Energy savings have become a global necessity, with the demand for greener appliances soaring across the globe. Solar and wind power technologies are beginning to proliferate in cities and states around the world. As a result, there is now more emphasis on energy-efficient electronic inverters and power modules.
Growth of renewable energy is being driven by the recent economic stimulus package in the U.S., which calls for spending $45 billion on alternative energy, including some $4.3 billion on smart electric grids[1]. Similar initiatives are also being deployed in Europe, Japan and China.
A new Energy-Related Products Directive (ErP) was recently approved by the 27 members of the EU and will take effect in January 2013[2]. The impact of this new directive 2009/125/EC will be dramatic for the efficiency of fans with electric input power in the range 125W to 500KW. A similar directive, 641/2009, covers motors used in recirculation pumps. Once the full impact of these standards occurs by 2020, 34 terawatt-hours per year of electrical energy will be conserved and an equivalent 16 Million tons of CO2 emission will be reduced.[3]
These more stringent government regulations require companies to design and market more advanced (and more affordable) energy-efficient solutions in appliances, in heating and ventilation systems, in water circulation (pumps) systems, and of course, in PV solar inverter units.
Today, engineers can select from a multitude of IGBT and MOSFET technologies to develop customized solutions for various applications. For instance, a number of process technologies have been developed to manufacture IGBTs and each can be selectively optimized for specific inverter topologies. As one specific example, field-stop (FS) trench technology is gaining momentum, enabling higher power density and rugged operation.
The overall effect of these innovations when applied to the analog-power stage of an inverter is an easy to develop, more flexible, higher performance, and less expensive product.
Large installations of heating and ventilation systems as well as water re-circulation pumps are used on manufacturing floors, in clean rooms and/or in large condominiums). Hundreds, if not thousands of individual EC fans and pumps can be installed, adding serious consequences, not only to the overall load balance, but more importantly for the line PFC quality. A specific example of a new integrated power solution (high frequency PFC front-end and IGBT based inverter stage) features increased power density and increased system integration enabled by the reduced losses and size of new advance IGBT trench technology. The package technology is flexible and ready to adapt to further system integration and/or new semiconductors (i.e. GaN switches and GaN Rectifiers) which are going to enable operation of the PFC stage at higher frequency with direct impact in size and weight of the passive components used in the boost circuit, as well as in the EMI filter.
IGBT TECHNOLOGY
The past few years have brought significant advances in the fabrication of IGBTs. The fundamental improvement has been the adoption and deployment in high volume manufacturing of Trench technology and Field . Trench FS IGBTs offer significant improvement in terms of loss reduction, over the last generation of Non Punch-Through (NPT) IGBTs. Fig. 1 compares different IR IGBT die cross-sections for different technologies, planar PT, planar NPT and Trench Field Stop. For the Gen6.x IGBT, the combination of trench cell geometry and field stop structure enables an optimized carrier concentration in the die. With this structure, by increasing the carrier density in the vicinity of the trench gate, it is possible to reduce substantially the V CESAT. In addition, by adopting specific implantation and annealing techniques, it is possible to deliver low carrier lifetime in proximity of the backside p-emitter that when combined with a reduced doping and optimized design, allows fast carrier extraction at turn-off with minimized current tail hence with obvious benefits for high switching frequency operations.
To have a correct perspective of the die size reduction made possible by the Trench Field Stop Technology, Fig. 2 illustrates the geometric dimension of two equivalent die of IGBT 600V in planar Gen5 technology and Trench Field Stop Gen6.x technology.
The die size reduction, while fundamental in multi-chip products as it enables more integration of functions or lower the cost of packaging, must be accompanied by significant power loss reduction to deliver the complete system advantage. The comparison of Vce(on) vs. Ice of Planar NPT IRGB8B60K (size 2.0), Trench FS IRGB4056D (Size 2.0) and IRGB4060D (Size 1.2 in Fig. 3) illustrates how the Trench field stop helps minimizing power losses [1]. While the first two IGBTs have the same die size, the latter is about 20% smaller. At the same current density (same die size) the conduction losses can be reduced as much as 30% and by taking advantage of a smaller die, it is still possible to achieve 15% conduction losses reduction (VCE(SAT)) of 1.3V vs. 1.55V vs. 1.8V).
Typically, Trench Field Stop IGBT die are thinned down (at wafer level) to thickness well below 100 µm, as compared to >250µm of older technologies. As a result, it improves the thermal resistance, reduces conduction losses (less material resistance), and ultimately it lowers the charge stored to improve switching performance. Furthermore, a thinner IGBT die also has a positive impact on the thermal performance of the transistor and in addition, FS does not need lifetime killing-type irradiation hence resolves the issues of process stability and high leakage hence enabling higher temperature of operation up to 175°C. Further process and design improvements for the Trench structure, back side annealing and further thinning of the wafers, are set to incrementally improve overall energy efficiency in application by reducing power losses.
POWER STAGE INTEGRATION
Combining the effect of the power loss reduction delivered by the FS Trench IGBT and the overall die shrink, the available real estate of the integrated module substrate is available to add the integration of a complete PFC stage (switch plus boost-diode and gate network and buffer driver). This new level of system integration is beneficial in applications where multiple EC Fans or pumps are installed as well as in HVAC integrated systems (inverter-based air-conditioning units). More specifically, the new IR Gate Driver, the IRS26302, drives the six FS Trench IGBTs used for the motor inverter stage and, in addition, the PFC switch.
The design of the module is highly flexible and enables both high frequency PFC stage, based on Super junction HV MOSFET technology (high frequency operation up to 130 kHz) and low frequency (<30 kHz) PFC stage, based on Trench IGBT technology.
The latter is generally preferable in an HVAC solution, were lower frequency of operation are required due to the EMI impact of large coupling to ground intrinsic of heat exchanger and condenser. High frequency operation is instead required in highly compact and efficient designs where the size of passive components are required to be minimized in order to obtain highly compact, integrated motor EC fans and pumps.
The design of the new integrated power module was intended to achieve the highest efficiency by adopting today's state-of-the-art silicon based switch and diodes for operation at high frequency but at the same time, with all the packaging compatibility to house more advanced semiconductors in the very near future.
The power stage schematic is shown on Fig. 4., Fig. 5, and Fig. 6 show details of the internal module assembly. A total of seven active switches and 14 diodes are assembled on the module substrate (not shown are the gate network for each switch).
In order to maintain the overall thermal budget of the module in check, the HF PFC stage was verified in switching operation to validate the switching losses and confirm the overall power dissipation (inclusive of conduction losses) less than 15W, which compensate the saving of ~2W per IGBT in the inverter stage (at nominal 5A RMS motor load current).
ENERGY-SAVING DESIGN
The PFC stage switching waveforms, under nominal condition of full load and line input (500 W and 220VRMS), are shown in Fig. 7. The total energy loss per switching cycle (E ON+EOFF) is less than 70µJ.
The entire system, front end PFC and Inverter stage is housed in a new SiP IRAM module package as shown on Fig. 8. The new IRAM module has been modified with 29-pin lead-frame design vs. the previous standard 23-pin by using a smaller pitch distance (1.27mm vs. 2mm). Between the high voltage pins, extra space is provided in order to satisfy the clearance and creepage distance requirements mandated by UL/IEC standards.
This new highly integrated solution enables a more compact and simpler design for the power stage of EC fans and pumps that have to meet the new energy efficient standards. However, substantial improvements that can help further reduce the power losses can be envisioned for the inverter stage. Also, by increasing the PFC switching frequency 3 to 4x the present frequency, a drastic reduction in size and cost for the passive components required for the EMI filter and boost stage is at reach. All these improvements require eventually the adoption of a new technology for the active components that uses advanced semiconductors based on III-V material like GaN and SiC and not any more based on silicon.
TECHNOLOGY TRENDS
All topologies based on Silicon MOSFETs have intrinsically limited improvement capabilities. Based on state-of-the-art active components and passive components, constrained integration opportunities pose a limit to the technology evolution. For future integrated modules, GaN based switches have a better potential figure of merit than other power components based on Si or SiC material. Fig. 9 shows the relative comparison of the specific R ON FOM for Si, SiC and GaN-based switches. The potential improvement exploitable from the GaN technology is large, based on the material limits[6].
To improve overall conversion efficiency, all these topologies require the power switch with the lowest possible specific RON×QG figure of merit. GaN-based MOSFETs show great potential in FOM improvement over the coming years.
Preliminary tests have proven that GaN-based power device technology has a positive impact on the overall efficiency and size in grid connected micro-inverters, power optimizers[7]. In conjunction with a radical improvement in parasitic charges QG and QGD when compared to the best in class silicon (IRF4321), GaN-based switches have delivered a power loss reduction >3x in the front-end of solar micro-inverters. Fig. 10 shows the efficiency improvement obtainable by the total power loss reduction from the adoption of GaN-based MOSFETs vs. traditional silicon-based MOSFETs.
SMALLER FILTER SIZE
It has been demonstrated[8] that by increasing the PFC PWM frequency in the range of 500 kHz, the EMI filter size and the boost inductor can be reduced by 45% and 80% respectively (compared with the baseline of the same components needed in PFC stage running at 100kHz). The overall volume reduction of the passive components can be >40% with obvious positive impact on cost and circuit layout. High voltage GaN and SiC MOSFET have potentially the FOM capabilities to enable such improvement and beyond.
The first generation of GaN based switch prototypes have 20X Lower QRR compared to IGBT Copak and more than 200x less than that of Super Junction body diode. EOFF (off switching loss) of these GaN Switches has been measured 72% less than for IGBT and 30% lower than HV Super Junction, as shown in Fig. 11.
Fig. 12. illustrates a relative comparison of PFC boost inductors as a function of PFC stage PWM frequency of operation. Fig. 13 illustrates the impact of higher frequency operation on the output inductor size for a 5 kW solar inverter[10].
For the inverter stage, where IGBTs are today the standard, the improvement of FOM: VDS(ON) × ETS (conduction voltage drop multiplied by total switching energy) provided by GaN based switches is almost 3X at nominal current density of 2A/mm2. Fig. 14 shows the progress of the FOM of IGBTs and GaN based switches.
Because GaN technology is characterized by an intrinsic lateral structure, it enables flip-chip packaging and virtual elimination of parasitic elements due to wire-bond stray inductance and parasitic resistance. In a PFC stage configuration, the possibility to minimize the emitter stray inductance is an important advantage for the reduction of unwanted ringing and has relevant direct EMI impact. Moreover, the device's lateral structure will eventually lead to the monolithic integration of multiple power devices and drivers. This, in turn will drive the MOSFSET technology roadmap to develop semiconductor packages that are both smaller and more efficient, reaping energy benefits for designers.
REFERENCES
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European Community http://www.erp-compliance.com/elt.pdf
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02-2010 Mag. Ebm-papst
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Datasheets of IRGB8K60, IRGB4056D and IRGB4060D, International Rectifier, http://www.irf.com
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Datasheet of IRAM136-1061A, International Rectifier, http://www.irf.com
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N.Ikeda et.al. ISPSD 2008 p.289
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“(GaN)-based power device technology and its impact on future Efficient Solar grid connected micro-inverters, power optimizers and string inverters”. Alberto Guerra and Jason Zhang. PCIM 2010
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“MHz PFC Study and New Architecture“, Chuanyun Wang, Ming Xu CPES 2007
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N.Ikeda et.al. ISPSD 2008 p.289
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Courtesy of Fraunhofer Institute