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Ceramic Capacitors Aid High-Voltage Designs

 Medium- and high-voltage capacitors find widespread use as snubbers or filters in applications, such as switching power supplies for audiovisual and business equipment (computers, modems and fax machines) and in lighting ballasts. Their excellent performance at high frequencies (several ten to several hundred kilohertz) has made them a choice of design engineers worldwide. In addition to their use in power supplies, these capacitors are widely used in industries related to telecommunications, medical, defense and aerospace, semiconductor and test/diagnostic equipment.

The definition of medium and high voltage varies for each manufacturer. However, the authors consider capacitors rated at or above 250 V as medium voltage and those above 2 kV as high voltage (although some would define capacitors at or above 100 V as medium voltage).

Table 1. Ceramic versus other capacitors.
  Ceramic Film Al electrolytic
Available capacitance for 250-V rating 10 pF to 1µF 10 pF to 47µF 10 µF to 1000 µF
Rated voltage available 0~50 kV 0~30 kV 0~630 V
Polarized No No Yes
Equivalent series inductance Low High Very high
Equivalent series resistance Very low Average High
Size (volume) Small Large Medium
Capacitance vs. dc voltage Changes Stable Stable
Capacitance vs. frequency Stable Stable Not stable
Cost (for high voltage, high cap) High Moderate Low
Thermal stability Very good Fair Fair

Notwithstanding some design changes, high-voltage capacitors essentially offer the same advantages as any other multilayer monolithic ceramic capacitors (MLCCs). These advantages include high-volumetric capacitance, extremely low impedance and ESR, and high thermal stability and reliability. Other benefits include the choice of several temperature characteristics (X7R, C0G and U2J), nonpolarity, a choice of chip-style or leaded packaging (chips are most common) and high performance-to-price ratio.

Because ceramic capacitors cover a broad spectrum, the scope of this article is limited to medium- and high-voltage capacitors. Furthermore, we will omit the discussion on tantalum capacitor replacement (MLCCs typically replace tantalums at capacitances up to 220 µF and voltages below 25 V.).

High-Voltage Capacitor Options

Table 1 shows a comparison of ceramic capacitors with other types available in the marketplace.

Today, ceramic capacitors account for more than 85% of the total volume of capacitors sold worldwide. As mentioned above, they offer the best cost-to-performance features. And for chip types, the mounting operation can be highly automated and efficient. Medium- and high-voltage MLCCs have replaced the film capacitors in most volume applications.

Low-voltage MLCCs are available in case sizes of 01005 and larger; however, the smallest size for a 250-V rated MLCC is 0603. Advanced manufacturing techniques and carefully simulated internal designs allow high capacitances in small packages with no external arcing. In fact, most of the MLCC medium- to high-voltage capacitors do not need to be coated externally. So much so that Murata now offers UL-certified safety-rated X1/Y2, X2 and Y3 MLCC chips. Yet, in some niche and specialized applications, ceramics are not the cheapest option available compared to aluminum electrolytic or film capacitors.

Recent ceramic capacitor developments have improved technical characteristics and performance. As these improvements are implemented in production models, ceramic capacitor costs are expected to decrease as volumes increase. Nevertheless, ceramic capacitors offer effective hybrid solutions. In other words, it's possible to improve performance by just replacing a few aluminum or film capacitors in an application and still keep costs manageable.

For some high-end applications (typically where space is a constraint), ceramics are replacing aluminum and film capacitors to achieve much higher performance. As is the case for any new product, these capacitors may not offer the cheapest option when first introduced, but history has shown their costs to drop almost exponentially once they are incorporated into the design.

New application requirements in automotive and other areas are driving development of high-voltage ceramic capacitors. Since 1998, for example, California has mandated sales of low-emission vehicles and plans call for these sales to reach 10% of total within a few years. The trend of low-emission vehicles has caught on in Europe and Japan as well, starting with electrically driven power steering and air conditioners. These and other applications such as “systems to stop engine idling” have led to the evaluation of dual-voltage batteries as power sources. The control circuits for such motors currently use organic film or aluminum electrolytic capacitors (1000 µF to 20,000 µF) as input capacitors for smoothing purposes.

However, as the density of electronics increases in an automobile, and the performance requirements become more severe, ceramic capacitors are being considered (and used in some niche applications) as replacements in such circuits. A bank of board-mounted capacitors can offer very high capacitance at high voltages. Fig. 1 shows one typical example consisting of six 45-µF, 250-V ceramic capacitors. Each capacitor measures 32 mm (L) × 40 mm (W) × 4.5 m (T), has a metallic termination and is surface mountable. The volume of each capacitor in this bank is 100 times that of the mass-produced 2220 case size. In addition, the ripple-current rating for this bank is 25Arms.

Recent Technological Advances

Self Heating. One requirement of any high-frequency capacitor is its ability to withstand high ripple currents. In almost all cases, the current rating is constrained by the allowable temperature rise of the capacitor. (A rise of 20°C or less is the industry norm). The heat generated in a capacitor is dependent solely on its ohmic resistance (i.e., its ESR). In case of an MLCC, the ESR is a function of the electrode resistivity and the dielectric loss.

Table 2. Specifications of ceramic and aluminum-electrolytic capacitors.
Type Size
MLCC 30 × 40 × 5 250 45 125 25
Aluminum electrolytic 50Φ × 100 350 1800 105 7

Fig. 2 shows the ripple current versus temperature rise for a capacitor tested in an inverter circuit. Ripple current is about 17 Arms for a standard X7R capacitor for a 20°C rise (at 20 kHz). However, as the figure shows, this temperature rise can be reduced significantly by tweaking the dielectric material. In this case, the newly developed paraelectric phase dielectric material has a much lower dielectric loss, suppressing the temperature rise to just below 20°C even for ripple currents as high as 25 Arms.

Piezoelectric Effect. One minor drawback for X7R dielectrics has been their susceptibility to piezoelectric-induced stresses. Although this effect is marginal and may be neglected for case sizes smaller than the 2220, for larger capacitors it can lead to catastrophic failures caused by cracking. Modifying the dielectric composition to avoid any piezoelectric effects within the range of operating frequencies may skirt this problem (Fig. 3).

Whereas the standard X7R material shows piezoelectric noise, this is almost absent from the new ceramic. These measurements were made under a 300-V bias at 90°C. Note this effect is not present in film or aluminum-electrolytic capacitors. With this new development, ceramics now offer a viable alternative to film and electrolytic capacitors in the large case sizes.

High-Frequency Performance

This section compares some high frequency characteristics of a ceramic capacitor versus an electrolytic type. Table 2 lists the sample parameters.

Although the rated voltages of MLCCs are lower than that of aluminum electrolytic, they are guaranteed to the same specifications because of the much higher breakdown voltage margins for MLCCs.

Fig. 4 illustrates the impedance and ESR characteristics of the two capacitors specified in Table 2. It is easily noticeable that the MLCC offers a much better solution, especially at frequencies exceeding 100 kHz. The ceramics achieve this at capacitances almost two orders of magnitude lower than the electrolytics.

Under identical measurement conditions, the rise in surface temperature for MLCC versus aluminum electrolytic was measured at 20 kHz and 15 A rms ripple current. The temperature rise was 0.8°C for the MLCC versus 3.1°C for the Al electrolytic. When considering the fact that the temperature rise inside the Al electrolytic is higher than at the surface, the advantages of the MLCC become obvious.

Noise absorption characteristics were evaluated based on the circuit shown in Fig. 5. Other conditions include a primary side voltage of 150 Vdc and a switching frequency of 20 kHz.

Figs. 6 and 7 show the voltage waveforms across each capacitor and emitter/collector. In spite of having 1/20th the capacitance under bias of the electrolytic, the superior surge absorption characteristics of the MLCC are self-evident. However, because of the lower effective capacitance of the MLCC, the ripple voltage is higher and its effect on the battery must be evaluated. Due to their extremely low ESR at higher frequencies, MLCCs have very high ripple-current capabilities. This, along with their higher rated temperatures, makes them an attractive choice. Their volumetric capacitance is higher compared to film capacitors, but smaller than electrolytics. However, the electrolytic capacitance is rated at room temperature and 1 kHz. Because at higher temperatures and/or frequencies the capacitance drops significantly due to high ESR for aluminum electrolytics, their effective capacitance is the same as MLCCs in actual operation.

Overall, MLCCs offer a better solution in terms of size and configuration flexibility compared to aluminum electrolytics and film capacitors (especially for primary snubbers in high ripple-current situations).

Although it was not possible to list all the types of medium- to high-voltage ceramic capacitors along with their applications in the scope of this article, the article illustrates the basic advantages of MLCC as well as recent advances in technology. As with all ceramic capacitors, high-voltage MLCCs boast very low ESR and ESL, have flexibility of size and configuration, and high thermal stability and volumetric capacitance. Their performance in the high-frequency region is excellent.

Of course, these capacitors cannot compete with aluminum capacitors (in terms of cost) in low-frequency regions where the most important characteristic is the bulk capacitance. In addition, the drop in capacitance with dc bias for X7R MLCC dielectrics is also a drawback compared to film or aluminum capacitors, but is an insignificant parameter at higher frequencies (although C0G and U2J dielectrics do not display any drop in capacitance). Medium- to high-voltage MLCCs are already widely used in the electronic industry, and their usage in field of power electronics is expected to grow significantly in the coming years.

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