Accurate Loss Calculation for SiC Devices
Ensuring Design Success in Power Electronics
Loss calculations are a critical part of power design. These include conduction and switching losses, and the impact of thermal losses.
The widespread adoption of Silicon Carbide (SiC) transistors across various industries, from automotive systems to industrial machinery, has revolutionised power electronics. These advanced devices offer significant advantages over traditional silicon (Si) counterparts, including superior switching characteristics, higher operating temperatures, and improved efficiency. However, unlocking the full potential of SiC devices hinges on one critical factor: accurate loss calculations. This article explores the importance of precise loss calculations for SiC devices and their profound impact on engineering decisions, particularly regarding component selection and overall design integrity.
Calculating Losses in SiC Devices
Losses within SiC devices significantly influence the design process. SiC MOSFETs and diodes exhibit inherent power dissipation during operation. These losses can be divided into the following elements.
Conduction Losses - When conducting current, SiC devices exhibit a small but finite resistance between the drain and source terminals, called Rds(on). This inherent resistance, which is temperature-dependent and commonly specified at 25 °C, leads to power dissipation proportional to the square of the drain-source current Id flowing through the device:
Pcond = Rds(on) * Id2
Switching Losses - Each time a MOSFET is turned on, a certain amount of power is lost throughout the process, and the same happens when the device is turned off. These losses are referred to as switching losses.
The average power dissipated is due to switching losses over time is proportional to the voltage and current between the drain and source, the switching frequency, and the sum of rise and fall times:
Pswitch = 0.5 * VDS * Id * (Trise + Tfall) * fsw
Gate Charge Losses – Gate charge loss refers to the power required to charge and discharge the MOSFET's gate capacitance over time. During each cycle of operation, both charging and discharging occur, leading to power dissipation. The average power loss due to gate charge can be calculated with the following formula:
Pgate = 2 * QG * VGS * fsw
QG is the electric charge through the gate, VGS is the gate drive voltage, and fsw is the switching frequency.
Dead Time Losses – In a half-bridge configuration, dead time represents the short period during which the high-side and low-side MOSFETs are intentionally turned off. This prevents cross-conduction, also known as shoot-through, where both transistors conduct simultaneously, causing a short circuit.
The average dead time losses over time, PD, can be calculated as follows:
PD = VD * IOUT * (Tdr + Tdf) * fsw
VD is the body diode voltage drop, IOUT is the output current, Tdr and Tdf is the dead time at rising and falling, and fsw is the switching frequency. Since SiC MOSFETs can switch faster than traditional silicon MOSFETs, they allow for shorter dead times and higher efficiency.
By estimating losses associated with different SiC device options, engineers can select components that optimize the trade-off between efficiency, cost, and thermal management requirements.
Thermal Management and Enclosure Design
Loss calculations directly influence thermal management strategies and enclosure designs for SiC-based applications. Since SiC devices generate heat during operation, efficient heat dissipation is essential to ensure reliable and long-term device performance. Loss estimates derived from the methods outlined above provide a critical foundation for designing effective cooling systems.
One of the most important thermal calculations involves determining the maximum junction temperature Tj of the SiC device. This parameter is crucial as it dictates the maximum allowable operating current and switching frequency.
The following equation relates device power dissipation PD to junction temperature, ambient temperature Ta, and junction-to-ambient thermal resistance θja.
Tj = Pd * θja + Ta
Thermal resistance θja is a device-specific parameter typically provided in the manufacturer's datasheet. It quantifies how easily heat flows from the SiC chip (junction) to the ambient environment. A lower thermal resistance indicates better heat transfer. For example, Nexperia’s SiC MOSFETs are designed with low thermal resistance packaging, enabling engineers to optimize cooling system designs and improve long-term reliability.
By calculating the expected power dissipation based on loss predictions, engineers can select appropriate cooling solutions, such as heat sinks or fans, to maintain the junction temperature within safe operating limits.
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With the ongoing shift to renewable energy and the electrification of transport and industry, the demand for higher power and greater energy efficiency in electronics is increasing. Wide bandgap technologies including SiC and GaN bring many advantages.
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Early integration of thermal considerations into the design phase is paramount. This approach allows engineers to optimize the size and cost of the cooling system while ensuring device reliability. Conversely, underestimating losses can lead to insufficient heat dissipation, resulting in premature device failure or derating (reducing operating parameters) to prevent overheating. This can significantly impact system performance and incur additional costs for redesigning the cooling solution.
The enclosure is essential in managing thermal resistance. The selection of materials with high thermal conductivity, such as aluminium, facilitates heat conduction away from the device. Additionally, integrating heat sinks equipped with fins and designed for proper airflow enhances the surface area for heat dissipation, thereby reducing the thermal resistance junction-to-ambient (θja). Furthermore, employing forced convection methods, such as fans, can significantly improve heat transfer by actively moving air across the heat sink.
When designing the enclosure, it is imperative to consider several factors. First, the enclosure size and cooling system must be adequate to handle the expected heat generation of the SiC MOSFET, ensuring proper power dissipation. Additionally, proper airflow within the enclosure must be maintained to prevent the formation of hot spots. Finally, the ambient temperature and humidity should be taken into account, as these environmental conditions will influence heat dissipation.
Tools and Challenges in Loss Calculation
Calculating losses in SiC devices presents engineers with practical challenges. Device characteristics can vary depending on the manufacturer and specific model. Additionally, factors like operating conditions significantly influence loss profiles.
Several tools and methodologies can be employed to overcome these challenges and achieve reliable loss estimations.
Datasheet Information - Manufacturer datasheets typically provide valuable loss data, including thermal resistance, junction temperature, and figures of merits at specific operating conditions. These values can serve as a starting point for loss calculations. However, datasheet losses are often measured under specific test conditions that may not perfectly reflect real-world applications, requiring the application of appropriate correction factors.
Simulation Software - Powerful circuit simulation software allows engineers to model SiC devices and their behavior within the designed circuit. These tools can incorporate user-defined loss models and operating conditions to predict switching and conduction losses with greater accuracy. Popular options include SPICE-based simulators and specialized power electronics design software packages offered by various vendors, including:
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STPOWER Studio (part of eDesignSuite), a dynamic electro-thermal simulation software for STMicroelectronics’ STPOWER devices
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Elite Power Simulator, a tool providing PLECS models for accelerating the design with onsemi’s EliteSiC, Field Stop 7 IGBTs, and PowerTrench T10 MOSFETs.
Empirical Testing - For critical applications or situations requiring the highest level of accuracy, empirical testing can be a valuable tool. By constructing a prototype circuit and measuring actual device voltages, currents, and temperatures, engineers can obtain real-world loss data specific to their application
In conclusion, integrating precise loss calculations into the design process optimizes efficiency and enhances the reliability and longevity of SiC-based systems. As SiC technology advances, it will be paramount in advancing the performance and cost-effectiveness of power electronics, and in driving innovations in automotive systems, industrial machinery, and beyond.
For information on SiC devices talk to the Avnet Silica Power Specialists.
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