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Issue #217 August 2008
Subcategory Winner - Microchip 2007 Design Contest
INTELLIGENT ENERGY SOLUTIONS
Electric Vehicle Inverter Design
Build A System For Powering AC Induction Motors
by Dan Hall, Tristan Kasmer, Doug Krahn, Adam McIntyre, and Dena Ponech
Start | Power Inverter | Gate Drivers | Control Board | Space Vector Modulation | PID Tuning | Altering Motor Control Parameters | HMI | Protocol | Firmware/Software | Sources & PDF
POWER INVERTER
Powerex CM400DU-12F insulated gate bipolar transistor (IGBT) modules and their companion BG2B gate-driver circuits provide a relatively low-cost method for generating three-phase AC power from DC power.[1] The DC power from the EV battery pack is converted to three-phase AC to drive an AC induction motor. We configured each phase of the IGBT module design with RCD snubber protection (see Figure 1).[2] The gate driver circuit can be seen in the application note for the BG2B universal gate drive board from Powerex using two VLA106-15242 DC/DC converters and two VLA503-01 gate drivers.[3] The gate driver boards are recommended by Powerex for use with the dual-IGBT modules and provide 2,500 VRMS of control signal isolation via high-speed optocouplers and desaturation detection to prevent short-circuit conditions on the IGBTs. The CM400DU-12F modules are rated with a collector-emitter voltage of 600 V and a continuous emitter current of 400 A with a peak rating of 800 A. The modules are able to provide switching speeds of up to 30 kHz, we used 20 kHz. The frequency range promotes efficient operation of the spatial vector modulation (SVM) algorithms used to drive the IGBTs and bring the switching noise out of audible range.
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| Figure 1 — This is a Powerex CM400DU-12F module with an RCD snubber circuit. |
Because the IGBT modules are capable of high switching speeds and operate at extreme power levels, transient voltage and current protection are important. An RCD snubber circuit was chosen for its ability to limit peak voltages and reduce total circuit losses, including switching and snubber losses. Low ESR and low self-inductance rated components are the heart of the protective design, eliminating the parasitic and residual inductances that can occur across the IGBT’s switches and across the DC bus.
The inverter can generate approximately 100 kW, so it requires an effective cooling system. The initial cooling system chosen was based on a liquid coldplate from D6 Industries that costs $180. The cooling system can be configured as a stand-alone unit with its own pump and radiator or as part of the vehicle’s heating and cooling system. The packaging of the IGBT modules enables all three to be mounted directly on a single coldplate for simple and effective thermal management. The gate driver circuits for the inverter are supported by an aluminum bar (see Photo 1). This cooling configuration is mounted to the inverter enclosure for a greater heat dissipation area.
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| Photo 1 —This photo shows the IGBT and gate driver assembly during the testing phase of development. Hall-effect current sensors are mounted on phase 1 and 3. The snubber board (not shown) is installed on top of this circuit. |
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