Posted on 27 May 2019

SMD Power GaN & SiC Devices on Power Boards Replace High Power 600V & 1200V IGBT Modules

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Power GaN and SiC Transistors with higher power densities and efficiencies enable corresponding improvements to system architecture. New technology enables optimum performance of compound semiconductors, accommodates system customization, and uses low cost commercial manufacturing techniques

By Courtney R. Furnival,

Modules verses SMD Boards

Surface mount package (SMD) high-power 600 V and 1200 V GaN and SiC devices offer extremely high efficiencies in high-performance packages. New SMD package architectures enable high-frequency operation with exceptionally low inductance, low DC/AC resistance and low thermal resistance. Last month’s article, “Power GaN and SiC Demands High Performance Modules” (1), focused on using the μMaxPak as building- blocks for smaller and more efficient conventional screw-terminal modules. This module solution allows high-speed compound semiconductors to operate more efficiently, while minimizing conduction and switching losses. However, conventional module packages, although readily accepted in the marketplace, still severely limit the realizable power density and possible cost savings.

In this article, the next step is taken to further increase power density using SMD assembly on open power boards similar to the familiar printed circuit boards, enabling more system integration and most of all reducing overall system costs. The foundation elements in this approach are the SMD power device. As explained in the previous article, the μMaxPak unique SMD package is built using a modified QFN package platform, which offers proven low cost commercial packages that can be assembled at most QFN assemblers. The μMaxPak accommodates co-packaged switches, paralleled die, cascode MOSFET and 3-D configured bump-chip gate drivers, all of which contribute to optimum compound semiconductor device performance when tightly integrated. The packaging technique is ideally suited for half bridges (HB), full bridges, and three phasebridges (3-Ph), while accommodating the full system with functions like protection, current sense, input rectifiers, gate drivers, and control circuitry. The Power Bridges are only one converter example. The μMaxPak packages are well suited for other power converters that contain both switches and diodes such as the boost, the power factor correction, and bi-level switch circuits. The high-density μMaxPak architecture is ideal for paralleling high-speed power GaN and SiC die. This SMD construction is much simpler than the conventional screwterminal module enabling assembly at typical printed circuit board (PCB) assembly houses. As a supply-chain streamliner, the μMax- Pak SMD can be assembled, tested, and controlled by semiconductor device manufacturers. Since the power board is similar to the printed circuit board, the end-users or system manufactures can tailor board design to their products and applications, with common PCB assembly techniques including coating or potting the power board to accommodate voltage and environmental requirements. The proper selection of board type is dependent on specific board properties and power level.

High-Performance Power SMD Boards

The μMaxPak SMD enables high speed and efficient power GaN and SiC performance at very high power density. It is important to select a power board to maintain that optimum performance and power density, and to maintain the low commercial manufacturing costs at the system level. Key factors at the system or board level are low thermal resistance, low inductance, system power density, simple SMD assembly, and easy user customization.

Low Thermal Resistance

The μMaxPak package can provide extremely low Rjc below 0.1C/W, with the mounting board or substrate providing the required heat transfer to the heatsink while maintaining the required high-voltage isolation. Insulated metal substrates (IMS) with excellent heat transfer from the μMaxPak case to system heatsink, have been used extensively for decades in Japan for high voltage IGBT sixpaks and power modules. IMS is much lower cost and simpler to assemble than DBC modules, and has become more available in the U.S. and Europe. As an example, Laird Technology’s premium Tlam2 HTD has an insulation layer that is more than 10 times more thermally conductive than FR4. At 0.004 inches thick the thermal resistance can be 100 times lower than a 0.040 inch FR4 PCB., This unique premium HTD thermal prepreg Tpreg and thermal via can provide Power PCBs (PPCB) with similarly low thermal resistance (Rcs), without a metal baseplate. That said, the metal baseplate can important because it not only provides a heat transfer and heat capacity, but also acts as package body, mechanical structure and mounting surface for SMD components, connectors and the complete system. The IMS and PPCB can accommodate power dissipation (PD) for HB μMaxPak’s with 5x5mm die switches for output power up to 25 kVA at 600 V, and 18 kVA at 1200 V. See Table 1 for details on power dissipation, efficiency, output power and associated conditions.

GaN & SiC Die in μMaxPak on Thermal Boards

Higher power levels can be achieved with high-temperature non-isolated PCBs with attached/soldered Al2O3 or AlN direct bond copper (DBC) substrate. The DBC provides excellent heat transfer and isolation. It can provide power dissipation (PD) for μMaxPak HB with 5x5mm die switch output of up to 50 kVA on Al2O3, DBC and 148 kVA on AlN DBC. See Table 1 for details on power dissipation, efficiency and output power. The Power PCB with attached DBC is more complex and expensive, but still accommodates SMD reflow assembly with all of the advantages of a single board system performance costs, relative to conventional modules.

SMD inverter

Low Inductance (L) and Resistance (R): Short traces and 4 oz copper layers on IMS. Provide very trace low L and R. SMD components and connectors can be easily placed and solder reflowed without additional jumpers or connections. The traces can be wide and thin to minimize DC and AC resistance, and the very thin dielectric layer to the baseplate or ground plane provides natural field cancelling, minimizing parasitic inductance.

Maximum Power Density: The simple SMD board accommodates simple small SMD component without additional electrical, mechanical and thermal structures. The boards enable 3-D structures with heatsink below the power board and control board/components above. Ideally, gate drivers and associated components are co-packaged in the μMaxPak. The following output examples are for 5x5 mm die switches, in 15 mm x 8 mm x 1 mm HB μMaxPak, with 99.0-99.5% efficiency. The maximum HB output based on the thermal limitations are; 1) insulated metal substrates in Figure 1b accommodate outputs of 25-50 kVA, 2) insulated PPCBs in Figure 1c also accommodate outputs of 25-50 kVA and 3) PPCBs with external DBC isolator in Figure 1d accommodate outputs of 75-150 kVA. The substrate thermal capabilities sometimes surpass the output capabilities of 5x5 mm GaN and SiC devices, but even higher device outputs are achievable with paralleled 5x5 mm die switches. Figure 1a shows the simplicity of a SMD inverter with three HB μMaxPaks containing one 5x5mm die per switch. The external connectors are nominal and will change with current levels. Connectors can be FASTON type at the lower current ranges, and solder at mid-range currents. This type of lead can carry 3-5 times the typical rated current on FR4 PCBs, because the IMS and PPCB can keep them cooler, and temperature is a major factor in their current ratings. At hundreds of amperes, screw terminals may still be required, but ideally only once per function exiting the full system. The inverter was used as a common example, but again it is most advantageous to maximize integration with power components on the IMS/PPCB, and other components above.

Simple SMD Assembly: The IMS or Power PBC accommodate easy SMD pick & place and solder reflow assembly, which is low cost and compatible with most PCB assembly houses. It minimizes mechanical and thermal hardware, and accommodates easy post potting or coating.

User Customization: These boards can be designed and modified with PCB type artwork, making it easy for the user to optimize their products and systems, and do not require the user to build around a large and awkward standard module. Likewise, the GaN and SiC supplier creates the μMaxPak with conventional QFN/DFN platform, and does not need to create multiple module configurations with associated mechanical structures, special materials, and costly equipment and tooling NRE.

Integration, Integration, Integration: The final key to Power GaN and SiC performance and high power density is integration. Smaller die, smaller SMD packages, high-speed and higher efficiencies enable more integration in smaller systems. The close proximity of all components in the system further reduces parasitics, interconnects, complexity and manufacturing costs.

Parallel Worlds

The first world being high-speed high-density digital devices like microprocessors, memory and interface, and the second world being power compound semiconductor services. The point being that today's digital devices were not possible in plastic dual in-line (PDIP) and small outline integrated circuit (SOIC) packages. Further, emerging power GaN & SiC devices cannot advance when limited by TO-220, TO247 and standard screw-terminal bridge modules. The reason is that power converters using high-speed high-density GaN and SiC devices must include leadless, wire bondless, 3-D stacked, integrated, and co-packaged structures. These packages do not just help power GaN and SiC performance, but are IMPERATIVE to the full performance of these devices. Further, most technologies and techniques developed for the highspeed high-density digital world are applicable to the high-speed high-density power compound semiconductor world.

(1)“Power GaN and SiC Demands High Performance Modules”, by Courtney R. Furnival, Semiconductor Packaging Solutions, published in Bodo’s Power Systems Magazine, May 2013, pp 56-58

(2) Laird Technology’s trademarked “Tlam” is a “Tpreg” with metal on both side (DSL), and that metal can be copper foil or can be an aluminum or copper plate on one side. The “Tlam ML” is a multilayer insulated metal substrate with DSL laminated to a metal base plate with Tpreg

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