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Packaging Performance

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Smart power is a feature of an increasing number of domestic and industrial appliances. Reduced power consumption leads naturally to operating cost and environmental benefits. Fairchild Semiconductor developers; Jun-Bae Lee, Jun-Ho Song, Dae-Woong Chung, Bum-Seok Suh, Motion Control System Team , Functional Power Group, describe how the adoption of direct bonded copper substrate packaging has enabled increased power ratings for its latest Motion-SPM smart power module devices.

Motors are the major source of energy consumption in domestic and industrial appliances such as air-conditioners. Since governments and users naturally wish to reduce energy consumption for cost and environmental reasons, inverter technology is increasingly being employed by a wide range of manufactures in the design of their products. Fairchild Semiconductor has developed a series of devices under its Motion-SPM trademark with the aim of supplying this demand with a highly efficient integrated smart power module (SPM) solution. The first and second series of the company's Motion-SPM in dual in-line packages (DIP, 60(31mm) and mini-DIP (44(26.8mm) packages have already been successfully introduced to the market [1]. Since then, a great number of SPM inverter systems have been implemented by top appliance manufacturers and these modules continue to run successfully, validating the module's excellent reliability testing.


Since their original introduction, Fairchild Semiconductor has taken the next step with the development of up to 15A-rated Motion-SPMs in single in-line packages (SIP). These highly specialised modules cover insulated-gate bipolar transistor (IGBT) inverter applications ranging up to 1.5kW. When dealing with such a high operating power, one of the most important requirements in the system is increasing "compactness" and improving the mass production process. Both these goals are accomplished in the SPM design by focusing on high quality and reliability, which results in a more cost-effective solution compared to discrete inverter solutions. These SPM modules have been developed for home appliances and in industrial equipment requiring higher efficiency and higher performance characteristics.


Design issues Figure 1 shows internal block diagrams of a highly integrated Motion-SPM in a DIP and a Mini-DIP. The first module, the Motion-SPM, is composed of three "normal" IGBTs, three sense-IGBTs, six freewheeling diodes, three high-voltage ICs (HVICs), one low-voltage IC (LVIC), and one thermistor.


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The SPM in Mini-DIP is composed of six normal IGBTs, six freewheeling diodes, three HVICs, and one LVIC. The built in IGBTs are designed with key electrical characteristics such as low conduction switching losses over all driving conditions, short circuit withstanding capability for the motor drive, and smooth switching waveforms that eliminates electromagnetic interference (EMI) noise caused by rapid changes in voltage (dV/dt). The sense-IGBTs in the modules have very good linear sensing characteristics, providing a simplified and cost-effective solution.


The most important characteristics of the freewheeling diodes are their softrecovery behaviour over the whole current and temperature range, and low forward voltage drop. System reliability is further enhanced by internal temperature  detection using built-in thermistor and integrated under-voltage lockout and short-circuit protection. The high-speed built-in HVIC enables the use of a single power supply without using a photo coupler. This results in both size and cost reductions for the inverter systems. In addition, the Motion- SPM has three divided negative DC terminals to monitor the inverter output current by using external shunt resistors, providing a lowcost sensor-less control solution.


The SPM package uses a narrow space, multi-die attach technology. These packages are designed to guarantee the best heat transfer from the power chips to the outer heat-sink. They also integrate larger power chips by using a direct bonded copper (DBC)-based package.


Therefore, an SPM can extend its current rating to up to 75A and provide reduced noise, smaller size and less mutual interference. Aluminium nitride (ALN) or ceramic-based DBC substrates have been used in power electronic applications for many years and have proven high performance ratings. Because of the leadframe structure, this packaging has a 2mm down-set shape, a structure that lowers thermal resistance. However, the distance between lead frame and the outer heat-sink can't be reduced any further since increased down-set thickness affects the reliability and assembly process. The optimisation of the bending depth has been obtained by doing simulations and experimental tests. The total thickness of the moulding is 5.5mm. Figure 2 shows the cross-sectional structure of a Motion-SPM in DIP.


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For the design of Motion-SPMs, pin-to-pin and pin-to-heatsink isolation distances should be considered. Clearance and creepage distances of pin-to-heatsink are more than 3mm and 4mm, respectively. Pin-to-pin distances are more than 3mm and 4mm, respectively. Both of these values meet the basic International Electrotechnical Commission (IEC) creepage and safety spacing requirements for inverters.


Higher performance and power ratings
The first and second series of the Motion-SPM used only ceramic-based substrates, mainly to reduce package cost. DBC substrate packaging was then adopted, based on the many cost and size advantages it offered, such as excellent thermal conductivity, electrical insulation, and improved integration capability. Subsequently, each current rating for both the Motion-SPM in DIP and Mini-DIP was almost doubled; and, moreover, the thermal resistance of each SPM was reduced by about half. And even though performance was increased, their compact external size remained exactly the same. Figure 3 shows real photographs of Motion-SPM in DIP and Mini-DIP.


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The power-carrying potential of a device is dependent on the heat transfer capability of the device. The Motion-SPM provides optimum thermal performance by using DBC substrate technology.


The total power loss in the Motion-SPM includes conduction and switching losses in the IGBTs and fast recovery diodes (FRDs). Loss during the turn-off steady-state can be ignored because it is negligible and has little effect on increasing the temperature in the device.


The conduction loss depends on the device's DC electrical characteristics (i.e., saturation voltage). Therefore, conduction loss is a function of the conduction current and the device's junction temperature. Conversely, the device's switching loss is determined by dynamic characteristics like turn-on/off time and over-voltage/current. Hence, to obtain an accurate switching loss value, we should consider the following: the DC-link voltage of the Motion-SPM system, the applied switching frequency, and the power circuit layout—in addition to the current and temperature. For the detailed equations for calculating both conduction and switching losses based on a pulse width modulated (PWM) power supplyinverter system for motor control applications refer to [2] and [3].


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It should be noted that the PWM modulation index MI=0.8, cos(=0.9, Vdc=300V, Vcc=15V, Tj=125°C, and sinusoidal output current are used as common simulated parameters in all these calculations. The results in this example are obtained by using a Motion-SPM (Figure 4). These graphs show the thermal impedance characteristics of Motion-SPMs. Figure 5 shows the maximum allowable motor output current versus switching frequency in the range 1-20kHz. (These values are obtained based on typical experimental data.)


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Application circuit and test board
The circuit configurations for typical applications of the Motion-SPM in DIP and Mini-DIP are shown in Figure 6.


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A single-supply (15V) drives the low-side IGBTs directly and charges the bootstrap circuitry for the HVICs. The LVIC blocks the command signals from the controller and generates a signal indicating failure mode, when failures in source current or supply under-voltage are detected. This signal line should be pulled up to the positive side of the 5V power supply with approximately 4.7k( to improve noise immunity for the Motion-SPM in DIP.


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The primary motivation for high-side gate driving is that it can provide design simplicity and cost reduction in the design, as well as high performance PWM control. It does this by means of an opto-coupler-less driving capability, a single-supply driving topology, and highspeed operation. To prevent a latched gate signal output that results in system failure, the impedance of the current loop is increased through the Vs terminal by inserting the component in series, as shown in Figure 7. This helps the parasitic diodes not to be turned on/off by the noise signal such as a steep dV/dt and extreme undershoot voltage. Therefore, dV/dt control can be achieved by adjusting the impedance. Motion-SPMs are designed to use the proposed driving method [4].


Conclusion
The performance and size benefits of Fairchild's highly integrated Motion-SPMs are a result of using DBC substrate technology. By using this packaging technology, the module's power ratings have been increased up to 600V/75A in DIP packages and 600V/30A in Mini-DIP packages, which are over twice the amount of the previous generation targeting up to 7kW and 3kW IGBT inverter-controlled motor drive applications. These high-performance modules offer appliance designers many advantages such as increased reliability, simple construction, easy assembly, and remarkable cost-effectiveness during the inverter power stage.


 


References
[1] Smart power module motion SPM user's guide, application note AN9035, Fairchild Semiconductor.
[2] F Casanellas, "Losses in PWM inverters using IGBT's," Proc. Inst. Elect. Eng.-Elect. Power Appl., vol. 141, no. 5, pp. 235-239, September 1994.
[3] K Berringer, J Marvin and P Perruchoud, "Semiconductor Power Losses in AC inverters," Conf. Rec. of IEEE IAS'95, pp. 882-888, 1995.
[4] JB Lee, BC Cho, DW Chung, BS Suh, "Design of a High-side Gate Driver using a Mini-SPM", IPEC 2004.

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