3.Excercises

Exercise 1

Прочитайте й перекладіть текст.

Exercise 2

Відповісти англійською мовою письмово на наступні запитання:

1. Які контакти є у транзистора?

2. Як підключається транзистор до навантаження у випадку, розглянутому у тексті?

3. Як регулюється змінний опір транзистора при регулюванні середньої напруги на навантаженні?

4. У чому полягає недолік регулювання транзистором як змінним опором?

Exercise 3

Викладіть у письмовій формі англійською мовою у чому полягає різниця у методах регулювання напруги при живленні від джерела постійної напруги.

Exercise 4

Відповісти письмово англійською мовою на запитання:

What are the differences between pulse width modulation and pulse frequency modulation?

Lesson 8.2

1. Text Choppers (transistors)

Obviously a mechanical switch (look lesson 8.1, Figure 8.1,b) would be unsuitable, and could not be expected to last long when pulsed at high frequency. So, an electronic power switch is used instead (Figure 8.2,a).

BJT. Historically the bipolar junction transistor was the first to be used for power switching. It has been widely used in inverters for drives, mainly in applications ranging up to a few kilowatts and several hundred volts.

A transistor can be used as an effective controllable resistor due to the resistance between collector and emitter depends on the current in the base-emitter junction.

Like mechanical switch the transistor would have to be able to provide infinite resistance (switch off) or zero resistance (switch on).

The main (load) current flows into the collector C and out of the emitter E, as shown by the arrow on the device symbol (Figure 8.2,a).

To switch the transistor on (i.e. to make the resistance of the collector –emitter circuit low, so that load current can flow), a small current must be caused to flow from the transistor base B to the emitter. When the base-emitter current is zero, the resistance of the collector-emitter circuit is very high, and the device is switch off.

MOSFET. Since the 1980s the power MOSFET has superseded the BJT in inverters for drives.

Like BJT, the MOSFET is a three-terminal device. The main (load) current flows into the drain D and out of the source S (Figure 8.2,b). Unlike the BJT, which is controlled by the base current, the MOSFET is controlled by the gate-source voltage.

To turn the device on, the gate-source voltage must be comfortably above a threshold of a few volts. When the voltage is first applied to the gate G, currents flow in the parasitic gate-source and gate-drain capacitances. Due to the capacitances have been charged the input current to the gate is negligible, so the steady-state gate power is minimal.

To turn the device off the gate-source voltage must be held below the threshold level.

IGBT. The IGBT is a hybrid device which combines the best features of the MOSFET (i.e. ease of gate turn on/off from lower power logic circuits) and the BJT (relatively low power dissipation in the main collector-emitter circuit). Therefore they are particularly well suited to the medium power, medium voltage range (up to several hundred kilowatts).