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MOTOR CONTROL BY POWER MOSFET TRANSISTOR    243


                    Gate Signal                            N- channel        P- channel
                    0 volts (LOW, logical 0)              Turns off          Turns on
                    5 volts (HIGH, logical 1)             Turns on           Turns off



                     Unlike bipolar transistors, which exhibit a drop in voltage when current is passed through
                   them, MOSFET transistors pass through nearly all of the volts to the motor.

                   The N-  and  P- channel MOSFET transistors you use should be complementary pairs, that is,
                   transistors that share similar specifications. This provides a balance in  current- carrying
                   capability. For example, you might use IRF530/IRF9530 or IRF540/IRF9540. Not all MOSFETs
           G       use such convenient numbering sequences to indicate pairing. You can consult a basic data
                   book to find complementary pairs, and be sure to read the specifications provided by online
                   retailers. Most retailers provide direct links to datasheets provided by the MOSFET
                   manufacturers, and you’re encouraged to review them and compare specifications.


                   There are many ways to build MOSFET  H- bridges, and each method has its distinct advantages.
                   Rather than trying to cover them  here— which  wouldn’t do them  justice— see the bonus  H- bridge
            ON THE
                   projects on the RBB Online Support site (refer to Appendix A for details). Included are several
             W E B
                   tested variations, from fairly simple to somewhat complex.

                   COMMON DESIGN GOALS FOR TRANSISTOR  H- BRIDGES
                   If you’d like to design your own MOSFET transistor  H- bridge, keep these basic design goals
                   in mind.
                     First, the transistors you choose must be capable of handing the current draw demanded
                   by the motor. Refer to the motor specifications to determine their maximum current draw, or
                   test it yourself using the steps in Chapter 21, “Choosing the Right Motor.”
                                Most motors draw at least 500 milliamps, and this exceeds the  current- carrying
           TO-92             capacity of MOSFETs that come in the smaller TO- 92 package. For most motors
                             you work with, stick with the devices in the TO- 220 or TO- 3 packages.
                     TO-220
                      Shown     On  larger- power MOSFETs, the case of the transistor doubles as the drain
                     approximate
                       scale  terminal. This is important if you mount the transistors on a common heat sink,
                  TO-3       and especially when you ground the heat sink to the metal frame of the robot.
                             Avoid the hassles of potential short circuits by getting a set of insulated
                     transistor- mounting kits. These insulate the transistors electrically but still allow the heat sink
                   to sink heat.
                     When using transistors in TO- 220 or TO- 3 packages, if you place the transistors close
                   together on a circuit board be sure none of the metal cases touch one another.
                     Most  H- bridge designs use flyback diodes placed in parallel with each transistor. Without
                   these, the back EMF from the motor could damage the transistor. You’ll know this has hap-
                   pened if the transistor constantly runs the motor, but at a reduced voltage. While most modern
                   power MOSFET transistors include a diode as part of its internal construction, many
                     robo- builders suggest adding your own external diodes. A  fast- acting Schottky diode, such as
                   the 1N5817, is usually a good choice when using small to medium-size motors.
                     And finally, remember that motors produce lots of noise. You’ll want to place an aluminum









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