Electronic component miniaturization and operation in harsh environmental conditions are growing trends in applications, such as on-board chargers, energy meters, capacitive power supplies, including connection in series with the mains, motor drives, wind and solar inverters. Current EMI (Electromagnetic Interference) X2 class suppression and DC-link power box film capacitors need capability improvement to meet these requirements.
Very high capacitance and dissipation factor stability are required during operational life in severe ambient conditions such as high temperature and relative humidity, while still meeting European and other Electrical Norms (ENEC and CQC), the criteria in the standard for automotive application (AEC-Q200) and the international safety requirement (UL). The moisture absorbed into the capacitor leads to corrosion of the electrode and accelerated degradation of the capacitor by increasing of capacitance loss. Temperature-Humidity-Bias (THB) is a standard test for accelerated stress testing of corrosion and other moisture-driven mechanisms for degradation. In this paper, we have studied the characteristics and performance under high temperature and humidity conditions of new capacitor designs in a miniaturized version of first to the market metallized EMI X2 class suppression and DC-link power box film capacitors. Three advanced KEMET series of metallized film capacitors have been stressed under an applied rated AC or DC voltage at 85°C and 85 %R.H. and the drop of capacitance and change of the dissipation factor have been monitored with the time for 500 and 1000 hours, respectively.
The paper was presented by Hristina Kostadinova Boshkova, KEMET Electronics Macedonia, North Macedonia at the 3rd PCNS 7-10th September 2021, Milano, Italy as paper No.5.1.
Progress in semiconductor technologies, such as the implementation of MOSFET Wide Band Gap (WBG) devices and the implementation of diode devices, emphasizes the size miniaturization and increased performance of electronic components. However, reliability remains a concern when components and devices downsize and become more compact. The utilization of WBG semiconductor components in power conversion systems allows for smaller footprints and greater efficiency with lower energy losses during the energy conversion. Other key advantages include reducing audible noise and the miniaturization of passive components, all with the benefit of printed circuit board (PCB) real estate reduction. However, due to the ever-increasing number of electronic components integrated into smaller geometries, miniaturized devices have become increasingly susceptible to electrical noise or interference. While the use of higher frequencies in WBG devices helps to minimize audible noise, it produces more high-frequency emissions and requires more complex designs to meet emission requirements by regulatory agencies. For these reasons, EMI suppression capacitors play a crucial role in the electronics industry, with the need for more miniaturized solutions under critical electrical and environmental applications.
Metallized film capacitors in EMI suppression
The safety EMI suppression capacitors, Class X and Class Y, are designed for AC line filtering, minimizing the generation of Electromagnetic Interference in the radio-frequency range and negative effects associated with received EMI/RFI in many electronic device applications. Class X and Y capacitors are directly connected to the AC power input in order to filter the noise emitted by the device to the electrical grid or to the power line. Because of the direct connection to the AC voltage, the capacitors may be subjected to overvoltage or voltage transients like lightning strikes and power surges. Class X capacitors are connected between line to line or line to neutral and Y capacitors are connected between line to ground. If a class X capacitor fails because of an overvoltage event, it is likely to fail short and this failure, in turn, would cause an overcurrent protective device, like a fuse or circuit breaker, to open. Therefore, a capacitor failing in this fashion would not cause any electrical shock hazards. If a Class Y “line to ground capacitor” fails short and this could lead to a fatal electric shock due to loss of the ground connection. For that reason, the class Y is designed to fail open in order to avoid a fatal electric shock hazard.
Class X capacitors can be further divided in two subclasses X1 and X2 according to the peak voltage of the impulse to which they may be subjected and which they can safely withstand. The X2 class capacitors can withstand peak impulse voltages up to 2.5 kV and X1 can withstand up to 4 kV. Similarly, class Y capacitors are divided into two subclasses Y1 and Y2, where Y2 capacitor can withstand max peak impulse voltages up to 5 kV and Y1 can withstand up to 8 kV. Also, X1 capacitors going to their higher peak impulse voltage capability can be substituted by Y2 or Y1 capacitors of the same or higher rated voltage whereas X2 capacitors can be substituted with X1, Y2 or Y1 capacitors of the same or higher rated voltage.
Metallized film capacitors in DC-Link applications
On the other hand, DC-Link capacitors form an essential stage in power conversion for many applications, including three-phase Pulse Width Modulation (PWM) inverters, photovoltaic and wind power inverters, industrial motor drives, automotive onboard chargers and inverters, medical equipment power supplies, etc. Demanding applications possess cost, harsh environmental, and stringent reliability constraints. Although circuit designs can use different approaches, the long-standing core of power conversion designs includes DC-Link capacitors. DC-Link capacitors can improve system energy density and resolve the challenge of ripple current introduced by rapid switching that is inherent to switching power conversions.
The automotive industry includes prime examples of power conversion in the hybrid and electric powertrains. Battery electric vehicles include a rechargeable bank of batteries to store energy for the drive system, an electric drive motor, and a power controller that includes an inverter. These all operate at high voltages extending from 48 VDC to as high as 800 VDC. Due to the physical limitations that limit current, high voltage correlates to high performance. The higher the operating DC voltage, the lower the required current flow for the same power output (P=VI). The automotive industry is well-known for requiring components that can operate with high reliability at extremely high temperatures, under continuous vibration, and where components are subject to harsh environmental conditions. The three-stage traction inverter converts battery power to drive the motor, and the DC-Link capacitor is key to this design.