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Posted on 30 April 2019

Power GaN and SiC Demands High Performance Modules

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New GaN and SiC Packaging Architecture Enables New Faster, More Efficient, and Higher Power Density Surface Mount Devices and Modules, like the μMaxPak and μMaxMod

By Courtney R. Furnival, Semiconductor Packaging Solutions, Lake Arrowhead, CA

Introduction

Power Module architecture for high-voltage (HV) power silicon controlled rectifier (SCR) and thyristor switches have changed little over the past four decades, slowly transitioning to bi-polar transistors and then to IGBTs and diodes. These modules have continued to be large and bulky with large screw terminals and high voltage (HV) spacing. They accommodate easy installation and replacement, but limit performance of ever faster and more efficient silicon IGBT devices. The advent of 600 V, 1200 V, and higher power silicon carbide (SiC) and gallium nitride (GaN) devices that are much faster, more efficient, and provide higher power density can no longer tolerate these limitations. The smaller die and lower losses offered by the compound semiconductor power switches enable new smaller structures, which accommodate high-speed switches, more integration, newer materials, and lower cost assembly. Power compound semiconductors extend the range of co-packaged surface mount device (SMD) power switches to power levels that were the exclusive domain of power IGBT modules, and create high-performance building-blocks for the next generation power modules. This article examines new criteria demanded by compound semiconductors, and offers new packaging solutions that enable smaller, faster, more efficient, and lower cost HV power compound semiconductor switches, bridges, and smart-bridges.

Legacy Modules

Standard industrial 600 V and 1200 V insulated gate bipolar transistor (IGBT) half-bridge (HB) modules are common, but single-switch (SS), full-bridge (FB) and three-phase-bridge (3ϕ) are available for many applications. These modules are bolted to large heatsinks, and high current terminals are bolted to power leads and bus bars. Usually fast-on leads are used for gate and sense connections, and for power leads at currents up to 50A. Figures 1a & 1b show such industrial HB power modules commonly used for 240 and 480 V ac applications with currents of 50 to 600 A and maximum voltages of 600- 1200V plus. These modules limit switching speeds due to high stray inductance of 40nH. These modules also have long leads and wire bonds that have high resistance that creates significant conductive losses.

The standard modules modified by some manufacturers better accommodate faster IGBTs, but performance improvements were limited, and turn-on and turn-off still creates excessive switching voltages. Inductances were reduced incrementally to 20nH with the low profile package shown in Figure 1c. These packages have larger footprint, simpler construction, and slightly higher current ratings, but the di/dt is even limited for faster silicon IGBT switches.

This discussion omits two product categories due to their uniqueness. First, there are the lower current 3-phase (3ϕ) bridges for home appliance applications. Due to their lower current rating in the 1 to 15 A range, they are less constrained by inductance and resistance. The high current 600 V electric vehicle (EV) and hybrid electric vehicle (HEV) modules are another exception.

Relative module and SMD sizes

Paradigm Shift for GaN & SiC

GaN and SiC devices can potentially be 1/10th the size and have 1/5th the loss1, and can operate at much higher switching speeds and temperatures. These devices require high density and high performance packages and modules, which enable their full performance and cost effectiveness. Early SiC and GaN power devices have not yet reached their potential, but the chart in Figures 2a & 2b shows the dramatic potential size reduction for 600V and 1200V GaN and SiC die, relative to Si die.

Potential size reduction for 600V and 1200V GaN and SiC die

Power GaN and SiC packages and modules must accommodate smaller devices with higher power and current densities, must enable much higher switching speed, and must provide significantly lower conduction losses. They need lower DC and AC resistance connections, very low stray inductance, and lower thermal resistance, while accommodating higher operating temperatures and the same high isolation voltages. Higher density functionality further enables high speed switching, through co-packaging of multi-switch bridges, gate drives, and integrated control, protection, and isolation.

Historically the HV power IGBT market has been 80% modules, and 20% discrete packages like D2Pak, TO220, and TO247. Faster, smaller and more efficient power GaN and SiC switches should increase the discrete and co-pak package bridge market share. However, with the growing demand for more efficient high power conversion, modules will certainly maintain a dominant position in the market.

Next Generation Power GaN and SiC Packages

Power GaN & SiC die require a quantum leap in packaging with higher power density and performance, but smaller die and higher efficiency provide new freedoms to package architecture, materials, and assembly. Smaller die can be soldered directly to copper leadframes, and can contact rigid molding compounds. This enables packaging with lower cost commercial package platforms, which are readily available and easily customizable to power application. Leadless and wire-bondless package platforms have been refined, are produced in high volume for high frequency computer and telecom applications, and they have all the attributes required for power devices.

New Semiconductor Packaging Solutions “μMaxPak” architecture for power GaN and SiC devices is “nearly chip-scale,” and the surface mount package is built on leadless and wire-bondless modified DFN/QFN platform. It removes heat from both top and bottom of thinned compound semiconductor die, and can provide an Rjc of 0.1 ºC/W with 5x5mm die. The power leads provide low inductance of 0.1 to 0.2nH and low DC resistance of 100-200μohms. The geometry minimizes resistance increases at higher frequencies from skin-effect constraints. The μMaxPak further reduces power lead inductance and resistance with parallel die, and in compact multi-chip HB, FB, 3ϕ and others power switch configurations. Figure 3b shows approximate size of 200, 400, and 600 A at 600 V and 100, 200, and 300 A at 1200 V HB μMaxPak SMD packages with one, two, and three 5x5 mm GaN die switches, respectively. The HB μMaxPak can include the gate drive ICs, which are a key enabler for optimum paralleled die switches. SiC die have similar potential, but generally require anti-parallel diodes.

The SMD μMaxPak does not include isolation and screw terminals, so you must use it on isolated boards or substrates. SMD μMaxPak on thermal PCBs or IMS are simple and economical, but at higher current levels, you must use the μMaxPak on open DBC assemblies or as building-blocks in higher power DBC modules. In all cases, the μMaxPak provides pre-tested building blocks with environmental and handling protection, and provides well controlled performance, parasitics, and die paralleling at higher currents. They are flexible and easy to use, and allow the enduser to mount and connect for specific applications.

The μMaxPak GaN and SiC SMDs enable 3-D internal architecture, allowing gate driver ICs to be assembled with output pads directly over the power switch gate pads. This is critical for switching speed, noise immunity, and tight voltage control and temperature compensation. Additionally, you can co-package isolation and control circuitry for optimum and consistent power switch parasitic, control, and protection. Integration or co-packaging in the molded packages is possible by reduced component and/or circuit size with high frequency GaN and SiC operation. Furthermore, pre-tested and well controlled building-blocks are essential for high yields with increased integration, and for well controlled parallel die switches.

Next Generation μMaxMod Modules for Power GaN and SiC

The GaN and SiC module size can and must be reduced with smaller compound semiconductor die and reduced power dissipation, but module size reduction is limited by screw terminal size and the HV spacing requirements of UL and other safety agencies. The package outline drawing in Figure 3a shows a projected minimum HB module size for 600 to 1200V GaN or SiC modules with currents of 200 to 600 A, while retaining standard terminals. In the same outline, the modules can also include integrated gate drivers and associated isolation, control, and protection functions. Table 1 compares the estimated module sizes and performance between new GaN HB modules and typical Si IGBT HB modules. The GaN μMaxMod modules, shown in Figure 3a, were modeled with the μMaxPak HB building blocks shown in Figure 3b. The μMaxMod GaN module is possible with chip and wire (C&W) construction; however, C&W increases inductance, DC/AC resistance, and thermal resistance, not to mention omitting all of the other advantages of a well controlled buildingblock. Table 1 shows the key metrics for comparing 200 to 600 A HB modules.

Metrics for comparing 200 to 600 A HB

Small size is important to lower inductance, resistance, and capacitance, and provide further inherent advantages to user and system integration. If the base plate is reduces by 10X to 20X, the smaller base plate not only minimize mounting force and screw size/quantity, but more importantly makes hi-performance metal matrix materials (like Al-SiC) very economically viable, The accompanying 6X reduced size isolation substrate makes high thermally conductive DBC materials (like AlN and Si3N4) also quite economical. Smaller packages not only contribute to lower stray inductance and lead resistance, but enables further reductions in system bus bars and power leads. Integration of gate drive, isolation, protection, and control devices are practical for Low Profile GaN and SiC modules, because of smaller component sizes with higher frequency operation.

Future Trends

The μMaxMod type modules improve performance, and minimize size and cost of high power GaN and SiC modules with traditional terminals, but large terminals and spacings still account for the majority of a module’s size and mass. Higher density module schemes are possible for specific applications, and can further improve performance and reduce material and manufacturing costs. Potential interconnect options include soldering, welding, and leadless or direct connects to final system locations, and the lead spacing can be reduced further with molded sleeves, potting, coating, and/or other insulation techniques. Smaller lower loss modules are more integratable into end-products. Examples of such integration are Motor Drives into motor housings and Solar Micro-Inverters into the panel connector boxes. The end user may do application specific customization, leaving the GaN or SiC device manufacturer to focus on module building-blocks like the SMD μMaxPak SS or HB because they can better control performance, parasitics, reliability, and cost effectiveness in their fully automated clean-room assembly and test facilities. Smart integration and co-packaging of more functionality continues, enabling higher speeds and lower losses.

[1] “Is it the end of the road for silicon in Power Conversion”, Alex Lidow, EPC, Inc., April 10,2019, pp 1,2 on EPC Website

For additional information please contact Courtney R. Furnival, CRFurnival@SPSpower.com

 

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