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Posted on 01 October 2019

The Power of Symmetry in Power Design

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All MOSFETs must share the load equally

Late last year, I began the conversion of my 1998 Chevy S10 pickup truck from gas to all electric. As an electronics engineer, the challenge of designing my own electronic systems was more than interesting to me. A low-voltage charger was needed for the 12-V system battery, a high-voltage three-stage charger was needed for the lead-acid battery bank for motive power (16 series-connected 6-V golf-cart batteries) and, the greatest challenge of all, the heavy-duty power controller for the motive system.

By Mark E. Hazen, engineer and technical writer, http://www.evhelp.com

 

These electronic systems are available off the shelf, but there is nothing more gratifying than designing and building your own. Both battery chargers were fairly straightforward, even the three-stage design for the main motive battery bank was not that difficult. However, the motive power controller became a quest for me. Not wanting to wait until I had time to complete my own power controller, I went ahead and purchased an industrial power controller to get me on the road. This strategy bought me time to design and build something unique – a nonbreadbox circular symmetrical power controller.

The breadbox industrial controller, which I purchased and had been using, is rated for a voltage range of 96 to 144 VDC and 500 A (maximum). Those design parameters are just fine for the on-road EV market. I have no complaint with this controller. It’s seems to be well designed and has plenty of thermal and electrical margin for my EV application. However, I wanted to create something even more robust and efficient – I wanted to climb the mountain myself. It wasn’t long until my thoughts turned to a circular and symmetrical design, which I have fondly named ‘Hazen’s Power Wheel’. Figure 1 shows the power wheel installed. It has all the characteristics of an early prototype or proof of concept.

Hazen’s Power Wheel Provides Thermal and Electrical Symmetry and Efficiency

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. This design does not ‘force’ all of the MOSFETs to operate equally – 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 of the MOSFETs, which means equal and symmetrical gate drive, power current flow paths and heat distribution and dissipation. These are some of the key problems that power designers face in applications that require high-power paralleled switches. Reasonable care must be taken to ensure that all MOSFETs share the load equally.

The key to the design, and the focus of this article, is the physical circular symmetry. This was accomplished with two juxtaposed aluminum discs, both visible in Figure 1.

Sandwiched between the two discs are 15 MOSFETs, mounted around the rim of the Drain Disc as illustrated in Figure 2. The MOSFETs chosen for this design are the IRFP90N20s from International Rectifier, each rated 200 V and 94 A (90 A package limit). These MOSFETs together deliver overall ratings of 200 V and 1350 A.

MOSFETs Evenly Spaced Around the Circumference of the Drain Disc

Each MOSFET of Figure 2 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. I made the disc in Figure 2 semitransparent so you can see the bus bar connection behind.

In Figure 3, you can see 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.

Aluminum Discs Bridge Provide Thermal and Electrical Symmetry

Note also the Gate Distribution Disc in the top center of Figure 3. This small disc even- ly 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 has eliminated the need for that.

Finally, Figure 4 provides another perspective that shows the disc sandwich and the means by which gate drive is symmetrically distributed. A single gate ‘super driver’, using a MOSFET half bridge, provides ample and equal drive to all MOSFETs. The switching frequency is a fixed 4 kHz. I designed and included a trimmer-adjustable current limit circuit that prevents the motor current from exceeding a maximum level in the range of 325 to 1350 A. I also included a watchdog circuit that shuts the controller down if the control resistor, which is connected mechanically to the ‘gas’ peddle, becomes open or disconnected.

One MOSFET Super Driver Feeds Center-Mounted Gate Drive Distribution Disc

Under the hood, the performance of this controller is very strong. From the beginning, I included in the physical design a 4” center-mounted fan to force air over the Drain Disc (see Figure 1 again). As it turns out, the Drain Disc becomes barely warm in normal operation. Nevertheless, the fan will remain to provide for additional thermal margin. Tests with the fan and fan platter removed show that the temperature is constant around the Drain Disc because of the symmetrical MOSFET placement and thermal mass of the aluminum Drain Disc.

Going forward, I will complete the controller by adding a weather/dust encasement and take the circuit boards to, shall we say, a higher level of sophistication – you know how disheveled proof of concepts can get.

For those of you who are interested, many more details regarding the conversion of my Chevy S10 are presented on my Web site, evhelp.com. I created evhelp to assist others in making the conversion from gas to electric. You will find many helpful tips and articles to explain most every detail of the process, including a generous FAQ section.

 

About the Author

Mark E. Hazen is an electronics engineer and professional writer. He has written several college-level engineering textbooks, a paperback on alternative energy and innumerable articles covering analog circuits and communications. He holds a patent on PWM motor control and obviously enjoys power design.

 

 

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