Power Electronics

Symmetrical Layout Enhances Power Controller

Configuring paralleled power MOSFETs in a circular layout sets the stage for the balancing of currents and heat dissipation in the power stage of an electric vehicle's motor controller.

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.

Fig. 3 shows the source disc in place with a representative MOSFET sandwiched between the two discs. The discs are separated with nylon spacers and nylon bolts. There is a small gap between the top surfaces of the MOSFETs and the source disc. The power cable that comes from the negative supply terminal of the motive battery bank connects to the -VSS bus bar, which delivers the current to the center of the source disc for unbiased distribution through the disc to the source leads of all MOSFETs. Again, the bus bar only contacts the disc at its center.

Also note the gate-drive distribution disc shown in Fig. 2 and in the top center of Fig. 3. This small disc evenly distributes gate drive to all MOSFETs via an interconnecting lead and small gate resistor for each.

As a side note, the source and drain leads of each MOSFET are pinned to the edges of the discs using brass washers and screws. Also, the electrical portion of this design does not use any electrical means of load balancing among MOSFETs. The physical symmetry of the design and the quality of the MOSFETs have eliminated the need for that.

The two juxtaposed aluminum discs are both visible in Fig. 4, which shows the Power Wheel installed in the pickup truck. As evidenced by this photo, the Power Wheel has all the characteristics of an early prototype or proof of concept.

Verifying Performance

Under the hood, the performance of this controller is very strong. From the beginning, I included in the physical design a 4-in. center-mounted fan to force air over the drain disc (Fig. 4). As it turns out, the drain disc becomes barely warm in normal operation. Nevertheless, the fan will remain to provide for additional thermal margin.

To assess the performance of the Power Wheel, temperature readings were taken around the aluminum drain disc at each MOSFET location (Table). Readings were taken with a handheld noncontact infrared digital thermometer. To obtain these readings, the vehicle was driven 8.5 miles in a city environment. The ambient temperature was 31°C.

The cooling fan and white disc cover, which can be seen in the under-hood photo (Fig. 4), were removed prior to the test. Also, as can be seen in the photo, the controller is mounted vertically and the bottom MOSFETs are within 15 cm to 20 cm of the electric motor, which during this test had an outer-case temperature of 52°C. The closeness of the electric motor causes the ambient air temperature to be higher near the bottom of the power stage.

Therefore, it is expected that the lower and back drain disc areas would be warmer because of their juxtaposition to the electric motor and vehicle firewall. It is also expected that, when the cooling fan is used, temperatures would be more even around the drain disc because the temperature of the flowing air would be more constant, forcing all MOSFET sites to increase temperature starting from the same ambient point.

What is interesting here is that all the temperatures are quite low (barely warm), which indicates that the cooling cover and fan are actually not needed. The reason that all the temperatures are low is that the MOSFETs are highly efficient with a low RDSON of 0.023Ω and the switching losses are also low at the 4-kHz switching frequency. The lack of hot spots around the drain disc indicates that the MOSFETs match fairly closely and that heat is dispersed quickly in the aluminum drain disc.

In addition, the controller has adjustable current limiting, which was set to limit the total drain current to 325 A maximum or 21.7 A maximum per MOSFET. The actual average road current is around 150 A, so the average current for each MOSFET is only 10 A, which is only 2 W to 3 W per MOSFET (P = I2R = 102 × 0.023 = 2.3 W plus switching loss).

Needless to say, this controller is nowhere close to being at risk — even on the hottest summer day. More details regarding the conversion of the Chevy S10 are presented at www.evhelp.com.

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