Electric vehicles are receiving widespread attention worldwide due to their low environmental impact. The performance of EVs depends on proper interfacing between energy storage systems and power electronics converters. In order to increase range, the different car manufacturers are putting all their effort into boosting the power density of the vehicle power electronics devices. The higher the power density, the lower the volume of the parts, which means more space for the battery. However, higher power density leads to higher power losses that must be dissipated in order to maintain a proper temperature operating range.
Power density: A must to increase electric vehicle range. Increasing power density is one of today’s major EV manufacturers’ challenges. The graph above shows how lower vehicle mass and higher energy density significantly impact EVs range. Making smaller, lighter, and more compact power converters (which are electrical devices in charge of converting electric energy from one form to another) is mandatory to achieve higher car range and shorter charging time. The transformer is one of the key converter’s subcomponent, as well as one of the heaviest, so reducing its size by half can be a game changer. However, reducing transformer size is not a piece of cake and engineers must take into account variables such as mechanical dimensions, Lux Research, November 2020. core saturation, efficiency, and temperature so as to find the most suitable solution for the customer.
Limiting factors: Size, efficiency, core saturation and temperature.
While operating, inductors heat up due to power losses generated in both the windings and the core. Wire losses, caused by Joule Effect, increase with current density. The thinner the wire used, the higher the current density and copper losses. However, a thinner wire means less winding space needed and smaller final inductor size.
Core losses depend on magnetic flux density and switching frequency. The higher the magnetic flux density, the higher the core power losses and the lower the core size. Flux density cannot be increased indefinitely. Magnetic materials reach saturation when the magnetizing field is too high, which would cause inductance to drop and device failure. The following graph shows BH curve for 3C90 ferrite material. (adequate design value should not be higher than 200 mT).
In order to keep the part operating within an acceptable temperature range, heat is usually dissipated using rather convection, conduction methods, or both. In the automotive industry, cold plates are widely used to refrigerate transformers, capacitors, transformers, and many other electrical devices within the DCDC converters and OBCs (On Board Chargers). These cold plates are heavy and bulky, and parts over them must be carefully designed in order to make heat go from the heat source to the coolant. As a result, devices must include aluminum housings and thermally conductive potting compounds, making them heavier a less mechanically efficient.
If inductive components are not properly cooled, they will reach high temperatures, which may punish their performance. In addition, the higher the temperatures, the higher the power losses in both winding and ferrite. The following graphs show the temperature dependency of Rdc for different conductive materials and the ferrite core loss density at different operation points. For typical operational temperature range, in both cases, power losses increase when the temperature increases
Power losses must be dissipated to keep the inductor operating in an appropriate temperature range. If refrigeration is insufficient, the temperature can increase indefinably, causing failure. One of the most common inductor temperature failures happen when the core reaches the curie temperature.
When this point is reached, the inductor loses its magnetic properties.
Even if temperature requirements are met, efficiency requirements might not be achieved because of high losses. However, vehicle global efficiency must be evaluated rather than isolated converter electrical efficiency. Building more power-dense transformers and chokes leads to higher electrical losses. However, size reduction leads to lower part weight, which reduces car power consumption as well as leaves more free space for battery, increasing the vehicle’s global efficiency. Furthermore, more compact components in combination with the latest converter cooling technologies such as Coolgap, reduce coolant pressure drop through the part, reducing cooling system’s pump power consumption, thus improving efficiency. In addition, smaller inductors mean lower raw materials expenses, which reduces part costs.
Power converters and liquid cooling.
Historically, the vast majority of power conversion solutions have utilized either natural or forced convection cooling. However, certain application situations, such as high power levels (e.g. XFC), may dictate conduction cooling to a liquidcooled heat dissipation element as the preferred solution. As previously
mentioned, there is a widespread usage of cold plates in the automotive industry. However, this solution will soon be substituted by more efficient cooling systems that remove heat directly from the heat source of transformers, chokes, capacitors, transistors, and PCBs, avoiding the implementation of unnecessary housings and thermal potting excess. Do not hesitate to visit coolgap.net to learn more about the latest converter thermal management solutions.