Downsizing and footprint reduction are constant requirements of the market for all applications or equipment and the relative components. Film capacitors are not an exception and time by time is required to reduce the components dimension maintaining the same rated voltage and performances. Decreasing the volume of the components, one of the consequences is a reduction of the power dissipation by its surfaces and therefore a final increasing of its working temperature.
The paper was presented by Evangelista Boni, KEMET Electronics, Bologna, Italy at the 3rdPCNS 7-10th September 2021, Milano, Italy as paper No.4.1.
To understand the real requirement of the market about temperature KEMET has focused the attention on 2 main segments where the temperature trend is increasing: the automotive and the WBG (Wide Band Gap) application.
One of the major current trends in the automotive electronics industry is pushing the temperature envelope for electronic components. The desire to place engine control units directly on the engine and the transmission control units either on or in the transmission will push the components working ambient temperature above 125°C.
However, the extreme cost pressures and the increasing reliability demands and the cost of field failures (recalls, liability, customer loyalty) will make the shift to higher temperatures occur incrementally. The coolest spots on engine and in the transmission will be used. These large bodies do provide considerable heat sinking to reduce temperature rise due to power dissipation in the control unit. The majority of near-term applications will be at 150°C or less and these will be worst case temperatures, not nominal. The transition to X-by-wire technology, replacing mechanical and hydraulic systems with electromechanical systems will require more power electronics. High-temperature electronics use in automotive systems will continue to grow, but it will be gradual as cost and reliability issues are addressed. This work examines the motivation for higher temperature operation, the packaging limitations even at 125°C with newer package styles and concludes with a review of challenges at both the semiconductor device and packaging level as temperatures push beyond 125°C.
One of the first application demanding film capacitors at high operating temperature is the electric compressor. Vehicles primarily use electric compressors in two ways: in E-charger systems and in air conditioning systems. The circuit design for each of these applications is similar, as is the harsh condition in which these systems must operate.
An E-charger functions as a turbocharger, but one that is driven by electronics, rather than mechanically driven by belts (often called “superchargers”) or exhaust airflow. E-chargers have particular and obvious advantages over traditional turbochargers in hybrid electric vehicles, given their significant electrical power.
The challenge for the E-charger application is that much of the electronics used to monitor and control these devices must reside in the engine compartment. Engine compartments are high heat, high humidity, high vibration – the real definition of harsh environments. Any components used must be capable of not just withstanding those harsh conditions, but also thriving in them. They must be as small as possible, but also have a long and reliable life.
Electric Compressor Design
EVs and HEVs electric compressors drivers use an inverter circuit to convert a high DC voltage to an AC voltage. Fundamentally, this circuit involves an EMI/safety stage, a DC-Link capacitor, and then a MOSFET inverter stage. The trend in EV and HEV components is simplification, miniaturization, and higher power density. DC-Link capacitors are also required to work at higher temperatures, for a longer lifetime, with every new generation.
OBC (On Battery Charger)
OBC market is essential component of automotive electrification and it is growing rapidly as the demand for battery powered EV is increasing worldwide. OBC system is composed of AC/DC converter with PFC cascaded with DC/DC converter to control power delivery to the battery. Here below a typical example how can be used a resonant film capacitor in LLC resonant converters in OBC design.
The resonance converter works on high frequency, and its temperature is rising additionally. Due to the miniaturization of OBC, the packaging size becomes smaller and it causes higher ambient temperature. Thus, for this LLC resonant converter must be developed components which has high temperature resistance.
WBG Wide Band Gap
The key factors for the electronic world designs to use WBG are greater efficiency, higher temperature, frequency of operation, and greater power density designs. Translating all those factors to the passive components needed for the final conversion system turn to the use of smaller components like capacitors and magnetics. For instance, a SiC module can perform at 25 kHz or higher vs. previous Si designs operating at the 8 to 10 kHz range, allowing the DC-Link capacitor bank to be reduced from 1,000 mF ranges, using power electrolytic and film canister solutions, dramatically to just hundreds of μF requirements, using power box film radial solutions. According to WBG producers, this switch of semiconductor technologies can bring design reduction factors up to 50%, and according to GaN Systems experts, they expect reductions of up to 3x improvement in power density using SiC and GaN in applications as On-Board Chargers.
Fig.3: Electronics industries and applications benefiting from WBG solutions advantages
In addition to this impressive power density increase and efficiency gain on switching from large electrolytic and film canisters to a bank of power box film technologies for the DC-Link power bank, the film technology requires to perform under critical levels of operational conditions. The high ambient temperature and humidity conditions require the designers to compensate the capacitor top performances (e.g. ripple, voltage capability) with derating factors to maintain a reliable DC-Link solution for extended operational hours under those environmental conditions and extended lifespan.