Posted on 12 September 2014

Semiconductor Advances for Lower-Cost EVs

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The world is getting ready to drive electric. Established car brands are adding new plug-in hybrids and full EVs to their ranges, and even supercar manufacturers are taking advantage of electric drive. Governments are also backing the move: a recent EU draft agreement proposes new national minimum targets for publicly accessible EV charging points, as part of a package of measures to help low-emissions vehicles move freely on the EU’s roads.

By Jack Marcinkowski, Sr. Manager - TMA, International Rectifier Corp.

Even with such favourable terms on offer, a plug-in hybrid or EV today is still more expensive than a comparable vehicle with a conventional petrol or diesel engine. Consumers need car makers to find other ways of reducing the price differential, if sales of EVs are to increase.

Barriers to Cost Reduction

Although the battery pack is typically the single most expensive part of an EV, battery prices are falling relatively slowly, if at all. Technological development is currently aimed at improving energy density and power delivery for greater range and better performance.

The second most expensive component is the traction inverter. Within the traction inverter, semiconductor power modules account for as much as 25% to 30% of the total cost of the unit, whereas the cooling system represents around 15% to 20%. Reducing the cost of these power modules could have an important impact on the overall price of the vehicle.

The extreme demands placed on an EV traction inverter expose limitations in thermal performance that present a barrier to significant cost reduction. The key challenge lies in extracting heat from the dies of the power semiconductor switches, which are usually IGBTs and Diodes or MOSFETs, to prevent overheating leading to device failure.

Conventionally, the die is attached to a DBC (Direct Bonded Copper) substrate, using either solder or sintered metal as the die-attach medium. The top-side of the device is used only for electrical connections, which are implemented with wirebonds. The bottom side is used for electrical connection and heat transfer. The DBC substrate is attached to a thermal baseplate and a heatsink. The baseplate may also incorporate Direct Liquid Cooling (DLC).

The heat transfer achievable through the bottom side of the die only is inadequate to allow full utilisation of the device’s switching capabilities. This forces designers to connect multiple devices in parallel to carry the maximum inverter current, which prevents significant cost savings. Moreover, since die size is closely related to device cost, the reduction in heat transfer associated with any reduction in die area of the latest generations of devices still prevents designers from full utilization of the devices.

Figure 1 shows how various combinations of DBC, baseplate and DLC, with single-sided die cooling, yield only limited reduction in thermal resistance from junction to coolant (Rth j-coolant).

Established cooling techniques yield only incremental improvements in thermal resistance

The addition of a baseplate and DLC, and choice of soldered or sintered die attach, deliver only incremental improvements that will not enable the cost of the module to be reduced significantly and instead may lead to cost increase.

Two-Sided Die Cooling

A real breakthrough can be achieved by using both sides of the semiconductor device to remove the heat. This is possible only by eliminating the traditional wire bonds. International Rectifier has developed an innovative packaging concept that reduces the thermal resistance by as much as 50%, and also improves reliability by an order of magnitude by eliminating the wirebonds.

The basic building block of this package is IR’s COOLiR2Die™ surface-mounted power switch, which comprises an IGBT die and a matching diode die mounted on a ceramic substrate. The resulting device has a voltage rating of 680V and current rating of 300A in a compact, 29mm x 13mm x 1mm assembly.

COOLiR2Die™ – a ”very large die discrete SMT component”

Two types of COOLiR2Die™ have been developed. As figure 2 shows, one has the emitter (E) side of the IGBT and the Anode (A) side of the diode attached to the substrate, while the other has the collector (C) side of the IGBT and the cathode (K) side of the diode attached to the substrate. Having both of these packages available simplifies inter-switch connections when building Half-bridges, H-bridges or custom power circuits. Typically, the COOLiR2Die building blocks are attached to a ceramic substrate to achieve best heat-transfer characteristics through the top and the bottom of the assembly.

Depending on the type and thickness of the substrate, junction to case thermal resistances of approx. 0.24°C·cm2/W are easily achievable for both the bottom and the top side of the package. Further improvement is possible by using thinner substrates or substrates with higher thermal conductivity. The main improvement however comes from using both sides simultaneously to cool the semiconductor die.

Breakthrough in Amps/mm2

With the COOLiR2Die technology, the heat transfer takes place through the bottom and top sides of the semiconductor switch, as shown in figure 3.

Dual-side cooled power switch cross-section and heat-flow paths

Theoretically, the thermal transfer capability can be doubled and the thermal resistance between the device and the coolant can be reduced by 50%. It is worth mentioning that this improvement can be achieved with existing thermal transfer and cooling materials and techniques. No other improvements are needed.

Even if cost-related factors or other practical constraints prevent designers maximising the performance of the top-side heat transfer path, a substantial improvement in heat transfer can still be achieved.

For example if the top-side cooling path is only 50% as effective as the bottom-side path [Rth j-coolant (top) = 2x Rth j-coolant (bottom)], the total Rth j-coolant is 33% lower than with bottom-side cooling only.

Lowering the overall thermal resistance from junction to coolant allows for better utilisation of the semiconductor die area. This reduces the total die area needed to achieve a given current rating, resulting in lower bill-of-materials costs. Figure 4 illustrates that a saving of up to 38% die area is possible, if the top-side cooling can be made as effective as the bottom-side cooling.

Die Area reduction potential due to reduced Rth junction coolant (assuming constant Irms current)

Alternatively, the same die area can support a higher current rating, allowing designers to build higher-power modules without increasing cost. Higher PWM switching frequencies can also be used. Figure 5 shows that the module current (Irms) can be increased by up to 61%.

Potential increase of switch current rating with reduced Rth junction coolant (assuming constant die area)

At the same time, the reliability and lifetime of the power module are increased since better heat transfer reduces the magnitude of temperature swings thereby reducing stress on the semiconductors and the packaging structure. The power-cycling ability of the dual-side cooled module is also increased.

Conclusion

Improved heat transfer with dual-sided cooling of IGBT switches delivers a breakthrough in cooling efficiency, in contrast to the incremental improvements currently achieved by refining single-sided implementations. This enables a significant increase in the current density of EV traction inverters, enabling smaller devices to be used for the same current rating, or boosting current-handling capability for a given die size and cost.

The bottom line is more Amperes per Dollar, enabling electric vehicles to move closer to price parity with conventional petrol or diesel cars.

 

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