- Corona Inception Voltage (CIV)
- Corona Discharge is a small but locally intense electrical discharge that injects charge into the insulating film adjacent to edges of foil/metallization or a location where air is trapped between foil/metallization and the film. The discharge is caused by a voltage gradient large enough to ionize molecules in either the film or small air pockets. Each discharge does some small but cumulative damage to the film. Corona is an important consideration for AC and/or pulse applications where the cumulative damage can rapidly accrue and cause dielectric failure.
- Corona Inception Voltage (CIV) is the AC voltage at which the damaging corona discharges start to occur. Obviously, a designer would like to know what voltage that is. He might make the mistake of assuming that the “rated AC voltage” listed in many capacitor company catalogs would be a safe way to do it. Unfortunately, this is not the case. In fact, most capacitor companies don’t have any way of measuring CIV. They infer their ratings of CIV through theoretical design and life testing at certain voltages for a set period of time. If the parts pass (what is defined as “pass” is anyone’s guess) then that becomes the rating.
- CIV Testing – SBE’s proprietary (CIV) testing capability is utilized for small sample film design validation of large AC capacitors. This is critical to ensure that capacitors will not operate in corona subject to the electrical stress applied at the nominal voltage.
- ESL Measurement – SBE has developed a test method for precise measuring of low value ESLs that can be used to determine the total inductance from DC Link capacitor windings to the switching device terminals. Supporting magneto-dynamic finite element analysis has also been shown to agree quite well with the measurements. The measurement and simulation results demonstrate that a 1000 μF 600V Power Ring Film CapacitorTM has an ESL of approximately 3 nH with a properly designed terminal structure. The test is simple in concept; the capacitor is charged at a low voltage and discharged with a short circuit located at the switch semiconductor DC supply terminals. At the instant the short circuit occurs, the voltage curve is extracted and analyzed. A very close estimate of loop inductance can be obtained from a classical circuit analysis for an underdamped series RLC circuit. The bus structure is shown to dominate the total ESL for both horizontal and vertical inverter topologies when using annular form factor film capacitors with an optimal terminal configuration. This highly accurate method for low ESL measurement allows the designer to have visibility over the inverter performance and to improve its waveform, size and cost, particularly in transient mode. (See technical paper: “Characterization of Equivalent Series Inductance for DC Link Capacitors and Bus Structures”, presented at PCIM Europe 2012 (pdf link))
- Overshoot Measurement – SBE has also invested in developing an overshoot measurement method that will provide an in-house testing capability using IGBT bridges to relate the switch overshoot. Key to all this is an accurate measurement of the total bus current, especially the rate at which the current decays as the IGBTs open (di/dt).
The design is more challenging that the ESL tester described above. Each component has been carefully chosen, the switching elements are standard IGBTs available in the market, and many thoughts have been put on the whole assembly design, mainly, the current probe placement and ensuring a uniform current distribution. The overshoot voltage measured, should allow us to make a fair comparison with the one obtained at the customer’s end. Direct optimization of the design (re-design of bus, elimination of snubbers, smaller IGBTs, etc.) can be applied based on the measured performance.
- Trise Testing – For any DC link application, knowing what the capacitor safe operating condition is for the desired reliability is critical. After months of engineering development, SBE has now developed a “state of the art” system, which used with the SBE advanced capacitor simulation tool and life test data will provide customers with invaluable data.
In a typical DC link application, the film capacitor rating is defined by the internal winding hotspot temperature. The sophisticated temperature rise testing system SBE develops helps characterize capacitor performance subject to realistic ripple current and cooling scenarios. A power amplifier is used to drive a coil which is air-coupled to a secondary coil in the capacitor test loop. The resonant frequency of the test loop is defined by the coil inductance and a series capacitor of much lower value than the test specimen. Typically, a resonant frequency of approximately 20 kHz is selected, which is higher than the typical value of an automotive DC link.The capacitor under test is instrumented with thermocouples and connected to a laminar bus structure which interfaces to the test loop. The capacitor/bus assembly is installed between two temperature controlled plates such that single or double sided cooling can be evaluated for steady state ripple currents up to 300 A rms. The thermocouple data is then correlated with the SBE advanced capacitor simulation tool and life test data to define a safe operating condition for the desired reliability.If you have a DC link application and are looking to understand the capacitor safe operating condition, whether it is for a Power Ring or a competitive product, talk to our engineering team! See our application notes for more information.
- Thermal cycling – For an improved configuration of connections and to guarantee a robust mechanical design, SBE has invested in tests that consist of subjecting the parts to thermal cycling with a temperature varying from -50°C up to 110°C. These tests have helped us develop better methods of finalizing the mechanical concept and improve cost benefits.