What is a voltage source converter

Stacked Ceramic Capacitors for High Power Conversion

Stacked MLCCs are particularly suitable for harsh, high temperature applications

FROM NICHOLAUS R. CHEAP

Product Marketing Manager

AVX, www.avx.com

Highly reliable capacitors that are specifically designed for use in harsh, high temperature energy conversion applications - including military, aerospace, and energy - are in particular demand. Manufacturers building equipment for such applications have typically relied on components that meet stringent military specifications to ensure high reliability in harsh environmental conditions. Emerging markets such as well oil and gas exploration areas increasingly require capacitors that can operate with high reliability at temperatures up to 200 ° C, well above the -55 ° C to + 125 ° C temperature range. Aluminum electrolytes with a nominal temperature of 135 ° C and stacked ceramics with a nominal voltage of 200 ° C are available today. However, the performance of these two capacitor technologies at extreme temperatures is often very different. Therefore, it is important for OEMs to work closely with their suppliers when specifying capacitors for applications with operating temperatures outside the MIL specification range, as the existing documentation for the devices may contain information about extended temperatures.

Military temperature requirements

Current military, aerospace and space specifications include a Qualified Vendor List (QVL) and Qualified Product List (QPL), as well as a specification for typical capacitance changes over the temperature range of -55 ° to + 125 ° C. The two most popular dielectrics for high reliability MIL capacitors are X7R and C0G (NP0). The X7R dielectric exhibits a capacitance change of ± 15% over the -55 ° to + 125 ° C, the C0G (NP0) ± 30 ppm / ° C. Currently, drawings commonly used for stacked ceramic capacitors in military, aerospace and Space applications used are MIL-PRF-49470, DSCC 87106 and DSCC 88011.

Emerging markets such as the "downhole" oil and gas exploration sector have forced passive electronics suppliers to increase the temperature capabilities of stacked switched-mode power supply (SMPS) multilayer ceramic capacitors (MLCC) and aluminum electrolytic capacitors - the two most popular passive components. Today's oil drilling rigs can reach depths of seven to nine miles underground, at which temperatures can reach 200ºC or more. The most common high temperature specification range for SMPS capacitors is currently 125 ° C to 200 ° C.

SMPS MLCCs tend to have robust, high temperature performance that is much better suited than aluminum electrolytic capacitors for applications in the demanding oil production market. MLCC dielectrics that can reach temperatures up to 250 ° C are currently being developed.

New stacked ceramic dielectric formulations

Stacked ceramic capacitors provide stable, reliable extended temperature characterizations in a range of -55 ° to 200 ° C. The C0G dielectric is the most stable for this market and actually exhibits a 0.3% increase in capacitance on the high temperature benchmark of 200 ° C whereas very high temperature (VHT) dielectrics (e.g. X7R) show a -50% decrease in capacitance at 200 ° C. Although X7R dielectrics experience a drop in capacitance at such high temperatures, they also offer a stable drop that is easily characterized and can be recorded (see Fig. 1).

Fig. 1: Typical characterization of the extended temperature of C0G vs. VHT SMPS capacitors.

Stacked ceramics vs. electrolytic aluminum

Aluminum electrolytic capacitors are typically an electronics designer's first choice in an SMPS application. Electrolytes offer extremely high capacitance per unit area on the circuit board and the cost of electrolytes is much lower than stacked ceramic capacitors. Electrolytes provide high capacitance per unit volume from an extremely thin alumina dielectric (Al2 O3) layer deposited on an aluminum metal foil that is etched to increase surface area and capacitance.

Stacked ceramic components use a Pd / Ag precious metal electrode system (PMS) along with either a mixture of titanates such as titanium dioxide (TiO2) and additives for C0G class 1 dielectric or barium titanate (BaTiO3) plus silicates and / or aluminum oxide for the X7R dielectric class 2, both are Ag-terminated. Ceramic capacitors have limitations on the thickness of the dielectric layer due to the limited tensile strength of the ceramic, so they have a multilayer structure and are stacked to increase capacitance.

The use of electrolytes has several disadvantages. Aluminum electrolytic capacitors are polarized and if the polarity of an electrolyte is reversed it will overheat, potentially leading to catastrophic failure by exploding. Ceramic capacitors show no sensitivity to the polarity of an applied voltage.

Electrolytic capacitors show significant changes in capacitance (usually an increase) at elevated temperatures along with higher leakage currents, especially above 105 ° C. And the operating time is reduced at higher temperatures. Stacked ceramics retain their X7R or NPO stability at temperatures of 200 ° C and higher.

The dielectric of an aluminum electrolytic capacitor heals itself after an overvoltage is applied - a benefit. However, even at favorable storage temperatures, electrolytes have an increase in DC leakage over time due to impurities in the material.

Stacked ceramic capacitors generally work much better at high frequency. An electrolytic capacitor typically has a higher ESR than a ceramic capacitor, especially at high frequencies. An engineer will sometimes connect many electrolytic capacitors in parallel to lower the ESR. With a typical switching frequency of 100 kHz, a 100 μF 50 V electrolytic capacitor could have an ESR of 280 mΩ, while the same value of a stacked ceramic capacitor would only have 2.5 mΩ (see Table 1).

Table 1: Typical ESR performance (mΩ) of capacitors with different dielectric materials

Applications of stacked SMPS MLCCs at 125 ° C to 200 ° C

The well, gas, and mineral exploration market is the largest portion of this high temperature electronic components market; But the military, aerospace and automotive markets are also increasing their interest in high-temperature ceramics.

Offshore plants have to bring large amounts of energy from onshore supply networks to platforms. Ac cables can carry power to an offshore platform for tens of kilometers, and increasingly complex systems are needed to extend the power over longer distances, and AC cables present problems on these longer distances. AC cables cause increased system capacity through ferroresonance, which can lead to overvoltage and destruction of the insulation, which leads to component failure. However, stacked ceramic capacitors can be used as filters on AC / DC converters or on rectifiers / inverters that are part of a DC cable transmission system, allowing power to be transmitted over distances of tens of thousands of kilometers. An example of a DC cable transmission system is a voltage source converter (VSC) used in a high voltage direct current (HVDC) system. Stacked ceramic capacitors with C0G dielectric are used for EMI filtering on the AC input side, while VHT (X7R) dielectrics are used for DC filtering of high frequency ripple from the switching inverter.

Fig. 2: AVX stacked ceramic capacitors.

Other downhole applications using 200 ° C stacked MLCCs include start-and-run capacitors in AC induction motors. These motors power the rotary drilling tables that provide torque to the drills, high pressure mud pumps, the operation of the pulling units, compressors for the riser tensioning and conveyors that convey the drilled solids, equipment, pipes and supplies onto an offshore oil platform.

By: NICHOLAUS R. BILLIG, Product Marketing Manager, AVX, www.avx.com