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INDUSTRIAL CONTROL HANDBOOK - 3.4 POWER TRANSISTORS

Transistors have become more widely used in ac variable-frequency motor speed drives and other power applications than in previous years because today larger transistors are available that can control voltage over 1000 volts and current over 1000 A. Transistors that are manufactured for these higher-power applications function exactly like the smaller-signal PNP and NPN transistors you have used in your electronic laboratory experiments, which will make understanding their operation much easier. When available, transistors are preferred over thyristors because they are much more versatile and can react to higher frequencies. One of the problems with SCRs, triacs, and other thyristors is that they must depend on reverse-bias voltages to help turn them off once they have been fired. This is not a problem with the transistor since it can easily be controlled by controlling the current to its base.

Typically you will find power transistors mounted in modules where their internal connections have been made during the manufacturing process. This ensures that the transistors used in pairs are matched and that critical connections will endure through all types of rugged operations. Cooling and other protection issues are also controlled when the transistors are packaged as a module. You will typically find transistors as power transistors, darlington pairs, or as specialized transistors called insulated gate bipolar transistors (IGBTs). These types of transistors will be discussed in the remainder of this chapter.

FIGURE 3-16 Typical transistor modules in the output section of a variable-frequency ac motor drive. The transistor modules are identified as Q1, Q2, and Q3. Each module has two transistors, one to provide the positive part of the ac wave, and the second to provide the negative part. The large bold lines are from the dc bus voltage that feeds the output section.FIGURE 3-16 Typical transistor modules in the output section of a variable-frequency ac motor drive. The transistor modules are identified as Q1, Q2, and Q3. Each module has two transistors, one to provide the positive part of the ac wave, and the second to provide the negative part. The large bold lines are from the dc bus voltage that feeds the output section. (Courtesy of Rock-well Automation's Allen Bradley Business.)

FIGURE 3-17 Output waveform for transistors used in three-phase variable-frequency drive. The top diagram is for variable voltage input (VVI) drives, and the bottom diagram is for pulse-width modulation (PWM) drives.

FIGURE 3-17 Output waveform for transistors used in three-phase variable-frequency drive. The top diagram is for variable voltage input (VVI) drives, and the bottom diagram is for pulse-width modulation (PWM) drives.FIGURE 3-17 Output waveform for transistors used in three-phase variable-frequency drive. The top diagram is for variable voltage input (VVI) drives, and the bottom diagram is for pulse-width modulation (PWM) drives. (Courtesy of Rockwell Automation's Allen Bradley Business.)

FIGURE 3-18 Example transistor modules.FIGURE 3-18 Example transistor modules. (Courtesy of Rockwell Automation's Allen Bradley Business.)

Fig. 3-18 shows a diagram of two different methods of connecting additional transistors to provide large-current control. These transistors are available in modules so that technicians do not need to worry about internal connections. The modules provide terminals to connect the transistors with other parts of the motor drive circuitry. These terminal points also provide a location for troubleshooting the PN junction of each transistor. A standard front-to-back resistance test can be made for the top transistor in Fig. 3-18a by placing the meter leads on the B and C terminals to test the base-collector junction, and by using terminals B and BX to test the base-emitter junction. The second transistor in the set can also be tested by using the E, BX, and C terminals. Most maintenance manuals for the drives will provide information that specifies the proper amount of resistance when the junction is forward and reverse biased during the test.

Fig. 3-19 shows a picture of a typical transistor module used in motor drives. The terminals shown on this module will be identified exactly like the ones in the diagrams in Figs. 3-16 and 3-18. These terminals will provide easy field wiring in the drive. The terminal connections will also make it simple to remove wires for testing and troubleshooting. If the module must be changed in the field, the technician can simply remove the wires and change the module.

FIGURE 3-19 Transistor modules that show the terminal layout. The terminals in this module match the diagram shown in Figs. 3- 16 and 3-18.FIGURE 3-19 Transistor modules that show the terminal layout. The terminals in this module match the diagram shown in Figs. 3-16 and 3-18. (Courtesy of POWEREX Inc.)

FIGURE 3-20 (a) Internal configuration for a darlington transistor pair. This diagram shows transistor T1 as the driver of transistor T2, which is the output for the darlington. (b) Electronic symbol for the darlington transistor. (c) Typical packages for the darlington transistor including the module, 1-07 case, and the TO-220 case.FIGURE 3-20 (a) Internal configuration for a darlington transistor pair. This diagram shows transistor T1 as the driver of transistor T2, which is the output for the darlington. (b) Electronic symbol for the darlington transistor. (c) Typical packages for the darlington transistor including the module, 1-07 case, and the TO-220 case. (Courtesy of Philips Semiconductors.)

 

3.4.1 Darlington Transistors

Darlington transistors are sometimes called darlington pairs because two bipolar transistors are packaged together to provide better operation for high-power and high-frequency applications in motor drives and power supplies. Fig. 3-20 shows the diagram of two transistors that have been manufactured as a darlington pair. From this figure you can see that the first transistor T1 is the driver of the second transistor T2. Transistor T2 is called the output for the darlington pair. The input signal is sent to the base of T1, which will act like a typical bipolar transistor. The larger the base current becomes, the larger the collector current will be up to the point of saturation. The emitter of TI is used to provide base current to the output transistor T2. The base current of T2 will be used to drive the collector current, which will be the output current for the darlington.

 

3.4.2 The Need for Darlington Transistors

In recent years, electronics have been integrated into motor speed drives and a variety of switching-type power supplies. This meant that standard discrete components needed to be altered to provide better characteristics. The need for the darlington pair grew from the limitations of SCRs and triac-type thyristors. Thyristors control current by delaying the turn-on time. The later the pulse is applied to turn them on, the smaller the amount of current they will conduct during each cycle. On the other hand, a transistor uses variable current (0 to saturation), which provides an output current that will be a duplication of the input. This means the transistors will produce an analog signal when an analog signal is provided to its base. The simple bipolar transistor has several limitations including slow switching speeds, low gains, and larger power losses due to the switching process. A family of high-gain transistors called metal-oxide semiconductor field effect transistors (MOSFETs) were produced to address the gain problem, but they did not have the capability of controlling larger currents, so the darlington pair was designed. The darlington pair can actually be two discrete transistors that are connected in the driver/output configuration, or they can be a single device that has the two transistors internally connected at the point where it was manufactured as a single package.

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