Film Capacitors for Power Electronic Applications

By Freddy Estaban, EPCOS Electronic Components S.A., Spain; Suresh Chandran, EPCOS, Inc. USA
PASSIVE COMPONENT INDUSTRY NOVEMBER/DECEMBER 2005

New power electronics for industrial applications demands higher performance levels from all involved components—semiconductors to passive components— in terms of both electrical and climatic operating conditions. Capacitors are not an exception to these performance demands, and challenging standards are being set, in which the reliability of the product is becoming more and more critical. In this context, our experience demonstrates that film capacitors offer significant advantages compared to other capacitor technologies. High current capability, low inductance, flexible design, different mounting possibilities, thermal and electrical stability, reliability, and long service life make film capacitors a suitable solution for these applications. The new developments in film capacitor technology allow the designers of power applications to use new design concepts and advanced control systems for high switching frequency semiconductors. They can now use film capacitors, taking advantage of their excellent performance, rather than other traditional technologies. EPCOS has understood this new trend and offers different series of film capacitors, in different operating voltages, in order to fulfill the technical requirements of every position in the circuit.

Film Capacitors As a Reliable Solution:
The high performance of film capacitors is based on different characteristics of the product, which are linked to their internal construction and the properties of the plastic material of which they are composed. To begin with, the self-healing capability of film capacitors is one of their most important features; it protects the capacitors against catastrophic operating failures and makes them a highly reliable product, in comparison with other technologies.

Film capacitors also offer excellent thermal stability, being able to work within a wide range of temperatures without affecting the product performance. In addition, due to the high electrical stability of this technology, the most important electrical parameters of film capacitors remain constant when modifying the voltage. Low ESR values and high Irms handling capability, which are needed to work with high frequency ripple current (up to 100KHz) in applications, are other important characteristics of film capacitors. All these characteristics make film capacitors an optimal solution for many applications in the power electronics field, where they were not previously used.

Self-Healing Property of Film Capacitors:
The self-healing capability of film capacitors could be defined as their ability to clear faults (such as pores or impurities in the film) under the influence of a voltage. The metal coatings, vacuum-deposited directly onto the plastic film, are only 20 to 50nm thick. If the dielectric breakdown field strength is exceeded locally at a weak point, a dielectric breakdown occurs. In the breakdown channel, the high temperatures reached (up to 6000K) transform the dielectric into a highly compressed plasma that forces its way out. The thin metal coating in the vicinity of the channel is totally evaporated by interaction with the plasma, retreating from the breakdown channel. The rapid expansion of the plasma causes it to cool after a few microseconds, thus quenching the discharge before a greater loss of voltage takes place. The insulated region thus resulting around the former faulty area will cause the capacitor to regain its full operational ability.

Figure 1: Schematic of the Self-Healing Area during Electrical


1. Dielectric
2. Metallized electrodes
3. Material displacing shock wave
4. Air gap with metal vapor
5. Plasma zone
6. Plasma zone
7. Boundary layer between gas phase dielectric and plasma
8. Breakdown channel
9. Gas phase dielectric
10. Zone of displaced metallization and dielectric (insulating
region)

Application: Drives
Introduction
The function of an electrical adjustable drive is to control the speed, torque, acceleration, deceleration, and direction of rotation of the motor of a machine. The drives could be direct current drives (DC drives) or adjustable frequency drives (AC drives). In this sense, any electrical drive typically consists of three basic elements:.

1. Drive Controller
2. Driven Motor
3. Machine

DC Drives
DC drives, because of their simplicity, ease of use, reliability, and favorable cost, have been the preferred solution for industrial applications for a long time. In DC drives, the thyristor (a silicon controller rectifier SCR) converts the fixed AC voltage of the power source in the input to an adjustable voltage, controlled direct current (DC) output, which is applied to the armature of a DC motor. The speed and torque of the motor is managed by modifying the output voltage value.

In general, DC drives could be classified in two groups, Nonregenerative DC Drives (which are able to control motor speed and torque in one direction only) and Regenerative DC Drives (which are capable of controlling not only the speed and direction of motor rotation, but also the direction of motor torque).

AC Drives
Adjustable frequency AC motor drive controllers, frequently called inverters, are typically more complex
than DC controllers because they must perform two power section functions: conversion of the AC line power source to DC voltage, and finally an inverter change from the DC to a coordinated adjustable frequency and voltage output to the AC motor. Therefore, the speed and torque is controlled
by means of the adjustable frequency and voltage level of that AC output voltage. In spite of its complexity, there is a clear trend in the market to use AC variable speed drives. This trend has
been driven by the desire to emulate the excellent performance of the DC motor (such as fast torque response and speed accuracy), while using rugged, cheap, and maintenance- free AC motors. This is the reason for the fast evolution of these products during recent years. A number of different types of AC motor controllers are currently in common use as general purpose drives: Pulse Width Modulated (PWM), Current Source Input (CSI), and the Load Commutated Inverter (LCI). Each type offers specific benefits and characteristics, and the selection criterion is based on the final application requirements in terms of voltage and power.

Basic Schematic
As an example, Figure 2 system block diagram represents a general basic schema of a 3-phase DC drive, which could be used to control a DC motor.
Figure 2: System Block Digram of a 3-Phase DC Drive
On the other hand, for AC motors, a 3-phase AC drive could be represented by the system block diagram in Figure 3:

Figure 3: System Block Diagram of a 3-Phase AC Drive


Obviously, the final topology of a particular drive will depend on the real application requirements: different configurations for the EMC filter, output filter, and DC-link stage. Therefore, some of the capacitors that are included in these diagrams might or might not appear in solutions for specific requirements.

Capacitor Requirements
EMC Filter
Normally, across the line and line to ground capacitors are used in this filtering stage. In most cases they have to be X2 and Y2 approved capacitors, in accordance with international regulations. Electromagnetic interference produced by the equipment, if not filtered out by those capacitors, could interfere with the functioning of other devices in the vicinity. These capacitors work with an AC voltage with 50 or 60Hz frequency. Depending on the voltage level of the 3-phase network (380 to 440V), in particular for X2 capacitors, the rated voltage will be between 275 and 305V, which includes a safety margin of 10%. These capacitors must be able to withstand transients that could suddenly appear in the mains, in order to protect the equipment and its end users against those over-voltages. With respect to Y2 capacitors, 250Vac is the typical rated voltage, although there is trend in the market to used this product with a higher rated voltage (up to 305Vac).
DC-Link, Switching and Smoothing
Capacitors in the DC-Link module have to support the DC voltage from the AC/DC converter, by supplying high peaks of current when it is required. They also have to work with high frequency ripple current—up to 100Khz—which is superimposed on the signal and is coming from the inverter. Finally, capacitors in this position have to withstand recurring and non-recurring peak voltages, which are induced by switching or any other disturbance of the system. According to IEC 1071, the peak voltages could be 50% higher than the capacitor rated voltage. Therefore, low ESR values, high RMS current capability, high insulation resistance, relatively high capacitance values, 50% higher than the capacitor rated voltage.

Therefore, low ESR values, high RMS current capability, high insulation resistance, relatively high capacitance values, and good self-healing characteristics are the basic requirements for capacitors for that filtering stage.

Figure 4: Peak Voltages


Output Filter
In order to protect the load after the inverter, capacitors in the output filter have to deal with voltage pulses with rapid voltage changes that lead to strong peak currents—it means high dV/dt-. Also the capacitors must protect the motor against high frequency components coming from the inverter. Consequently, high pulse handling capability, good AC performance, and good self-healing properties are required characteristics for these capacitors.

Snubber Capacitors
Snubber capacitors are connected in parallel with semiconductor components in order to damp high peaks of voltages that are provoked by their switching operation. Again, high pulse handling capability and good thermal behavior means that low self-healing properties are requirements for these capacitors. In addition, good self-healing properties would further improve the protection level that any snubber capacitor could offer to the semiconductor, to overcome any other unexpected over-voltage. Finally, regarding the ambient conditions in this typical industrial application, 85°C is the most usual figure for the maximum operating temperature. Only in some rare applications, under stringent environmental conditions, could the
maximum temperature reach 100°C.

Figure 5: Snubber Capacitors

Additional Resources For This Story: (1) Paper & Plastic FILM Capacitors: World Markets, Technologies & Opportunities: 2008-2013 ISBN # 0-929717-87-2 (2008).