Electric vehicle
Electric Vehicle Technology Explained, Second EditionTable 7.1
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| 160 Electric Vehicle Technology Explained, Second Edition Table 7.1 Key data for the main types of electronic switches used in modern power electronics equipment Type Thyristor MOSFET IGBT Symbol d s g g c e Maximum voltage (V 1000 Maximum current (A 50 Switching time ( μs) 10–25 0.3–0.5 has some understanding of their advantages and disadvantages. Table 7.1 gives the main characteristics of the most commonly used types. The metal oxide semiconductor field effect transistor (MOSFET) is turned on by applying a voltage, usually between 5 and 10 V, to the gate. When on the resistance between the drain (d) and sources) is very low. The power required to ensure a very low resistance is small, as the current into the gate is low. However, the gate does have a considerable capacitance, so special drive circuits are usually used. The current path behaves like a resistor, whose ON value is R DS ON . The value of R DS ON for a MOSFET used in voltage regulation circuits can be as low as about 0.01 . However, such low values are only possible with devices that can switch low voltages, in the region of 50 V. Devices which can switch higher voltages have values of R DS ON of about 0.1 , which causes higher losses. MOSFETs are widely used in low-voltage systems of powerless than about 1 kW. The insulated gate bipolar transistor (IGBT) is essentially an integrated circuit combining a conventional bipolar transistor and a MOSFET, and has the advantages of both. The IGBT requires a fairly low voltage, with negligible current at the gate to turn on. The main current flow is from the collector to the emitter, and this path has the characteristics of a p–n junction. This means that the voltage does not rise much above 0.6 Vat all currents within the rating of the device. This makes it the preferred choice for systems where the current is greater than about 50 A. IGBTs can also be made to withstand higher voltages. The longer switching times compared with the MOSFET, as given in Table area disadvantage in lower power systems. However, the IGBT is now almost universally the electronic switch of choice in systems from 1 kW up to several hundred kilowatts, with the upper limit rising each year. The thyristor has been the electronic switch most commonly used in power electronics. Unlike the MOSFET and IGBT, the thyristor can only be used as an electronic switch – it has no other applications. The transition from the blocking to the conducting state is triggered by a pulse of current into the gate. The device then remains in the conducting state until the current flowing through it falls to zero. This feature makes it particularly useful in circuits for rectifying AC, where it is still widely used. However, various variants of the thyristor, particularly the gate turnoff (GTO) thyristor, can be switched off, even while a current is owing, by the application of a negative current pulse to the gate. |