Thermal management implications for SiC-based designs
Silicon carbide (SiC) -based switching devices characteristics are key to meeting the challenges of solar inverters and electric vehicles, say onsemi’s Didier Balocco and Paul Klausner.
As the world seeks environmentally sound energy solutions, electric power is rapidly rising in importance due to its ability to replace fossil fuels which have a finite availability and are demonstrated to be harmful to the environment. For decades, the semiconductors that form the basis of every electronic device have been largely silicon-based. As design goals become ever more challenging - particularly in respect of efficiency, power density and size - new wide bandgap devices are replacing silicon in many designs.
Two application areas that have become popular and show potential for significant additional growth are solar power and electric vehicles (EVs). Both of these application areas require an inverter. In solar this is used to convert the DC voltage from the photovoltaic (PV) panels into an AC grid voltage. In EVs, the battery voltage feeds an inverter that provides a multi-phase drive to the traction motor.
One of the primary ways to increase power density and reduce size / weight is to generate less waste heat during operation. This is achieved through more efficient designs which, in turn, is heavily reliant upon the semiconductors used in the design. Size and weight are primary considerations for inverters as they may be rooftop-mounted in solar power installations, while in EV applications space is always limited, and any additional weight reduces the all-important vehicle range.
In any semiconductor-based switching design, which includes inverters, there are two primary types of loss, i.e. conduction and switching. The conduction loss is proportional to the on-resistance of the semiconductor device and is calculated as the product of the drain current and drain source voltage (ID * VDS).
When comparing the voltage drop (VDS) characteristics of an SiC FET against a silicon (Si) IGBT, it is noticeable that VDS is generally lower for a given current in the SiC device. It is also worth noting that, unlike IGBTs, VDS in SiC FETs is directly proportional to ID, meaning that it is significantly lower at lower currents. This is significant in modern power applications as it means that efficiency is significantly better, with lower losses and heat generated, even at lower power levels where solar and automotive inverters spend a significant proportion of their working life.
The switching losses in a power system relate to the amount of charge required to switch the device. This gate charge (Qg) is required every switching cycle and is larger for Si MOSFETs than for SiC devices. As designers increase switching frequencies – so that the size and weight of magnetic components can be reduced – the difference in switching losses between Si and SiC devices becomes more pronounced.
One aspect of the heat created by losses is its impact on component density, which will directly impact the size of the end application. Components do not only raise their own internal temperature, they raise the ambient temperature of the entire application. Ensuring that the temperature rise does not lead to component failure relies on thermal management at a system level.
There are many factors affecting the amount of thermal management required in any design. While the amount of waste energy being dissipated due to losses in the circuitry is probably the most significant factor, maintaining a safe operating temperature is the ultimate aim.
SiC devices are able to operate at higher frequencies and temperatures than Si devices. This tolerance for higher operating temperatures means that the need for thermal management is reduced because it is permissible to allow a greater thermal rise in the devices themselves. This means that when comparing a Si-based design with the equivalent SiC-based design, there is a significantly lower thermal management burden, based on the combination of lower conduction and switching losses as well as the higher operating temperatures.
Primarily there are two approaches to thermal management, active or passive. The passive approach uses heatsinks in various forms to channel excess heat from the heat generating device to the ambient air. The capacity of a heatsink increases as its size increases and its ability to dissipate heat rapidly is proportional to the available surface area, which explains the sometimes complex designs that aim to get the greatest surface area in the smallest volume.
Active approaches generally involve some form of air-moving device such as a fan. As these strategies use forced airflow, they can produce a greater cooling effect within a smaller space. However, there are some inherent drawbacks including the reliability of the fan itself, the necessity for an opening in the inverter housing to let air in and out and the fact that the fan will also need electrical energy to operate; all of which will have an impact on overall system efficiency.
By way of comparison, a typical SiC diode exhibits 73% lower losses than the equivalent silicon diode when operating at 80kHz. As a result, in the high power inverters found in solar applications and EVs, the efficiency benefits of SiC devices will have a very significant impact in reducing the thermal management needs of power systems, potentially by 80% or more.
Total cost comparisons
Although SiC devices have been shipping to customers for some time, the perception that SiC-based designs will ultimately cost more than a Si-based design has slowed the adoption in some quarters. However, a direct comparison of the relative cost of Si and SiC devices without considering the impact of each technology on the overall system cost can lead designers to the wrong conclusion.
Consider a Si-based power solution at around 30kW. The semiconductor devices used for switching add up to about 10% of the bill of materials (BoM) cost. The primary passive components (i.e. inductors and capacitors) account for the majority of the remaining expenditure, at 60% and 30% respectively.
While it is true that SiC devices have a unit cost that is greater that their Si equivalents, the performance of the SiC devices allows the values of the inductors and capacitors to decrease by 75%, significantly reducing the size, weight and cost. This alone can reduce the total SiC-based BoM to a point that is lower than the equivalent Si-based solution. As explained, however, the cost of thermal management in a SiC-based solution is also significantly lower. Adding in this cost saving means that the SiC design is more efficient, smaller, lighter and lower cost by some margin.
The latest 1200V and 900V N-channel SiC MOSFETs from onsemi, for example, include a fast intrinsic diode with low reverse recovery charge to significantly reduce losses, even when operating at higher frequencies. The small chip size helps with high frequency operation as does the reduced gate charge (as low as 220nC) which lessens switching losses.
Available with ID ratings up to 118A, these new SiC MOSFETs improve overall system efficiency and EMI while allowing designers to use fewer (and smaller) passive components. Where higher current handling is required, the devices can also be configured to operate in parallel, due to their positive temperature coefficient / temperature independence.
With power designers being challenged to deliver designs and systems that are more efficient, reliable and smaller, they are looking to new technologies such as SiC to help them meet these challenges and also to achieve a lower total cost.
SiC-based switching devices offer designers the ability to operate at higher temperatures and frequencies with lower losses which are all key in meeting these challenges. Furthermore, these electrical performance benefits mean that the thermal management requirements and component values of passive devices are significantly reduced, delivering further cost and size / weight reductions. As a result, SiC designs are able to achieve higher levels of performance than Si ones at a reduced size and cost.