When specifying inverter output filter capacitors, the additional heating generated from the harmonic content of the system must be accounted for. If not, capacitor life will be shortened considerably. The filter capacitors selected should be designed to minimize losses in order to be able to dissipate the increased power generated by the harmonic currents. The increased peak voltage, caused by harmonic voltages superimposed on the fundamental waveform, should be examined as part of the design process. If the capacitor is not suitably sized, the dielectric can be damaged, causing premature failure.
All design engineers who consider using aluminum electrolytic capacitors want to know how long they will last and how many they can expect to fail. Many engineers do not realize that these are actually two different but related questions. In this paper we define life and reliability in a manner that will hopefully make the distinction clear.
Impedance modeling of aluminum electrolytic capacitors presents a challenge to design engineers due to the complex nature of the capacitor construction. Unlike an electrostatic capacitor, an electrolytic capacitor behaves like a lossy coaxial distributed RC circuit element whose series and distributed resistances are strong functions of temperature and frequency. Existing public domain Spice models do not accurately account for this behavior. In this paper, a physics based approach is used to develop an improved impedance model that is interpreted both in pure Spice circuit models and in math functions.
Aluminum electrolytic capacitors are widely used in all types of inverter power systems, from variable-speed drives to welders to UPS units. This paper discusses the considerations involved in selecting the right type of aluminum electrolytic bus capacitors for such power systems.
Snubbers are suppression circuits which are placed across IGBTs and switching transistors in power conversion circuits to suppress voltage transients and protect semiconductor devices from overvoltage. This paper shows how to design resistor-capacitor (RC) damping snubbers and the resistor-capacitor-diode (RCD) turn-off snubbers.
With so many types of capacitors available, circuit designers are faced with the challenge of selecting a capacitor that will be suitable for a specific snubber application. It is essential that the designer know the approximate conditions to which the capacitors will be exposed. The most important being peak voltage, temperature, dV/dt, and frequency. The designer may also be faced with constraints such as size, maximum allowable inductance, and cost. Once these conditions and constraints are identified the designer can begin the selection process.
Large-can aluminum electrolytic capacitors are widely used as bus capacitors in variable-speed drives, UPS systems and inverter power systems. Accurate thermal modeling of the capacitor's internal temperature is needed to predict life, and this is a challenge because of the anisotropic nature of the capacitor winding and the complexity of the thermal coupling between the winding and the capacitor case. This paper translates analytical models for heat flow in bus capacitors into an equivalent three-loop, seven-resistor, lumped-parameter thermal circuit model.
A comprehensive thermal model for screw-terminal aluminum electrolytic capacitors is developed in this paper. The test methodology and data upon which the model is based are discussed. Exact one-dimensional solutions, multi-dimensional heat equations, and finite-element analysis (FEA) model simulation results are presented. The effects of conduction, heat sinking, natural (free) convection, forced convection, and radiation are quantified and compared. Complex issues, such as anisotropism and multi-phase heat transfer, are discussed. A comparison of model results to test data is presented. Varying capacitor construction techniques are evaluated.