Symmetrical Layout Enhances Power Controller

Nov 1, 2007 12:00 PM
By Mark E. Hazen, Engineer and Technical Writer, evhelp.com, Ocala, Fla.

The conversion of a gas-powered pickup truck to an all-electric power train requires the introduction of several electronic subsystems to support battery-powered operation. I became familiar with these requirements late last year when I modified my 1998 Chevy S10 pickup truck to operate as an electric vehicle (EV).

A low-voltage charger was needed for the 12-V system battery, while a high-voltage three-stage charger was needed for the lead-acid battery bank (16 series-connected 6-V golf-cart batteries) that provided the truck's motive power. The truck's new electric motor also demanded a heavy-duty power controller to deliver power to the motor, an industrial-grade series-wound dc motor from Advance DC Motors.

To complete the conversion and get the EV running, I initially used an off-the-shelf breadbox industrial power controller. This power controller contains a pulse-width-modulated (PWM) controller, gate driver and power MOSFETs, as well as protection functions such as adjustable current limiting, low-voltage cutoff and overtemperature protection. With a voltage range of 96 Vdc to 144 Vdc, and a maximum load current rating of 500 A, this off-the-shelf controller was certainly adequate for the EV application.

However, I decided to create a novel design that would be even more robust and efficient, offering greater electrical and thermal margin than the purchased power controller. The resulting power controller, which I have dubbed Hazen's Power Wheel, employs a circular design that permits a symmetrical configuration of all the power MOSFETs.

The idea behind the circular and symmetrical concept is to distribute electrical and thermal currents evenly to help ensure that all MOSFETs are treated equally. The Power Wheel design does not “force” all the MOSFETs to operate equally; rather, it requires highly controlled semiconductor manufacturing conditions and/or somewhat sophisticated electronic controls to do that.

Instead, the physical design sets the stage for operational fairness for all the MOSFETs, which means equal and symmetrical gate drive, power current flow paths, and heat distribution and dissipation. Although the Power Wheel targets the EV motor-drive application, the same concept may be applied in other high-power applications where multiple switches are paralleled.

Power Controller Design

Like the off-the shelf power controller, the Power Wheel design includes the PWM circuit, MOSFET gate driver, MOSFET power stage and protection circuitry. However, in this article, the focus will be on the power stage, which consists of 15 MOSFETs that are physically configured in a circular symmetrical layout (Fig. 1).

The MOSFET gate driver is actually a single-gate “super driver,” consisting of a MOSFET half bridge, which provides ample and equal drive to all MOSFETs. The switching frequency of the power controller is a fixed 4 kHz. As shown in Fig. 2, the control circuit includes a trimmer-adjustable current-limit circuit that prevents the motor current from exceeding a maximum level in the range of 325 A to 1350 A. The control circuit also includes a watchdog circuit that shuts the controller down if the control resistor, which is connected mechanically to the gas pedal, becomes open or disconnected.

Fig. 2 includes a side view of the power stage. From this view it can be seen how the 15 MOSFETs that drive the motor are sandwiched between two large aluminum discs that connect to the drain and source of each transistor. The 15 MOSFETs are actually mounted around the rim of the drain disc as illustrated in Fig. 1.

The MOSFETs chosen for this design were International Rectifier's IRFP90N20s, each rated at 200 V and 94 A (90 A is the package limit). Together, these MOSFETs deliver overall ratings of 200 V and 1350 A.

Each MOSFET in Fig. 1 is mounted directly to the drain disc with 4-40 hardware and thermal compound for good heat conductivity. A copper bus bar (-M_(FEED)) collects the total motor-drive current at the center of the disc. The only physical contact between the bus bar and disc is at the center. The disc in Fig. 1 is illustrated as semitransparent to show the bus-bar connection behind the disc.

Want to use this article? Click here for options!
© 2007 Penton Media, Inc.