Explosion Robust IGBT Modules in High Power Inverter Applications

Low Inductance, Explosion Robust IGBT Modules in High Power Inverter Applications Lance Schnur ADtranz Transportation, Inc. Lebanon Church Rd. West Mifflin, PA 1236 USA Gilles Debled, Steve Dewar ABB Semiconductors AG Fabrikstrasse 3 CH-6, Lenzburg, Switzerland John Marous ABB Semiconductors Inc. 7 Epsilon Dr. Pittsburgh, PA 1238-2838 USA Abstract - Inverters in the MW class can greatly benefit from the use of IGBT modules with low internal inductance and with resistance to the effects of catastrophic explosions. The Flat Low Inductance Package (FLIP ) technology enables production of High Power IGBT modules from 18A, 18V to 12A, 33V with an internal inductance as low as 3nH. This represents an improvement of a factor of 2 compared to conventional modules at this power level. This is particularly important in high current, fast switching applications in order to minimize voltage overshoots during turn on and turn off and therefore reduce the voltage overhead needed by silicon for a given DC link voltage. In addition, the FLIP s new terminal arrangement enables an improved high power inverter construction which results in a total parasitic inductance of only 4nH for an inverter in the MW range. An additional problem with the application of high power inverters in the MW range is the behaviour of the packaging under catastrophic failure conditions. Typically, failure of conventional modules can result in particles being emitted from the module, which involves risk of bus bar shorts and causes significant mechanical damage to the bus bar structures and gate circuitry. The FLIP packaging is designed to avoid emission of metal parts under such conditions, which will limit mechanical damage and make repair less costly and time consuming. I. Introduction In high power inverter applications such as MW range transit car propulsion systems, the use of IGBT modules is widespread due to its advantages in terms of fast switching, ruggedness, simplicity of gate drive and ease of use, but also to the easy mounting and built in package isolation features. Reductions of 4 to % in inverter cost, weight, and volume are possible compared to alternative solutions. However, the fast switching operation requires special attention to reduce the circuit inductance to avoid overvoltage at switching. Another issue worth consideration is the ability of the module to manage faults without allowing extensive rupture or explosion. While the IGBT can limit and commutate current due to normal fault events, the traction environment may present situations where extensive energy is conducted through the module. In the event of such a high energy fault, the module must not explode to the point that it emits conductive particles. If conductive particles were to be emitted, inverter repair is lengthy and costly, and the risk of power bus shorting. The FLIP module concept was developed to address these concerns as well as to offer the performance and reliability required of MW class transit propulsion systems. It features low internal inductance and allows low inductive connection of bus bars. In addition, it has the significant benefit of robustness in the event the ultimate fault limit is exceeded. This paper discusses these benefits and the issues relating to the design of such modules. II- Overvoltages and Optimum Use of Silicon The accurate control of motor parameters combined with environmental specifications for mass transit equipment requires high inverter switching frequency (~Hz -1kHz). For power electronic systems in the MW range, high power IGBTs offer fast switching operation, reduced switching losses, and offer a realistic implementation of such high frequency operation. High speed of commutation necessarily involves di/dt high enough (2-6kA/us) that any unclamped inductance leads to voltage overshoot during commutation. IAS St Louis page 1 of October 22

The transient overvoltage reduces the margin to the safe operating area limit and in worst case requires a device with an higher voltage rating for a given commutated power. It results therefore in an lower efficiency of the equipment. Clamping of overshoots using a snubber circuit is not suitable because it adds additional complexity, losses, and cost, and it reduce overall reliability and efficiency. Another consequence of inductance is the increase of the rate of rise of voltage at recovery of the diode, for given gate drive and diode conditions. This dv/dt may stress motor insulation and increase peak reverse recovery power in the diode. Moreover, the inductive loops responsible for transient voltage overshoot also act as EMI generators and, in conjunction with parasitic capacitive elements, can cause oscillation. The inverter designer has therefore to struggle to manage this voltage overshoot at turn off and to minimize the inductance of the filter capacitors responsible for it. The high power IGBT module designer, on the other hand, must be concerned with the many IGBTs and diode chips connected in parallel inside the module. More uniform connections in the module offer more even distribution of current and commutation voltage conditions between chips, which help to keep the components inside the module at similar temperatures and enables better use of silicon and minimizes potential early-wear out failures due to degradation over module lifetime. III- FLIP Module Design The ABB Flat Low Inductance Package module, so-called FLIP [2], is designed for low internal inductance by making use of wide parallel short sheets for collector and emitter terminals. This structure forms an internal laminated bus structure, making use of mutual inductance cancellation, and limits skin effect [1]. The internal construction of the module is exhibited in Figures 1 and 2, showing the elements making the internal bus structures and their respective locations on the baseplate. Connection extends to the chip by multiple wire bonds of equal lengths. These multiple wire bonds not only reduce the inductance of this connection but provide, at the same time, redundancy of the contact. There is no need for terminal stress relief as in the other module types, which affects internal inductance. Auxiliary terminals can also be connected by wire bonding to reduce gate noise sensitivity and EMI issues. IGBT and diode chips are mounted on eight substrates, located evenly on the module base-plate. This arrangement allows an optimum use of footprint area to evenly spread the power dissipated in both acceleration and braking operation modes, and it avoids any significant difference in junction temperature of the paralleled chips. The ceramic substrates are 1mm thick and are made of AlN which permit low thermal conductivity together with required isolation and partial discharge parameters. The small size of the substrates permit to have satisfactory performance in terms of the difficult thermal fatigue [3] requirements. The chips have a metallized molybdenum buffer plate on the emitter that enables a durable wire bond, which is also needed to enable acceptable device life in the strenuous power cycling environment characteristic of traction applications. The substrate has an unpatterned metallization, which provides the lowest possible inductance for any given geometry. The outside connections consist of multiple terminals in parallel, which provide sufficient cross section for high current. And since they are at different heights (for the emitter and collector) the inductance of the connection to the external bus is low. Rivets Case Gate & Aux. PCB Upper terminal sheet Insulator sheet Lower terminal sheet Lower insulator sheet Ceramic substrates Base plate Fig.1. FLIP Module construction (SNA 12F33D). The main terminal sandwich, which is connected via screws to the external equipment bus bars, is mechanically decoupled from the ceramic substrates to the equipment bus works, thanks to the wire bonds. This structure supports the vibration and shock responses of this module. The base-plate consists of an 8mm thick copper plate with a slighly convex shape. This offers low steady state and transient thermal impedance, as the thick plate acts as a thermal buffer thanks to its heat capacity. This function is useful for management of short power transients, and it provides a stable flat contact when mounted onto the heat sink. The large footprint of the module is favorable for low thermal contact resistance module to heat sink. IAS St Louis page 2 of October 22

IV- Internal and Circuit Inductances using FLIP module FLIP modules cover the ratings from 18A,18V to 12V, 33V. They are offered in several configurations, including a complementary pair such that inverter bus connections have minimum inductance external to the module. Figure 2 shows a sketch of the cross section of a FLIP module and the arrangement of the power electrodes. The internal inductance achieved using the FLIP module construction is as low as 3nH, which represents an improvement by a factor of 2 compared to conventional modules at this power level. Chart 1 illustrates the breakdown of the internal inductance and shows the comparison with modules having conventional construction. It shows that major contributors to inductance are power terminals, with.8 to 1 nh for upper terminals and 2nH for the lower side. The multiple bonding arrangement contributes only for.2nh, which is favorable to efficient internal paralleling of the chips. For conventional modules, the breakdown of internal inductance exhibits similar high relative contribution of terminal inductance to total internal inductance and also shows the much higher magnitude of inductances. 7 6 4 3 2 1 nh IGBT FLIP low IGBT FLIP high IGBT Conven. Diode FLIP low Diode FLIP high Aux. emitter to aux. collector Aux. emitter to emitter Aux. collector to collector Chart 1. Inductance of FLIP and Conventional Modules Diode Conven. Collector Emitter Insulator Auxiliary Emitter Auxiliary Collector Chip Baseplate Substrate Gate Drive High Side Module Heatsink Gate Drive Low Side Module Capacitor Fig. 3. Schematic of the construction of an inverter using complementary FLIP modules as implemented in Traction Fig. 2. Cross Section of FLIP module showing the power electrodes arrangement Note : The measurement of such low values of inductance need the use of specific measurement methods to get acceptable accuracy. The method developed to get this data is detailed in [4]. An equivalent current source (6kA) generates a constant di/dt (3kA/us) applied to the IGBT under test while it is in on-state. Under such di/dt, one nh results in a voltage drop of 3V which is measurable by a 1:1 voltage probe. The voltages are measured at different sensing points (power collector, auxiliary collector, auxiliary emitter, and power emitter) to permit the components of the internal inductance to be determined. Fig. 3. shows the construction of an inverter leg with the low inductance bus arrangement that can be realized with the two level terminal outputs of the FLIP module and complementary package approach. Such layout for a MW class inverter yields an inductance of 4nH. In addition, the space available directly on top of the module due the side location of the electrodes can be used to locate the gate drive unit, realizing short gate connections and improved immunity with respect to noise. The turn off switching waveform and short circuit waveform, shown respectively in Figures 4 and, shows the small voltage overshoot consequence of the very low inductance arrangement. The overshoot at short circuit can be also limited to some extent by a soft shutdown scheme applied to the gate of IGBT. However, this is not applicable in normal turn off because it results in significant increase in turn off losses. IAS St Louis page 3 of October 22

V. FLIP Module with Regard to Explosion The IGBT is a very rugged semiconductor device which can limit short circuit current and is able to turn off short circuit current levels after several microseconds, with 1us being the industry standard for minimum short circuit withstand capability required of the module. However, loss of control of the gate and/or the inadequate device protection, which can happen in exceptional situations, may lead to conditions exceeding safe short circuit current levels and to breakdown of the IGBT or of the diode. mechanical damage can result to the bus structures, requiring expensive and time-consuming repairs. The FLIP module is designed to delay the occurrence of case rupture and to minimize the effects of a catastrophic failure. This construction permits, in a case of defects previously explained, relief from the internal pressure due to arcing and therefore allows an increased fault current prior to explosion, in relation to conventional modules. And even when explosion occurs, the main electrode arrangement assures that metal parts (from the electrodes) will not be emitted with explosions of specified energies. Figures 6 and 7 show the explosion test circuit and results of an explosion with 7kJ and 2kJ of energy through the fault. IC (A), VCE (V) 3, 3, 2, 2, 1, 1, IC VCE VGE 2 1 1 () (1) Gate Voltage (V) As opposed to the conventional module, in which epoxy material is used to tightly maintain the housing to the terminals to achieve mechanical strength, the FLIP module does not use hard filling materials. The purposes of the outer shell are only to cover the entire module and to provide an additional clamp to the electrode sandwich. Large openings in the top of the shell provide a pressure release mechanism, although cover plates are epoxied over the openings. (2) (1) 1 2 3 4 6 7 8 9 1 11 12 13 14 1 16 17 time (µs) Fig. 4. Turn off waveforms of a 12A,33V FLIP module. Conditions: VDC=26V, IC=12A (1) 3, 2 3, 1 2, 1 IC (A), VCE (V) 2, 1, 1, IC VCE 1 2 3 4 6 7 8 9 1 11 12 13 14 time (µs) Fig.. Shortcircuit waveforms of a 12A,33V FLIP module. Conditions :VDC=26V,tp =1 µs The short through situation, when not controlled, results in flow of charge associated with the DC link capacitor energy. The fault current can rise locally the point of wire bond rupture, and the energy dissipated can raise the silicon gel temperature and can lead it into the plasma state []. This gel causes pressure to build at the fault location inside the module. VGE () (1) (1) Gate Voltage (V) Fig. 6. FLIP IGBT after explosion test at an energy Etot= 7.2kJ Conditions :VDC=1V, Cfilter=6.4mH, Lstray=2nH While this module shows extensive failure in the 2kJ case, as shown in Figure 7, the bus structure remains intact. Conductive particles were not emitted, as would have happened in the case of conventional modules with topmounted terminals and pressure-retaining hard epoxy fill inside the module. If this pressure builds to a sufficient point, it reaches the point of case rupture, and if energy is sufficiently high, then catastrophic explosion occurs. If charged or conductive particles are emitted, they can short inverter bussing, causing more extensive faults. And if metal parts are emitted, IAS St Louis page 4 of October 22

VI- Conclusion The FLIP module has been designed with very low inductance required to operate IGBT modules with the highest DC link voltage allowable by the particular chip being used. It also offers less severe consequences in the event of an unusual fault event such as chip or isolation failure. It therefore addresses two important issues related to high power, MW class inverters using isolated module components and gaining the benefits accorded of such module components. It also offers benefits for future applications involving high di/dt, such as soft switching resonant converters and pulsed discharge or power modulator systems. In addition to the low internal inductance, the allowed top-mounted gate drive system can offer high gate currents and high rates of change of gate current, which may be of importance with these high power module applications of the future. Fig. 7. FLIP IGBT after explosion test at an energy Etot= 2.3kJ Conditions :VDC=13V, Cfilter=24mH, Lstray=2nH References [1] D.Braun, R.Lukaszewski, D.Pixler, G.Skibinski, Use of coaxial CT and Planar Bus to improve IGBT Device Characterization,IEEE Trans. Power Electronics, 1996, pp. 17 116 [2] T.Stockmeier, R.Bayerer, E.Herr, D.Sinerius, U.Thiemann, Reliable 12Amp 2V IGBT Modules for Traction Applications, IEE colloquium, London, 199, Proc. pp.3/1-3/13 [3] E.Herr, T.Frey, R.Schlegel, A.Stuck, R.Zehringer, Substrate-to-base solder joint Reliability in High Power IGBT Modules, Microelectronic Reliability, 1997. [4] T.Stockmeier, U.Schlapbach, 12A, 33V IGBT Power Module exhibiting Very Low Internal Stray Inductance, proc. PCIM Hong Kong, 1997. [] D.Braun, D.Pixler, P.LeMay, IGBT Module Rupture Categorization and Testing, IEEE IAS meeting, 1997. IAS St Louis page of October 22