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POWER SUPLIES Increasing power density in welding machines By Fabio Brucchi and Forrest Zheng The demand for portable low cost welding machines, especially in developing countries, is increasing. Discrete IGBTs and MOSFETs are broadly used in the Manual Metal Arc (MMA) and Tungsten Inert Gas (TIG) types with power range from 1.5kW up to 6kW. Mostly, these machines use current mode PWM controls and simple topologies, like Two Transistors Forward (TTF), Half Bridge (HB) and Full Bridge (FB), typically with Zero Current Switching (ZCS) turn-on and hard-switching turn-off. For these configurations, high frequency is one of the most important design trends to improve performance and to reduce cost at system level. Infineon´s Trenchstop 5 IGBT technology, thanks to the dramatic turn-off losses reduction, is the most promising candidate, which is fully capable to accomplish the strong technical requirements of the welding machines. Trenchstop 5 IGBTs improve the performances compared to the previous generation IGBTs, while operating at higher switching frequency. They are also suitable to directly replace, in proper layouts, conventional High Voltage MOSFETs reaching switching frequencies up to 100 kHz. The operation at higher switching frequency leads to the reduction of the magnetic components´ size and of the capacitors´ number. However, a simple “plug-and-play” replacement of former IGBTs is not always possible due to potential issues induced by higher di/dt and dv/dt, such as high voltage overshoot at turn-off, oscillation at turn-on or degradation of EMI figures. Improvements in half bridge topology The drastic reduction of the turn-off losses may result in substantial mechanical changes of the primary side of the converter, thus to a simplification of the mechanical solution. This leads to even further improvements of the PCB layout and the gate driver design. Consequently, the machine’s dimension Fig. 1: (a) 4.5 kW Half-Bridge welding machine demonstrator and (b) related waveforms at time scale 10μs/div. Green waveform, IGBT collector current at 20A/ div. Blue waveform, IGBT VCE at 100V/ div. Purple waveform, output current at 100A/div. Red waveform, IGBT VGE at 10V/div and weight can be significantly reduced. Figure1 shows a welding machine demonstrator designed for this purpose. It is a single phase 4.5kW half bridge MMA/TIG welding machine. In this case it is possible to replace two 40A/600V IGBTs per switch with a single IKW50N65H5 Trenchstop 5 IGBT thanks to adequate layout improvements both in power loop and signal loop. Furthermore, due to the reduction of the switching and conduction losses, the temperature of the devices is strongly reduced, even allowing the use of isolation foils. Figure 2 depicts the case temperature profiles for different technologies of Infineon IGBTs. A noteworthy difference in case temperature, between diverse technologies, can be observed. In particular, Trenchstop 5 outperforms former Trenchstop silicon by 40K. The test is performed dimensioning the gate resistance RG(off) to keep the voltage overshoot at turn-off within 80% of the breakdown voltage, thus limiting the collector emitter voltage at a maximum value of VCE=520V. The lower the stray inductance of the board, the lower is the RG(off) that can be chosen in order to meet the limits imposed. Also, the maximum Gate-Emitter voltage oscillations are considered in the test. Indeed, the acceptable value in this test is -25V<ΔVGE(max)<25V per less than 200ns. Alternatively, it is possible to use Trenchstop 5 in non-optimized layouts by adjusting the passive gate network. In such a case, by introducing a larger gate resistance for the turn-off and a CGE/RCE gate clamping structure, it is once again possible to keep VCE and VGE overshooting within acceptable values. However, it results in a strong reduction of the benefits deriving from the use of Trenchstop 5 IGBTs. This highlights the importance of an appropriate layout. An opportunity to reduce the stray inductance in the power boards even further is to use Trenchstop 5 IGBT technology in surface mount assembly on isolated substrates. This results in a more compact solution with a single heat sink for both high- and low-side IGBT. As a consequence, a special IGBT isolation like IMS or Al2O3 ceramic with an additional reinforced isolation is required. The introduction of these technical changes leads to a significant reduction of the dimensions and weight of the entire machine. An example is given in Figure 3. Here, the second half bridge MMA/TIG welding machine demonstrator, thanks to its new design, causes a reduction of 35% in dimensions and 15% in weight compared to the former demonstrator. This concept allows achieving an overall stray inductance of 40nH, which can be further reduced by 20nH if a different package assembly combination and a full bridge topology design is Fabio Brucchi is Application Engineer at Infineon Technologies AG, in Villach, Austria – www.infineon.com – He can be reached at fabio.brucchi@infineon.com Forrest Zheng is Application Engineer at Infineon Technologies China Co. Ltd, working in Shanghai. He can be reached at forrest.zheng@infineon.com 30 Electronic Engineering Times Europe November 2014 www.electronics-eetimes.com


EETE NOV 2014
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