Electric Vehicle Powertrain Hotter Than the Sun ?

I recently heard Professor Phil Mawby from Warwick University quote at the Future Powertrain Conference 2016 that the heat flux from an IGBT (the current preferred switching device for high voltage electric and hybrid vehicle drives), is the same as that at the surface of the sun which is around 400 W/cm^2. This is the second time I have heard this claim, and to be honest the first time I was sceptical, but decided it was worth some further investigation. Sure enough, on researching it further, the figures are indeed correct, so when I heard Professor Mawby mention this last week I knew he was right!

How is this possible? Surely the device should burst into flames and dissolve into a melted pile of silicon, plastic and metal?

A typical EV traction power inverter (one of the names used for the electronics module that controls the motor by switching current to its phases) will have what is known as a "Full Bridge" consisting of 3 sets of IGBTs (this is of course assuming the motor is of the 3 phase variety!). for each phase there is a pair of IGBT modules which could be single modules, or could be multiples arranged in parallel. An IGBT is a solid state switching device, they are turned on and off in sequence to control the torque and speed of the motor.

Although the electric powertrain is very efficient at around 90% compared to an internal combustion engine at less than 50%, they are not 100% efficient so there is still some energy lost as heat. The IGBT switches on and off at high frequency resulting in losses due to the internal resistance of the device which varies as it goes from open circuit to closed circuit and the high current flowing through it. There are also loses in other components such as capacitors, inductors etc.

The unique engineering challenge here is that whilst the amount of energy to be dissipated is relatively low, the devices themselves are physically very small, so that waste heat is very concentrated, giving the very high level of heat flux. The waste heat needs to be managed and dissipated to prevent damage to the switching device and other components. 

These high heat fluxes can lead to localised hot spots inside the power electronics modules and is also why the designers and manufacturers of these traction inverters will quote a relatively high minimum coolant flow rate through the controller. This is also why when coolant flow in such a system is not correctly balanced there can be issues with random air locks appearing in an apparently air free system! Ensuring a relatively high flow through the power electronics ensures that localised boiling of the coolant is avoided but must also be managed to ensure thermal stress across the module is minimised and the power consumption of the pumping devices is not an excessive parasitic drain on the finite and precious battery power.

The issue is further complicated by the need to maintain a relatively low temperature inside the power electronics module in order to maximise component life and performance, due to the thermal resistance inside the module a max coolant temperature of less than 65degC is not uncommon in order to ensure that device temperatures are maintained well below their typical 125degC max operating threshold. A more detailed explanation can be found in a whitepaper on our website.

This challenge will inevitably get bigger as the power density of the EV traction and power management systems goes up, there are some ambitious targets out there for increasing power density. This means that effective thermal design of the devices, circuit boards and heat sink's is critical to reduce the thermal resistance. As is the external cooling circuit, as the design of the inverter heat sinks become more elaborate system pressure rise increases, and problems with coolant system bleeding due to surface tension effects in what are effectively now micro channel heat exchangers emerge.

Wide bandgap devices offer some scope for electronics operation at higher temperatures which will allow higher coolant temperatures, which in turn makes the end heat rejection somewhat easier. However at elevated temperatures with very high heat fluxes the local thermal management around the devices becomes even more critical leading to very complex cold plate designs and the need to move away from solder based circuit construction methods.

Another challenge in the EV is how to keep the occupants at the right temperature without excessively using energy, as whilst there is a high heat flux the total amount of energy available and the ability to convert it into useful heat for the occupants is limited!

AVID is at the forefront of efficient thermal management system design for Electric and Hybrid vehicle powertrain. We are pushing the boundaries of system efficiency and effectiveness with efficient electric pump and thermal system design including "inside the box" and external thermal management.

About the author; Ryan Maughan is the Managing Director of the AVID Technology Group Ltd. AVID is based in the North East of England and is a leader in the design and manufacture of electrified ancillary and thermal management systems for electric, hybrid and conventional vehicles and machinery. The companies high performance electric motors and electronics power pump, fan and heat exchanger solutions provide the thermal management for many leading high performance and heavy duty electric and hybrid vehicles.