Discrete Versus Module IGBTs for Automotive Traction Inverters


Automakers naturally strive to improve the efficiency, performance and reliability of each new model in their hybrid electric vehicle (HEV), plug-in hybrid electric vehicle (PHEV) and all-electric vehicle (EV) fleets. Accomplishing this requires considerable effort and automakers turn to their power electronics suppliers since these devices play a key role in improving the overall efficiency, reliability and performance of automotive powertrains.

Fairchild has been a top supplier of automotive-grade power electronics for over two decades, and one of the key decisions we see designers having to make is between using discrete components or power modules for a power stage. The choice between using discretes and modules is natural in any automotive power electronic application, including traction inverters.

A core component of all HEVs, PHEVs and EVs, traction inverters convert the battery’s electricity from DC into the three-phase AC required to control the torque and/or speed produced by the motor. Though the overwhelming majority of traction inverters in production use modules, the number of those using discrete IGBTs is growing.

Does this mean that discrete IGBTs will eventually supplant modules in traction inverters and other powertrain components?

No. Though discrete IGBTs have several compelling advantages, modules have their own advantages too. Given this situation, it is unlikely that one will eliminate the other.

So, when should you choose one over the other for a new traction inverter design?

The answer depends on several factors.

One consideration is the power level required for your traction inverter. If you know with exact certainty what the level needs to be, then going with the appropriate module makes sense. However, if there is any uncertainty, employing discrete IGBTs is a better option due to their inherent flexibility. This is because discrete IGBTs can be easily added or removed to increase or decrease the current, which allows an automaker to adjust a traction inverter design during the development phase.

The flexibility of discrete IGBTs is particularly advantageous when it comes to customization, which is another factor to consider.

For example, discrete IGBTs makes sense if your traction inverter is for a modular, baseline powertrain meant to support different types of electric vehicles with differing power levels. This single powertrain can be customized for specific vehicles simply by adding or reducing the number of discrete IGBTs.

On the other hand, there is little need for customization if the powertrain will not be used in multiple vehicles with a wide range of efficiency and performance requirements. Here again the module shines.

Having an appropriately staffed engineering team well versed in power electronics is another important consideration. The flexibility and customization advantages of discrete IGBTs are compelling, but increase the complexity of the design. Consequently, engineers must be sufficiently experienced in such power electronics topics as thermal design (to efficiently spread and remove the heat), PCB layout design (to optimize current sharing between the parallel IGBTs) and manufacturing techniques (to ensure reliability in high volume production).

From a system-level standpoint, designing with modules is typically easier than with discrete IGBTs. This is because the power stage is contained in the power module and the designer doesn’t need to focus on minimizing parasitic inductance or power stage layout, but can focus on the heat sink and cooling system.

Modules also generally allow for a smaller total system form factor, which can be an important differentiating factor as designs become standardized and markets get crowded.

So if flexibility and customization are key requirements, and you have an engineering staff with the right experience, then discrete IGBTs are for you. Conversely, modules are the way to go if your product design does not have any need for flexibility or customizing, or if space is tight.

Cost is another factor to consider. From a component standpoint, power modules are more expensive than discrete components. From a system-level standpoint, traction inverters developed with discretes can be comparable in cost to those which are developed with modules. This is because discrete solutions typically require electrically insulated thermal interface material, complex busbar designs, component matching and a more complex assembly process.

Complementing their design flexibility, discretes are also more flexible from a supply chain standpoint as they are usually available from multiple suppliers, so automakers can have a second source. On the other hand, automotive-qualified power modules from different suppliers are usually not pin-to-pin compatible. This implies that once a module is designed into a certain traction inverter, replacing it with a module from a different supplier will require a system-level redesign.

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