at the capacitance value left under nominal voltage (working voltage), a simple calculation demonstrates it’s the same when using this ceramic and when using a X7R or 2R1 ceramic dielectric.
Furthermore the dissipation factor is very low, typically less than 0.05% what makes the heat dissipation in use not significant.

Under working conditions the capacitance values of this new range of products are equivalent to X7R values with the unrivaled advantage of no heat dissipation. Figure 5 demonstrates this performance in comparison with X7R material at 400Hz. Opposite to X7R, the C48X capacitors don’t suffer a temperature increase, what makes them more reliable.

Figure 5: Temperature increase of capacitors of the C48X range working at 400Hz in comparison with X7R material

This ceramic is clearly much better adapted to low frequencies applications (typically 50Hz and 400Hz) than the X7R materials. That’s why it’s by now for example widely used in plane electrical network.
Moreover, it can also withstand very high dV/dt, up to 10kV/µs what makes it perfectly adapted for pulse and charge/discharge applications. 

Ranges

The capacitors using this “C48” ceramic have been developed from 200V to 5kV with chip sizes ranging from 1812 to 16080, what allows a maximum chip capacitance value of 10µF 200V whereas the stacked versions are proposed with a maximum capacitance value of 47µF 200V.Taking into consideration the dissipation of this capacitor is very low, such a product appears to be perfectly suited for power applications.
Regarding the mounting of these capacitors, many configurations (Table 2) are possible to be compatible either with surface mounting or through-hole mounting. All these versions can be suitable for space use and can be designed in order to avoid any whisker growth risk (no use at all of any tin without minimum 10% lead).

For SMD mounting, the components can be mounted directly on the board or, what is recommended for the biggest sizes, using ribbons (“R” version) or DIL connections (different shapes – “P”, “PL” and “L” versions) which will absorb most of the thermo-mechanical stresses and prevent ceramic cracking.

Through-hole mounting can be processed via the help of DIL connections (version “N”) or using classical cylindrical leads to be soldered by hand or with a wave. For these through-hole mounting components different coatings are available according to the level of environmental protection which is required

Table 2: Summary of the different configurations proposed for the C48X range

New Developments

The performances of this “C48” material are (except variations of capacitance with temperature) very close to NPO characteristics. So, it’s assumed that, for applications where the temperature stability is not the key parameter, the capacitance of NPO capacitors can be multiplied, using this ceramic, by a factor of 5. In a reverse way, for a given capacitance value, the capacitor volume could be reduced by 5, what is a very promising way for miniaturization whatever the application of NPO capacitors would be.

This is of course a development way to be considered also for space applications and Exxelia Technologies is working on it. For this evaluation, the ranges proposed will be extended to lower voltages and lower sizes.

Conclusion

This new range of high voltage capacitors manufactured using a ceramic dielectric whose characteristics are intermediate between NPO and X7R (2R1) materials looks very promising for challenging power electronics applications. The common trend in power electronics with new semiconductors is demanding new components being able to handle higher power with more compact sizes for better system integration. 

The material used in this C48X range is also of interest for lower voltages applications and Exxelia Technologies is working this way.

More information: Exxelia Technologies    Source: Bodo's Power Systems, May 2016