Symmetrical Layout Enhances Power Controller

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

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 -V_SS 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 R_DSON 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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