Primarily, charging can be either AC or DC. The battery requires DC to charge, so, in AC charging, a conversion is needed between the charging socket and the battery, while with DC charging the vehicle is plugged directly into DC that can be fed to the battery. The various ‘levels’ relate to the amount of power that can be delivered, and this, in turn, relates to the power source. The higher the level, the more power that’s available and the shorter the charging time.
The other key difference is where the charger is located. In DC charging, the charger is external to the vehicle and all power conditioning (including rectification) is done outside the vehicle. DC charging tends to have the highest power ratings and is used for many commercial / public charging stations, such as those found on the forecourts of gas stations or beside the highway.
AC charging typically includes an on-board charger that moves around with the vehicle. The AC charger receives the mains via a charging cable and connector and converts this to a DC voltage at the appropriate level.
Charging typically breaks down into three types, based upon the activity / lifestyle of the vehicle user. ‘Main harbour’ charging refers to charging at or near a home or workplace while ‘destination’ charging includes charging when the vehicle is parked somewhere while an activity takes place. Examples include restaurants, shopping malls, sports stadia, etc. Both of these categories typically provide AC power and rely on on-board charging while the final charging type (‘range extension’ charging) uses DC power at very high levels. Range extension charging is similar to fuel stations and allows vehicles to be rapidly recharged during a journey.
Benefits of on-board charging
The major benefit of on-board charging is that it uses readily-available AC power and, via an extension lead, the vehicle can be plugged into any of the billions of outlets installed in every building.
Level 1 AC charging makes use of a single-phase supply and it’s this that limits the power available to around 1.9kW with a 120V supply, and 3.7kW with a 220-240V supply. This is the most common type of charging available in private residences.
Businesses, however, often have three-phase electricity available and using this for charging will increase the available power to around 20kW which offers far more rapid charging than Level 1.
AC charging is the most flexible, as charging points are available and can provide the entire charging needs for some users, depending upon their lifestyle and how the vehicle is used. If the vehicle is solely used for commuting during the day, then charging the vehicle overnight is very convenient. AC charging is far less suited to range extension charging, where the distance to travel exceeds the range of the vehicle, as the charging times are simply too long.
Requirements for an on-board charger
The primary role of an on-board charger (OBC) is to manage the flow of electricity from the grid to the battery. This means that the OBC must comply with the requirements of the grid in locations where it will be used. The primary requirement is not to inject reactive power back onto the grid, which is achieved by having a power factor (PF) of >0.9. The OBC must also suit the types of charger available, meaning that it has to support single-phase and 3-phase operation.
There will also be requirements to provide isolation from the power source and a maximum current that the grid can deliver, which must be factored in to the design. As with all power systems, there is a potential to generate electromagnetic interference, so all relevant EMC standards must be complied with. At this power level, the ability to communicate with the grid is also necessary.
Figure 3: A Block diagram of a typical on-board charger
Because the OBC is permanently mounted, the weight must be minimised, to reduce its impact on the range of the vehicle. Efficiency is also important, and there are other benefits to efficiency too, such as requiring less thermal management which will reduce the size, weight and cost of the OBC.
In future, it may be possible to use the vehicle as a portable energy store, using the energy stored in the battery to power the home during times of peak demand or high electricity costs. The battery would then be replenished at times of cheaper electricity. This would save money for the home owner and help the electricity companies by balancing the load on the grid. In order to facilitate this, the OBC would need to return energy to the grid through an inverter.
The benefits of wireless charging could apply equally to vehicles as they do to smaller portable devices such as smartphones or tablets, especially to add a ‘top up’ charge to extend their range. Wireless technology will be particularly appropriate to vehicles that follow pre-determined routes, or often wait at a specific place, because the charging stations will be fixed. This would include buses that follow specific routes stopping at the same bus stop and taxis that wait at a taxi stand (perhaps at an airport or railway station).
Design challenges
Designing for power systems is generally a constant challenge to meet high efficiency requirements in a small space and this is particularly true in the case of OBCs. The efficiency of the OBC is particularly important as it can reduce charging time.
The more efficient the OBC design, the less waste heat is generated during charging which reduces the need for thermal management. If it becomes necessary to add heatsinks to an OBC design then the size and weight will increase, neither of which are desirable within the confined spaces of a modern car – especially as weight decreases the overall efficiency of the vehicle.
Regulatory compliance is a challenge with OBCs, particularly with the need to meet power factor rules for the grid when the vehicle is plugged in for charging. Commonly a boost converter is used for power factor correction (PFC) to rectify the AC input and deliver a high-level DC voltage to a DC-DC converter that’s used to charge the EV battery.
Any design for an automotive application has to take account of the harsh environment of the vehicle. Designers are required to produce designs that are able to cope with prolonged vibration, heat, cold and significant amounts of conducted and radiated electrical noise.
Passive components for OBCs
Passive devices such as magnetic components and capacitors play a pivotal role within all aspects of OBCs. The boost converter that forms the front-end of the PFC stage will contain a common-mode EMC filter, filter capacitors, PFC coils and a DC Link capacitor that provides a charge reservoir between the boost stage and the DC-DC converter.
LLC converters are widely used in industrial and consumer applications and, although there is no specific output choke used, magnetic components are used for an isolating transformer and output EMC filter, along with various capacitors.
The potential to adopt wireless charging brings a wider requirement for passive components including the coils for power transfer (transmit and receive), such as the one shown on the right, and proximity detectors to ensure that the vehicle is correctly aligned with the charger.
While many recent advancements in power electronics have been centred on semiconductor devices such as MOSFETs and IGBTs and their associated control, very few of these would be able to achieve their full potential without corresponding increases in performance of the passive devices that they rely on as well as connectors and cabling.
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In many applications, multiple resistors are used in parallel, simply to handle the required power dissipation. While this provides a solution at the circuit level it increases the component count, cost and board space required – none of which is ideal in an automotive setting. One recent innovation is the first high-power resistors that are offered with an AEC-Q200 qualification (left). These 1% tolerance devices are supplied in an insulated package designed to be mounted directly to a heatsink where they are rated at up to 800 W. This high-power dissipation allows multiple lower powered devices to be replaced by a single resistor, or (due to their pulse capability) larger wirewound resistors can be replaced, saving board space.
Inductors are one component that, if not selected carefully, can be damaged by prolonged exposure to heat and vibration. However, rugged types that are qualified to AEC-Q200 are available such as metal composite power chokes that are needed for step-up and step-down operation as well as filtering. The latest versions offer high vibration resistance and are able to operate at temperatures as high as 150°C (including self-heating) while maintaining excellent inductance stability over this extended temperature range. The shielded construction virtually eliminates flux leakage, thereby minimising any EMI issues.
Summary
On-board charging is, and will remain, an important aspect of all EVs for the foreseeable future as, while it is slower than the rapid DC chargers, it allows greater flexibility to charge (or top-up) vehicles from commonly available power points. However, while the vehicle is in motion, the OBC has no function and if it’s excessively large or heavy, then all it does is reduce range.
Consequently, designers are being challenged to design OBCs that are not only highly efficient in operation but also small and lightweight. They must be able to cope with the rigours of the automotive environment (heat and vibration among others) as well as being able to be produced at a cost point that meets the aggressive demands of auto makers.
While passive components may be simple in nature, they have a pivotal role in helping designers meet these challenging objectives, and the market is developing rapidly to meet increasing demands in this space. The choice on offer is substantial, and choosing appropriately is key.
Below we’ve highlighted our leading suppliers of components for on-board charger applications.
If you require advice on selecting the right components for your design, our technical specialists are on hand to help.
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