Semiconductors for Compact and Cost-Efficient on-Board Chargers
On-board chargers (OBC) are required in any fully electric or plug-in hybrid vehicle. This article explains what factors are important in the development of an OBC and how these can be achieved with suitable semiconductor components.
Battery electric and plug-in hybrid vehicles will shape the street scape much more strongly in the next few years than they do today. Since these vehicles have no, or very low, local CO2 emissions according to WLTP, manufacturers are increasingly relying on the sales of these vehicles in order to achieve the CO2 emission targets in the EU.
Both types of vehicles use lithium-ion battery packs with nominal voltages typically in the range of 350V – 360V. Today the battery capacities of new electric vehicles are between 40kWh and 80kWh, allowing ranges up to 400km. Plug-in hybrid vehicles integrate smaller battery packs that are around 10kWh since the pure electrical range usually does not exceed 50km.
In the case of the electric vehicles, the charging of this battery by an external source is a basic requirement; with the plug-in hybrid, charging by an external source is in addition to the charging by energy generation of the internal combustion engine.
Regarding the charging infrastructure, three basic types of charging can be distinguished: charging at the regular location (“main harbor charging”), charging at the destination (“destination charging”) and charging to extend the range (“range extension charging”). [Fig. 1]
Charging scenarios and charging power
Main Harbor Charging refers to charging at locations where the vehicle is usually kept, typically at home and at work. It is where the OBC comes into play. Since the vehicle is typically at the charging point for several hours, the charging time plays a minor role in the overall charging ecosystem. This is reflected in the installed charging power, which seldom exceeds 2,3kW (230V, 1-phase, 10A) or 3,6kW (230V, 1-phase, 16A) today. However, these power levels are only sufficient to charge either plug-in hybrid vehicles or electric vehicles with batteries less than 40kWh. To charge batteries with capacities greater than 40kWh overnight, or during working hours, the charger will require an output power of 11kWh, or more, which necessitates 3-phase power inputs.
Destination Charging occurs at locations such as supermarkets, shopping centers, movie theaters, etc. It lasts for shorter periods of time than Main Harbor Charging, and it is used to partially recharge the battery. In order to sufficiently charge the battery in this limited time higher charging power is required. Today the typical charging station at such locations has an output power of 22 to 50kW, and the AC to DC power conversion is integrated into the charging station, especially at the higher powers.
Range Extension Charging is comparable to refueling the classic automobile. In the shortest possible time the battery is charged for longer distances. Today, charging power in the range of 50kW to 120kW is installed. Over the next few years up to 350kW are planned to charge even larger batteries,for example 80kWh in less than 20 minutes. For this type of charging only DC charging columns with integrated voltage conversion are used.
Since the OBC combines the conversion from AC to DC voltage and the control of the battery charging it needs to ensure compatibility with single and three-phase power networks in addition to the various required communication protocols. At the same time, active power factor, isolation and EMC requirements need to be met in the input side, as well as supplying the proper charging current and voltage on the output side.
More and more is being discussed about electric vehicles being used as energy storage for the domestic solar installation, in order to capture excess power generated during periods of strong sunlight and supply this power to the home later. This requires bi-directionality in the OBC: in addition to the charging function for the battery in the vehicle, the OBC must also provide its energy in the other direction to the outside. However, such systems are rarely used today.
Optimize power density and costs
What parameters are to be considered when developing an OBC? Space plays a big role – especially in plug-in hybrids. Therefore, the power density of the OBC should be as high as possible. At the same time, it is important to keep the costs as low as possible. This is especially true because the OBC is not a differentiator for the vehicle manufacturer. Cost pressure often means that vehicle manufacturers are not willing to pay more for higher efficiency.
An onboard charging system consists of two subsystems: the active power factor correction stage (PFC) and the DC-DC voltage converter (DC-DC) [Fig. 2]. The PFC stage secures an active power factor of >0.9 on the line side, implements the rectification of the AC line voltage to an intermediate DC circuit voltage and communicates with the charging station. The DC-DC converter controls the battery charging current and separates the mains from the battery side by galvanic isolation.
In principle, various topologies are available for each section. However, these are usually limited to a few in the automotive sector in order to meet the above-mentioned requirements:
- For PFC: Boost converter, Totem Pole
- For DC-DC: LLC, ZVS Phase Shift
In addition to the choice of topology, the focus in system design is also on the power semiconductors. In addition to the passive power components, these have a major influence on power dissipation, maximum achievable switching frequency, and robustness. This in turn directly affects power density, cost, and efficiency. For example, reducing power dissipation reduces the size and cost of the cooling system and the OBC as a whole, and also increases efficiency. Higher switching frequencies allow the use of smaller passive components.
Each of the circuit topologies mentioned above require different types of semiconductors. For example, due to the high switching frequency of an LLC converter (up to 500 kHz) MOSFETs are the best choice. A totem pole PFC operating in continuous conduction mode requires fast switching IGBTs due to hard switching of the devices. Silicon carbide (SiC) MOSFETs and diodes can, in principle, facilitate higher efficiency.
Infineon Technologies offers the full range of power semiconductors specifically designed for in-car on-board charging systems.
Automotive CoolMOSTM CFDA and Automotive Driver IC AUIRS2191S
Resonant DC-DC topologies such as LLC or ZVS Phase Shift are operated with switching frequencies up to 500kHz and thus require the fastest possible commutation between high and low-side switches. Typically, these topologies use soft switching of the MOSFETs (i.e. either in zero-voltage or zero-current switching mode). However, under certain circumstances, a hard commutation of the MOSFETs may occur. Often, the reverse recovery of the MOSFET body diode is not taken into consideration, and this can lead to the desired switching frequency or load current not being achieved. However, if a hard commutation occurs in the field, the fast removal of the reverse recovery charge could destroy the MOSFET in extreme cases. So-called “fast body diodes” help here. Infineon’s CoolMOSTM CFDA technology significantly simplifies the design by providing a fast body diode with a recovery delay 40% below average. Fig. 3 shows the resulting turn-off behavior close to the ideal switch: no overvoltage peaks occur, and oscillations remain negligible. Thus, efficient resonant topologies with high switching frequencies can be realized reliably over the lifetime without having to invest in additional filter circuits. In addition, the good controllability of the CoolMOSTM CFDA with different gate resistance values simplifies circuit design and reduces the burden of evaluation testing.
In order to minimize the total bill-of-material costs of the driver circuits, it makes sense to use integrated driver ICs. The AUIRS2191S is the perfect complement to the CoolMOSTM CFDA based products in resonant topologies. With the two channels for a high-side switch and a low-side switch, the driver stage for a half bridge can be realized in a space-saving manner. A typical propagation delay of 90ns and driver current of ± 3.5A are sufficient to drive power semiconductors up to 3kW up to 200kHz without additional components.
Automotive TRENCHSTOPTM 5 Fast Switching IGBTs and Automotive Gate-Driver IC AUIRS2191S
Totem Pole based PFC stages are often used to enable bidirectional charging or increase efficiency with acceptable overhead. As mentioned above, operation in the CCM necessitates the use of fast switching IGBTs. Infineon’s TRENCHSTOPTM 5 technology products are ideally suited for this, as they each have the lowest switching and forward losses in their class, allowing for the highest possible efficiency. The switching frequency also plays a further role in component selection: Totem pole stages are usually operated at 70kHz, TRENCHSTOPTM 5 supports frequencies up to 120kHz and thus ensures reliable operation over life. In order to keep the circuit complexity low, the AUIRS2191S can be used again as a driver stage.
Automotive CoolSiCTM MOSFET and Automotive Driver IC AUIRS1170S
Efficiency improvement plays a major role today, if it increases the power density and thus space can be reduced, assuming the increased costs are acceptable. In the future, it is quite possible that new energy usage criteria for electric vehicles will also include the energy usage of the OBC itself. In China and Japan, discussions are currently taking place on this topic – in order to be able to present a more correct overall consumption of electrical energy per electric vehicle. This would mean that efficiency becomes more important in OBC than it is today, even justifying additional costs.
Silicon carbide, compared to silicon as a semiconductor base material, offers the advantage of producing active power devices with lower switching and forward losses and thus enabling higher efficiency. Taking as an example a Totem Pole PFC stage, today with the use of the latest fast switching TRENCHSTOPTM 5 IGBTs with integrated Rapid-Diode over 97% efficiency can be achieved. By replacing the rapid diodes with the current generation CoolSiCTM diodes from Infineon, this can be increased to values above 98%. Even higher values can be achieved with a completely SiC-based implementation, using CoolSiCTM MOSFETs instead of the TRENCHSTOPTM 5 IGBTs, but the cost-benefit ratio is significantly less favorable. Fig. 4 shows the efficiency improvement over a power range up to 3,3kW, CoolSiC MOSFET 650V scaled from 1200V.
Another possibility for improving the efficiency is provided by the step from passive to active rectification on the secondary side of the DC-DC converter. At the same time, bidirectional operation is made possible. In contrast to passive rectification with diodes,the associated heat sink is usually not required. This benefits the space and reduces costs in production. Infineon’s AUIRS1170S is the only driver component in automotive electronics that was specifically developed for synchronous rectification. The drain to source voltage measurement on the power semiconductor switch integrated intodriver eliminates the otherwise necessary activation by a microcontroller. Measurement results with the CoolMOSTM CFDA technology show an improvement of 0.5% compared to passive rectification.
AURIX™ Microcontroller
Electric and plug-in hybrid vehicles are connected to the outside world via a communication interface in the OBC;via Ethernet, CAN (FD) or Power-line, the OBC communicates with a charging station or wallbox, for example, to request information about the installed power supply or to communicate the battery charge status. Like any other communications interface, these interfaces of an OBC can also be usedinappropriately. Since the OBC is integrated into the E / E architecture of the vehicle via the CAN (FD) bus, improper intrusions can have serious consequences. Therefore, the OBC must be protected by appropriate encryption algorithms and secure user authentication. Here, the widely used AURIXTM microcontroller series from Infineon with the integrated “Hardware Security Module” (HSM) offers the perfect solution;not only is the implementation of a security system possible that is deemed fully compliant to the EVITA standard, but also safe communication with AutosarSecOC can be implemented.
Integration of the high-voltage low-voltage DC-DC converter in the OBC seems technically and commercially feasible and could be used more frequently in the future. In addition to higher demands on computing power, functional safety would also be more important here. Together with the TLF35584 power supply IC, the products in the AURIXTM family enable system solutions that not only deliver the required performance, but also conform to ISO26262 to ASIL-D standards.
