Power Electronics Cooling: How to Reduce the Cost of Inverter Modules by Reducing SiC Area
Updated: Oct 8, 2020
Q: How can we capture the high performance of silicon carbide (SiC) while delivering a cost-effective power module?
A: Using enhanced cooling techniques to reduce the number of SiC chips, at the same total current, while sustaining safe operating temperatures.
Inverter power modules are a critical component for power conversion processes in electric vehicle traction, renewable power, and other high growth technology sectors. At the heart of these high-power inverter modules are small silicon carbide (SiC) transistor chips to provide the switching function for DC to AC conversion. While SiC offers incredible advantages over pure silicon (Si) due to its enhanced semiconductor properties, SiC chips can be very costly, often in the neighborhood of $15-50 per cm^2 of chip area.
Therefore, making the most out of each SiC chip is of utmost importance for fielding cost-competitive power modules, of which elite thermal management is a key driver. To begin understanding this, let us take a quick look at what these power modules look like under the hood:
Figure 1: Image of an inverter power module with its cover removed. Each transistor contains many chips connected in parallel to support high current while keeping junction temperatures low. Image source.
A typical three-phase inverter utilizes six switches. However, as seen by the number of wire bonds in Fig. 1, each of these switches is often made up of many chips in parallel. This is done because modern power modules are handling higher and higher currents, and the losses per chip increase as the current increases. These losses become waste heat, requiring thermal management solutions to remove the heat and operate safely under the recommended SiC junction temperature of ~175°C. For example, using only a single chip per switch would result in a very high current, a very high thermal load, and a temperature far above 175°C. Thus, more chips are added in parallel until the current is low enough on each chip to sustain the 175°C operating temperature, while still handling the application's total current.
But of course, this comes with the key tradeoff: each chip adds more SiC area, and therefore increases the cost of the power module. So, how can we reduce the number of SiC chips without the devices overheating?
One method is via better cooling. Advanced thermal management techniques can take away more heat per unit area; a perfect match for chips with higher currents. As a brief example, for a total thermal load of 5kW over 25cm^2 of SiC chip area, a cooling technique capable of 200 W/cm^2 would be required. On the other hand, by using a 500 W/cm^2 cooling technique, the SiC area requirement for a 5kW heat load would shrink down to just 10cm^2. To make matters more challenging, heat dissipation in transistor chips is nonlinear with increasing current, so the importance of high-power density cooling becomes increasingly necessary.
This then begs the question- how does reduced SiC area translate into cost savings of the module? Table 1 shows an example landscape for a few different configurations of an example three-phase inverter. Of course, as fewer chips are used, the thermal losses become higher and the thermal management technique must become increasingly sophisticated.
Table 1: Example cost savings table for a three-phase inverter (six transistor switches). Assumes 0.5 cm^2 SiC chips at a cost of $25/cm^2. The contents of this table are by no means universal and must be evaluated for each individual application.
Silicon carbide offers great benefits for increased performance out of inverter power modules, but comes at a high unit cost. By utilizing high performance cooling techniques, each chip can support higher current, and therefore, fewer chips can be used in each module. This allows power module designers to take advantage of the high performance of SiC while offering highly cost effective solutions.
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