DC-Link capacitors for DC filtering and energy storage are expected to operate at higher temperatures, in more extreme conditions, and for longer lifetimes, than ever before. Automotive applications are leading those demands for better performance, but most existing power box, DC-link film technologies are not suitable for these applications and struggle under those severe conditions.
Vehicles primarily use electric compressors in two ways: in air conditioning and electric turbocharger systems (E-turbo). The circuit design for each of these applications is similar, as is the harsh condition in which these systems must operate.
For our application, we will specifically consider an electronic-driven turbocharger rather than one mechanically driven by belts (often called “superchargers”) or exhaust airflow. E-turbos have particular and obvious advantages over traditional turbochargers in hybrid electric vehicles, given their significant electrical power.
The main goal of an E-turbo is to run the system more efficiently and provide a faster response during idle or low RPM conditions. They accomplish this by positively pressurizing the air at the intake to the combustion chambers, forcing more air into the chamber for each combustion, increasing the engine’s efficiency.
The challenge for the E-turbo 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 very definition of harsh environments. Any components used must be capable of withstanding those harsh conditions and thriving in them. They must be as small as possible and maintain a long and reliable life.
HVAC Compressors Application
EV and HEV electric compressors use an inverter circuit to convert a high DC voltage to an AC voltage that powers the electric induction motor of the HVAC pump. Fundamentally, this circuit involves an EMI/safety stage, a DC-Link capacitor, and then a MOSFET inverter stage. Trends in EV and HEV HVAC components include simplification, miniaturization, and increased power close to the compressor exposed to high operating temperatures. Some of the critical applications of this type of HVAC compressor include electric water heaters, air heaters, and coolant heaters that control the temperature condition of the battery and interior of the vehicle.
The Role of the DC-Link Capacitor
The DC-Link capacitor is found in power converters (in the DC part of the circuit between the input and output stages). This capacitor is critical to filter the DC voltage and store energy to provide instantaneous current to downstream circuits. The DC-Link capacitor must withstand high power, high ripple currents, and a large amount of charge/discharge cycles. Furthermore, in this particular application, they must also withstand high temperatures and harsh conditions in the engine compartment of a vehicle.
Traditionally, electrolytic capacitors have been chosen for power conversion applications due to their low cost per capacitance and high energy storage (capacitance) per volume. But the trend of higher ripple current and higher voltages is forcing designers to consider more film technology. Power film capacitor technologies bring advantages to their designs, including:
- Lower DF = lower ESR = low losses; higher current ripple current capability
- Dry construction = no concern for evaporation and C and DF degradation in time = extended life without needing replacement or continual maintenance and monitoring
- Higher voltage = reduce the need for capacitors in series; no need for balancing resistances (fewer losses and control); more cost-space efficient