Power Film Capacitor Application Guide PAGE CONTENTS DC Capacitor Overview 153 Construction 153 Metallized Capacitors 153 Film/Foil Capacitors 153 Hybrid Capacitors 153 Custom Designed Film Capacitors 154 Applications for Power Film Capacitors 154 DC Link for Inverter Applications 154 Advantages of Film vs. Aluminum Electrolytics for DC Link Apps 154 DC Output Filtering 154 IGBT Snubber 154 Definitions 154 the dielectric system vaporizes the metal deposit in the area of the fault, a process known as clearing. The result of “clearing” is a tiny amount of capacitance loss while allowing the capacitor to continue to operate without any adverse effects. If a condition arises that causes multiple clearings, such as overvoltage, or dielectric aging at end of life, the capacitor will continue to self heal and lose capacitance. The capacitor is considered to have failed when it loses 3% or more of its capacitance. Changing the metallized electrode thickness alters the properties of the capacitor. Lighter metallization, higher ohms per square, result in higher energy density designs. While light metallization improves the voltage capabilities, it compromises the rms and peak current carrying capabilities of the capacitor. Patterned metallized layers add another dimension to the mix of options. Metallized patterns have built in fuses that further enhance the self healing capabilities and dielectric voltage withstanding properties of the system. Film / Foil Capacitors --- Foil DC FILM CAPACITORS FOR POWER ELECTRONICS AN OVERVIEW --- Film Film capacitors are widely used in power electronics applications --- Film including but not limited to DC Link, DC output filtering, and as IGBT snubbers. The dielectric most often used is polypropylene because it has low dissipation factor (DF) that permits high AC currents with low self heating, and it performs well over the temperature range and frequencies in power electronics applications. Other materials such as polyester (PET) may be used for light duty filtering but its high dissipation factor makes it a poor selection where high AC current or high peak current at high rep rates are encountered. Materials such as polycarbonate and PPS have desirable characteristics for power electronics applications but they are in scarce supply and are therefore relatively expensive and exotic materials. --- Foil Aluminum foil electrodes are used where very high peak and rms currents are required. IGBT snubbers, for example, are designed to handle the high peak currents encountered during IGBT switching. Typical IGBT applications, such as those encountered in high power inverters, have voltage rise times exceeding 1000 V/µs with switching rates of 10 kHz or more. The end connections of a capacitor employing a simple metallized electrode system would deteriorate with repeated exposure to these conditions. Foil capacitors use electrodes that are about 5 microns thick to handle the high current pulses. The capacitor’s electrode system is an important design consideration. There are three basic options for electrodes used with polypropylene capacitors. A description of each follows: Foil electrodes are also used where the capacitor will see high rms current, especially where the capacitor size is small. As an example, tank circuits for induction heating devices typically require capacitors less than 100 µF that must handle hundreds of amperes. The main benefit of the foil electrodes is to reduce the heat rise by reducing ESR. Cooler operation prevents thermal runaway and dielectric failure from self heating. The main disadvantages of foil electrode capacitors are their inability to self heal and low energy density relative to metallized types. Metallized Capacitors Hybrid Capacitors This catalog features only polypropylene types which cover the vast majority of power electronics applications. CONSTRUCTION --- Metallized Film --- Foil --- Metallized Film --- Metallized Film Metallized capacitors use a thin layer of vapor deposited aluminum, zinc or alloy (aluminum/zinc) blend as the electrode system. The metallized layer is only hundreds of angstroms thick, so it takes up little space in the capacitor winding relative to the dielectric thickness, measured in microns. Metallized capacitors offer the highest energy density of all of the available film constructions. Metallized capacitors also self heal. A fault in --- Film One scheme that combines the benefits of metallized and foil electrode types, is the hybrid series capacitor. It has foil electrodes that connect to the external leads of the capacitor and afree-floating metallized electrode wound in a series configuration. The result is a self healing capacitor that handles high current pulses. 1 With all of these variables at play, the choice of dielectric, electrode metals, electrode thickness and metallized pattern must be considered to optimize the capacitor’s performance for a specific application. CUSTOM DESIGNED FILM CAPACITORS This catalog offers standard capacitor products for use in common power electronics applications. Inverter design engineers are often challenged with finding a capacitor that takes them out of the realm of standard catalog items. Packaging a capacitor to accommodate a physical bus structure is a common design challenge. Designing for a specific ripple current rating and life expectancy at a given ambient temperature is another consideration that usually requires custom solution. CDE is highly experienced in custom capacitor design and manufacturing. Where possible, we “repackage” standard materials to meet specific customer requirements. APPLICATIONS for POWER FILM CAPACITORS The most common applications for DC film capacitors in power electronics are DC Link, DC Filtering and snubbers for IGBT modules. A brief description of each application follows: DC Link for Inverter Applications Large value capacitors are used as the energy storage element or DC-Link at the DC input to the inverter. The size of the DC Link depends on the amount of AC energy it must absorb to maintain required ripple current at the DC line and the level of rms current it can handle because of ESR heating. DC Link Capacitor Aluminum Electrolytic capacitors offer greater capacitance per unit volume and higher energy densities compared with film. The trade-off is that the much higher ESR of aluminum electrolytic capacitors often results in capacitor banks that are oversized to handle the ripple current requirements. Polypropylene film capacitors have much lower ESR to handle the AC ripple without overheating. Film technology advantages over electrolytics are listed below. Advantages of Film Capacitors versus Aluminum Electrolytics for DC Link Applications IGBT Snubber As with all switching devices, IGBTs are subjected to voltage transients during turn-off operation. Voltage transients result from energy trapped in the circuit’s stray inductance. The amount of voltage overshoot is dependent on the amount of stray inductance and the switching speed (dV/dt) of the IGBT. Fortunately, there are ways to protect IGBTs and other switching devices from over voltage; the most common is to use a snubber capacitor across the switch to divert the inductive current. IGBT snubbers may be terminated with lugs for direct mount across the IGBT to minimize added inductance. DEFINITIONS Capacitance (C) Nominal capacitance typically given at 25 ºC and 1 kHz in units of microfarad (μF) Capacitance Tolerance Range in percent for which the capacitance may differ from rated capacitance as measured at 25 ºC and 1 kHz. This range results from variances in materials and manufacturing processes rather than from temperature and or frequency characteristics. The standard manufacturing tolerance for polypropylene capacitors is ± 10% or “K” tolerance. Tighter tolerance of ± 5%, “J” tolerance, can be achieved for polypropylene, usually at a slightly higher cost. Temperature Coefficient The temperature coefficient is the average capacitance change per ºC over a specified temperature range. 2 • • • • Two times the voltage capability frees you from series capacitors and voltage balancing resistors. Three times the ripple current capability frees you from needing excess cap to handle ripple. Dry construction frees you from the explosive failures with liquid electrolyte. Solid encapsulation delivers higher shock and vibration withstanding. Non-polar dielectric delivers reverse-proof mounting and AC withstanding. DC Output Filtering Film capacitors are widely used for DC filtering in power supplies. Their function is to smooth out the DC voltage waveform after rectification. 2 1 Cap Change (%) • Capacitance Change vs Temperature 0 -1 -2 -3 -55 -25 0 25 Temperature (ºC) 50 75 100 Capacitive Reactance (Xc) The reactance is the capacitor’s opposition to passing AC current. It is inversely proportional to frequency and capacitance. 1 X= c . . . 2 πfC Equivalent Series Resistance (ESR) The total ohmic resistance that contributes to power loss, represented by a single resistance in series with the ideal capacitor. Typically given at 25 ºC at 10 kHz and 100 kHz in units of milliohms (mΩ) Equivalent Series Inductance (ESL) The total series inductance of the capacitor winding including any internal connections, typically low and given at 25 ºC in units of nanohenries (nH) Insulation Resistance (IR) The ratio of the applied voltage to leakage current (DCL), typically given in megohms (MΩ) or in the discharge time constant format MΩ x μF. The formula for insulation resistance: Vdc IR= DCL Insulation Resistance vs Temperature IR x C (MΩ x µF) 1000000 Rated DC Voltage (Vdc) The maximum operating peak voltage for which the capacitor has been designed for continuous operation at rated temperature. Rated AC Voltage (Vrms) The maximum operating AC rms voltage for which the capacitor has been designed for continuous operating at rated temperature, typically given at 60 Hz. Peak Current (Ipk) The peak current amplitude for which the capacitor is designed, given in units of amperes (A). The Peak Current is related to dV/ dt by the formula: Ipk=C. dV / dt Where C is rated capacitance. RMS Current / Ripple Current (Irms) The maximum operating rms current, typically given at a specific reference frequency and temperature in units of amperes rms (Arms) Thermal Resistance (θcc , θca) The total thermal resistance from core to case (θcc) and case to ambient (θca) defined as the temperature change per dissipated power (ºC/W) 100000 10000 The formula for thermal resistance: 1000 0 25 50 Temperature (ºC) 75 100 Dissipation Factor (DF, tanδ) The ratio of the capacitor’s ESR to capacitive reactance Xc , the DF of a capacitor is frequency and temperature dependent and is usually specified at 25 ºC and 1 kHz. DF= ESR Xc θca+ θcc= ΔT Irms . ESR 2 Where ΔT is temperature rise in ºC. Life Expectancy The life expectancy formula for the power film capacitors in this catalog* is given in terms of applied voltage and temperature. DF change with temperature and frequency are given for polypropylene in the curves below. Dissipation Factor vs Temperature Dissipation Factor (tan δ x 10-5) 30 LifeS LifeR VR VA F TR TA 25 20 15 = Service Life = Rated Life = Rated Voltage = Applied Voltage = Voltage Acceleration Factor = Rated Temperature = Ambient Temperature Expected Lifetime vs Core Temperature and Applied DC Voltage 10 -75 -25 0 25 50 Temperature (ºC) 75 100 1.7 -4 Applied / Rated Voltage Ratio (V A/VR) Dissipation Factor vs Frequency 100 Dissipation Factor (tan δ x 10 ) -50 10 1.6 1.5 1.4 50ºC 1.3 60ºC 1.2 70ºC 1.1 85ºC 1 0.9 0.8 100 1000 10000 100000 1000000 Expected Lifetime (h) 1 1 10 Frequency (kHz) 100 1000 * Life Expectancy curves for the DC Link types 944U and 947C are given on their datasheets. 3 Notice and Disclaimer: All product drawings, descriptions, specifications, statements, information and data (collectively, the “Information”) in this datasheet or other publication are subject to change. The customer is responsible for checking, confirming and verifying the extent to which the Information contained in this datasheet or other publication is applicable to an order at the time the order is placed. All Information given herein is believed to be accurate and reliable, but it is presented without any guarantee, warranty, representation or responsibility of any kind, expressed or implied. Statements of suitability for certain applications are based on the knowledge that the Cornell Dubilier company providing such statements (“Cornell Dubilier”) has of operating conditions that such Cornell Dubilier company regards as typical for such applications, but are not intended to constitute any guarantee, warranty or representation regarding any such matter – and Cornell Dubilier specifically and expressly disclaims any guarantee, warranty or representation concerning the suitability for a specific customer application, use, storage, transportation, or operating environment. The Information is intended for use only by customers who have the requisite experience and capability to determine the correct products for their application. Any technical advice inferred from this Information or otherwise provided by Cornell Dubilier with reference to the use of any Cornell Dubilier products is given gratis (unless otherwise specified by Cornell Dubilier), and Cornell Dubilier assumes no obligation or liability for the advice given or results obtained. Although Cornell Dubilier strives to apply the most stringent quality and safety standards regarding the design and manufacturing of its products, in light of the current state of the art, isolated component failures may still occur. Accordingly, customer applications which require a high degree of reliability or safety should employ suitable designs or other safeguards (such as installation of protective circuitry or redundancies or other appropriate protective measures) in order to ensure that the failure of an electrical component does not result in a risk of personal injury or property damage. Although all product-related warnings, cautions and notes must be observed, the customer should not assume that all safety measures are indicated in such warnings, cautions and notes, or that other safety measures may not be required. 4