High-power dc-dc converters play a critical role in the electrical systems of hybrid electric vehicles (HEVs). These complex modular components provide the link between the low-voltage (14 -V) bus and the high-voltage bus used in the power train. The latter dc bus carries a supply voltage somewhere in the 200 V to 450 V range, which powers (among other functions) the inverter that drives the electric motor. If that supply range seems wide, it's because the high-voltage dc bus, like so many other electrical characteristics, varies from hybrid to hybrid.
International Rectifier (IR) is one of the vendors that develops dc-dc converters for HEVs. According to Henning Hauenstein, program manager of IR's automotive group, these modules are designed to deliver a typical output power from 1.5 kW to 3.5 kW. With the amount of silicon required to support these power levels, the finished dc-dc converter may be nearly the size of a small shoe box. IR's converters are typically bidirectional, capable of transferring energy in either direction. However, Hauenstein notes that some car makers employ two unidirectional dc-dc converters in their HEV applications.
The choice of the dc-dc converter topology depends on the strategy established by the carmaker or tier 1 supplier that is ordering the dc-dc converter, says Hauenstein. That customer will typically specify system level parameters as well as the converter topology and switching frequency. It's then up to a company like IR to work out the details of a custom design. In the process, IR will develop the silicon devices (driver ICs, MOSFETS and IGBTs) and modules needed to complete the design.
“The dc-dc converter is one of the most customized electronic elements of the hybrid electric vehicle today,” says Hauenstein, who notes that there are many variations of dc-dc converter architectures as well as many variations of hybrid drive trains from one OEM to the next. Hauenstein contrasts this situation with that of the HEV inverter hardware, which he categorizes as more of an application-specific standard product.
Although the fully custom approach gives the power supply designer a great opportunity to optimize the power converter for the HEV, this approach is likely to change. In order to eliminate the premium on hybrid vehicles, car makers will need to develop a hybrid drive train that is robust and flexible enough to be used in several hybrid models, says Hauenstein. This demand will produce a more standardized approach to designing the dc-dc converter.
At the same time, there is pressure to shrink the power electronic modules to half or even one-third their current size. This will enable the converter to migrate from either the passenger or trunk compartment to a spot under the hood. That move multiplies the design challenge since power density will be doubled or tripled at the same time the converter is exposed to much higher temperature and vibration.
Semiconductor developers will address these requirements partly through improvements in silicon and packaging. For example, IR is currently developing trench IGBTs that will reduce switching and conduction losses from those currently obtained with its planar IGBTs. Meanwhile, the company is also working on power device packaging that will eliminate the need for the wirebonds that now attach the silicon die within power module assemblies.
Getting rid of those wirebonds will help to meet challenging customer demands for reliability. Removing the wirebonds has the secondary benefit of reducing parasitic inductance, enabling faster device switching and less ringing.
Improvements made in the dc-dc converter can have a large impact on other parts of the hybrid's electrical system. For example, hybrid vehicles commonly add a second liquid-cooling loop for cooling the inverter and the dc-dc converter. As the dc-dc converter's and dc-ac inverter's performance improves in the future, cooling requirements may be reduced. In some cases, it may even become possible to eliminate the secondary cooling loop, allowing air cooling of the power electronics.