Medium power applications are the “ideal application” for Wolfspeed WolfPACK power modules
【Introduction】With the increasing adoption of power electronics, design engineers are constantly challenged to create systems that are more efficient than ever before. Often, one of the major challenges is selecting the most appropriate device for a new design, which is critical to meeting converter specifications without adding unnecessary system cost. Wolfspeed understands this challenge and continues to expand our product portfolio to better meet customer needs. Our goal is to provide application engineers with a broad range of products that, when adopted, enable their designs to be more efficient, reliable, and configurable than the competition.
After more than 30 years of research and development in silicon carbide (SiC), our current portfolio includes a wide range of SiC Schottky diodes, MOSFETs and power modules covering a wide range of power requirements. Superior current-carrying capacity and lower switching losses compared to silicon (Si) transistors enable converters with higher efficiency and power density, ultimately resulting in the best solution for medium power converters (10 to 100 kW) plan. That’s why Wolfspeed recently introduced the WolfPACK™ family of power modules. This family of power modules is an ideal alternative to converters traditionally paralleled with discrete components.
Power Modules vs. Discrete Transistors
In medium power applications, discrete solutions typically require multiple devices per switch node location. Whether paralleled or interleaved, these additional components further increase design challenges in board layout, thermal management, isolation, electromagnetic interference (EMI), and reliability. A key advantage offered by power modules is that they are designed to reduce the complexity of such challenges and can greatly reduce the system design burden (especially when replacing a row of discrete transistors). Figure 1 conceptually shows the power output range suitable for the WPAC series; beyond 10 kW, the discrete solution complexity increases, making the WolfPACK series more attractive in terms of cost.
Figure 1: The WOLFSPEED WOLFPACK module is designed for power beyond the power rating of a single discrete MOSFET and simplifies thermal management and system layout design.
Typical Challenges of Discrete Solution Design for Medium Power Systems
When designing converters for discrete solution devices, designers must carefully consider the required transistor specifications such as blocking voltage, current rating, on-resistance, and switching energy. Often, a significant design issue is device selection, and discrete devices limit scalability due to packaging constraints. This means that increasing system power requirements (or designing converters with higher output power) often requires extensive redesign. Additionally, repeated device selection requires new, higher voltage/current transistors, and often new thermal management, PCB layout, and mechanical design to accommodate the package.
Choosing to use more transistors in parallel creates a new set of challenges. For example, the adoption of new devices and their thermal management and peripheral components such as gate drivers and passive components will require more space. Additional layout challenges arise due to higher losses, voltage overshoot, and reduced lifetime due to inductance imbalance between parallel transistors. In other words, dramatically scaling the output power of a discrete solution converter is as challenging as designing a new converter.
As converters aim to achieve greater power density through higher switching frequencies, the challenges of designing discrete devices also increase. Faster slew rates required to achieve high switching frequencies can introduce EMI to the control system, especially if the PCB is poorly designed. Electromagnetic interference can directly cause transistors to turn on incorrectly, which can lead to additional losses, shortened device life, and even converter failure. Faster slew rates also increase gate driver cost because of the large amount of isolation required to protect sensitive control systems from power transients. The cost of such gate drivers is also related to the number of parallel transistors required.
Avoiding Common Failure Modes for Medium Power Designs: Reducing Stray Inductance
Reducing stray inductance is critical to converter design. PCB traces, packages, connectors, interfaces, leads and wires all cause stray inductance and care must be taken when designing power and gate loops. Of particular importance are the inductances that couple the gate and power loops together, which are shared by the power and signal source connections (ie, common source inductances). Packages with separate Kelvin connections are usually preferred as they eliminate any external source parasitic inductance (LCS). While the above considerations have always been important in converter design, these stray inductances play an even more critical role when utilizing high di/dt SiC transistors. This is because the di/dt generated by the MOSFET switching creates a voltage on the parasitic inductance (VL = L × di/dt), which increases the voltage peak at the MOSFET drain. Therefore, the required headroom between the bus voltage and the MOSFET blocking voltage is directly related to switching speed and parasitic inductance. Since switching speed is also related to switching losses, reducing parasitic inductance is far more beneficial than reducing switching speed. These effects are exacerbated when devices are connected in parallel, as significant current imbalances can occur at the switching instants.
Using power modules eliminates many of these design challenges, and optimization of power and gate loops is easier because much of the necessary engineering is already done in the module. This reduces the complexity of the converter design and simplifies changes to the layout. Designers can also find solid block layout rules of thumb in Wolfspeed’s Design Library.
Avoiding Common Failure Modes for Mid-Power Designs: Simplifying Thermal Management
Typically, discrete devices require voltage isolation between their thermal interface and the thermal management system. The reason is that the heat sink or cold plate will be grounded and the discrete components will be exposed to high voltages.Power modules are fabricated by mounting the device to a suitable copper-faced ceramic (commonly referred to as a direct copper-clad substrate). [DBC]), eliminating the need for additional insulation design. The traditional stacking method used in power module design, the DBC is attached to a metal (or composite) backplane with mounting points for securing the module to a heat sink or cold plate. Care must be taken when installing the backplane as uneven pressure or insufficient/excessive thermal transfer medium material (TIM) can increase the thermal resistance between the module and the thermal management system.
Achieving good heat transfer between these interfaces requires two main factors: thermal resistance (Rth) and coefficient of thermal expansion (CTE).
Rth is a model of the ease with which heat is transferred from one interface to another – Rth The higher it is, the less thermal energy (or power loss) is extracted from the heat source. The size of the thermal resistance depends on the contact area, the thermal conductivity of the material and the layer thickness. In a power module with a backplane, the RJC (that is, the thermal resistance between the transistor chip and the backplane case) and the thermal resistance between the case and the heat sink must be considered. To reduce RJC, the new Wolfspeed WolfPACK modules are backplaneless, enabling direct cooling of the DBC substrate. This increases heat transfer from the transistor, reducing the junction temperature of the chip for a given power level (Figure 2).
Figure 2: Comparison of the classic board-mounted chip structure (left) and the boardless WOLFSPEED WOLFPACK module structure (right).
Typically, the CTE of SiC chips (4.0 10–6/K) matches the CTE of the ceramic substrate, which is typically composed of aluminum nitride (AlN: 4.5 10–6/K) or alumina (Al2O3: 8.2 10–6/K) composition. But for mechanical reasons, the bottom plate is generally made of copper (Cu: 16.5 10–6/K) or Al-SiC composites (8.4 10–6/K) made. This mismatch, combined with the rigid bonding layer between the DBC and the ceramic, results in increased stress at the material junction. These thermomechanical stresses acting on the large contact interface between the DBC and the backplane can lead to fatigue and fracture of the solder joints. Sufficient thermal cycling can cause delamination of the solder joints (which greatly increases thermal resistance) and even fracture of the brittle ceramic DBC, resulting in module failure.
Wolfspeed WolfPACK’s unique baseless design eliminates rigid connections to mismatched materials, which in turn eliminates this point of mechanical failure. The baseplate mounting bolts are replaced with metal tabs that pull on the plastic housing to distribute the force evenly across the baseplate. Since the interface between the DBC and the heat sink is a flexible grease (rather than a rigid solder), differential expansion between materials can be tolerated without significant stress. In addition to better reliability than manual and automated soldering (see Table 1), these press-fit metal tabs greatly reduce power module assembly costs. This mounting method simplifies thermal system design by allowing any number of modules and other components to be mounted on a single heat sink or cold plate.
How can designers extend power with the Wolfspeed WolfPACK?
Compared to discrete components and traditional power modules, the high power/high current capability of Wolfspeed WolfPACK modules greatly simplifies the design of mid-power converters (up to 100 kW), making it easy to make them more scalable with a small footprint, Higher power density can be achieved. Wolfspeed WolfPACK modules are available in a range of different sizes and configurations, allowing many power systems to be developed quickly, easy to build and maintain, and highly reliable in the field. With a maximum drain-to-source voltage (VDS) rating of 1,200 V and a continuous drain current (ID) of 30 A to 100 A, these modules easily form unit modules for medium power systems. In addition, Wolfspeed WolfPACK modular solutions are very scalable because, thanks to the modular design, it is much simpler to expand the system by interleaving and paralleling.
Medium power DC/DC converters are required in countless applications such as electric vehicle charging, solar energy conversion/storage, and power supply. For example, a multiphase interleaved, bidirectional DC-to-DC converter can be implemented by connecting any number of power legs in parallel to extend the maximum output current/power capability while reducing current ripple (Figure 3). The gate signals of the three-phase interleaved DC/DC converter switches are phase shifted by 120° to eliminate low frequency harmonics. Interleaving can be achieved with only minor changes to the controller and thermal system. The output power can reach over 60 kilowatts, which is still well below the maximum junction temperature of the SiC chip, allowing the system to operate reliably over its lifetime. Interleaving is an excellent strategy to avoid some of the challenges of paralleling devices, while also improving system performance and reducing the size of the output Inductor.
Figure 3: Basic schematic diagram of an interleaved DC/DC converter.
This interleaving approach can be applied to a variety of converter and inverter architectures to reliably scale power without sacrificing electrical and thermal performance. Combined with the advantages of SiC technology and the simplified thermal management of the backplaneless Wolfspeed WolfPACK family, designing a family of converters with a wide output power range has never been easier!
Simple scalability is one of the hallmarks of the Wolfspeed WolfPACK™ family of backplaneless power modules. As previously mentioned, one way to increase the power capability of a system is to interleave or parallel modules. But one of the easiest ways to extend the power rating of an FM3-based system is to plug a GM3 into your solution. But scalability isn’t just about power, it’s about choices – choices that enhance the performance of your current solution, depending on what you’re trying to achieve with a scalable system.
To help understand what the benefits of inserting a GM3 can be in a system, let’s consider a typical 2-level flat grid-tied inverter or AFE system with the following parameters: 800 VDC bus voltage, 480 VAC Line-to-Line RMS Gate Voltage (RMS), Ambient Temperature Tamb = 50 °C, line inductance L = 100 µH. Each leg represents a half-bridge FM3 or GM3 Wolfspeed WolfPACK™ power module.
Figure 4: 2-level grid-tied inverter or AFE system.
In this study, we consider the CAB011M12FM3 (11 mΩ) as our FM3-based reference benchmark. Using the system parameters defined above and operating at a relatively high switching frequency (50 kHz), a power rating of 75 kW can be achieved due to semiconductor losses before reaching a maximum junction temperature of 150°C.
Inserting the CAB008M12GM3 (8 mΩ) into the exact same 75 kW / 50 kHz system still shows high system efficiency (up to 98.9%), but reduces the device junction temperature to only 114°C. Operating the device at this lower temperature can extend device life or improve reliability, as well as provide higher overhead and overload capability. Also, it is particularly evident that there is room to increase the junction temperature and thus increase the power rating of the system, in this case up to 100 kW (50 kHz/Tj = 150°C).
Now plugging in the CAB006M12GM3 (6 mΩ) further enhances the system discussed above. Likewise, at a given power rating, the operating junction temperature of the device can be lowered, or the additional junction temperature margin can be further utilized by increasing the power rating of the system, or even increasing the switching frequency. A summary of this comparative study is given below to demonstrate the scalability options offered by the GM3 platform.
Figure 5: Results of a comparative study showing the scalability of GM3.
Obviously, plugging in a larger GM3 platform, as shown, increases the power rating of an FM3-based solution, but the benefits for a scalable solution go beyond that. Depending on your design goals, reducing operating junction temperature to increase system robustness, or increasing switching frequency to reduce magnetics size/cost and improve control range are ideal ways to enhance performance for your scalable system. All in all, the GM3 platform offers designers the option to easily scale power electronics systems.
Figure 6: Easily expand your system with the GM3 WOLFSPEED WOLFPACK™ platform.
In addition to increasing module size or active die area, another way to improve scalability is to choose the material stack of power modules. In the absence of a substrate, this choice is really only two degrees of freedom and has a significant impact on the overall thermal resistance of the module (thermal transfer medium material (TIM) and substrate ceramic material). As can be seen from the figure, the TIM layer can account for the overall node-to-heatsink thermal resistance (RthJH) of 60%. While the end user can choose from a wide variety of TIMs, even a very high performance thermal paste material cannot significantly affect the effect of the TIM layer. In another degree of freedom, ceramic substrates can significantly reduce the overall thermal resistance value, which we will explore next.
Figure 7: Typical RthJH distribution.
A typical must-have ceramic substrate for the WolfSpeed WolfPACK series is alumina (Al2O3), ideal for this type of backplaneless module family due to its excellent price/performance ratio. But customers understand that aluminum nitride (AlN) can greatly improve performance with relatively low cost increases. The thermal conductivity of AIN is higher than that of Al2O3 7 times higher, so it’s not hard to understand the effects: lower thermal resistance, lower T for a given lossjincrease the PC service life under a given loss, and improve the utilization of SiC performance.
Consider at 800 VDC Bus voltage, 480 VAC Line-to-line RMS grid voltage, ambient temperature Tamb A 2-level grid-tied inverter operating at = 50 °C and line inductance L= 100 µH also proves this again. As with the previous scalability study, using 6mΩ GM3 on an AlN substrate, it is possible to scale across three variables: power, switching frequency, and junction temperature. This provides a solution for customers who need more usable current capability, or in some use cases, lower operating junction temperature for longer life, or support higher heatsink temperature (lower cost).
Figure 8: Results of a comparative study showing scalability of GM3.
Wolfspeed WolfPACK offers a new design built on a long-term investment in SiC technology
The Wolfspeed WolfPACK Power Portfolio is the culmination of decades of investment in SiC R&D, coupled with a substrateless design, giving OEMs and design engineers more options to support a wide range of industry applications.
By assembling several SiC MOSFETs in a container that provides a press-fit, solderless pin connection to an external PCB, it provides designers with greater flexibility and scalability. The Wolfspeed WolfPACK family of power modules provides optimized pin assignments for specific applications (eg half-bridge, full-bridge, six-tube integration, and buck/boost circuits, etc.) based on the internal arrangement of the MOSFETs. The bottom of the Wolfspeed WolfPACK housing uses a ceramic substrate instead of the bottom plate. The ceramic substrate is used for electrical isolation between the bottom of the module and the metal gasket. The metal mounting tab is connected to the heat sink by spring force. This method evenly distributes pressure to the bottom of the module, ensuring adequate thermal contact with the heatsink, while also providing a strong mechanical connection between the heatsink, module, and PCB.
The higher power density in a backplaneless package, combined with SiC technology, enables compact layouts, faster and simpler switching, which can reduce the size of designers by up to 25%. In addition to power density benefits, Wolfspeed WolfPACK modules simplify system layout and assembly. This allows engineers working in mid-power applications to greatly increase power density while minimizing design complexity.
The inherent simplicity of the Wolfspeed WolfPACK supports a high degree of scalability, helping to speed up the production process, reduce system assembly costs, and provide a wide range of options. These new Wolfspeed WolfPACK modules feature full SiC MOSFET half-bridge and full SiC MOSFET six-tube integration, with a choice of different on-resistances.
Power modules that offer a wide range of options and reliability
The new line of Wolfspeed WolfPACK modules provides designers with a power portfolio for a variety of applications, whether working on single kilowatt designs or anywhere in between megawatt systems.
Built on Wolfspeed’s industry-leading SiC technology, these modules deliver ultra-low losses in a small package, ideal for large-scale automation and manufacturing to deliver clean, reliable power to energy conversion systems.
The Links: CM1000HA-24E 2MBI100U4A-120