Page 77 - Rashid, Power Electronics Handbook

5

Power Bipolar Transistors

Marcelo Godoy 5.1 Introduction........................................................................................ 63
Simo Äes, Ph.D. 5.2 Basic Structure and Operation................................................................ 64
Engineering Division 5.3 Static Characteristics............................................................................. 65
Colorado School of Mines
Golden, Colorado 80401- 5.4 Dynamic Switching Characteristics.......................................................... 68
1887 USA 5.5 Transistor Base Drive Applications .......................................................... 69
5.6 Spice Simulation of Bipolar Junction Transistors....................................... 71
5.7 BJT Applications .................................................................................. 72
References ........................................................................................... 74

5.1 Introduction with high-current and voltage devices are restricted to cyclo-
converter-fed and multimegawatt drives [1].
Power-npn or pnp bipolar transistors used to be the tradi-
The ®rst transistor was created in 1948 by a team of physicists tional components for driving several of these industrial
at the Bell Telephone Laboratories and soon became a semi- applications. However, insulated-gate bipolar-transistor
conductor device of major importance. Before the transistor, (IGBT) and metal-oxide-semiconductor ®eld-effect transition
ampli®cation was achieved only with vacuum tubes. Although (MOSFET) technology have progressed so much that they are
there are now integrated circuits with millions of transistors, now viable replacements for the bipolar types. Bipolar-npn or
¯ow and control of electrical energy still require single pnp transistors still have performance areas in which they still
transistors. Therefore, power semiconductor switches consti- may be used; for example, they have lower saturation voltages
tute the heart of modern power electronics. Such devices over their operating temperature range, but they are consider-
should have larger voltage and current ratings, instant turn- ably slower, exhibiting long turn-on and turn-off times. When
on and turn-off characteristics, very low voltage drop when a bipolar transistor is used in a totem-pole circuit, the most
fully on, zero leakage current in blocking condition, enough dif®cult design aspects to overcome are the base drive circui-
ruggedness to switch highly inductive loads, which are try, i.e. the required circuit for driving the base. Although
measured in terms of safe operating area (SOA) and ES=b bipolar transistors have lower input capacitance than those of
(reverse-biased second breakdown), high-temperature and MOSFETs and IGBTs, they are current driven. Thus, the drive
radiation-withstand capabilities and high reliability. The circuitry must generate high and prolonged input currents.
right combination of such features restricts device suitability The high input impedance of the IGBT is an advantage over
to only certain applications. Figure 5.1 depicts those voltage its bipolar counterpart. However, input capacitance is also
and current ranges, in terms of frequency, at which where the high. As a result, the drive circuitry must rapidly charge and
most common power semiconductors devices can operate. discharge the input capacitor of the IGBT during the transi-
The ®gure actually gives an overall picture of where power tion time. The IGBT low-saturation voltage performance is
semiconductors are typically applied in industries: High analogous to bipolar power-transistor performance, even over
voltage and current ratings permit applications in large the operating-temperature range. The IGBT requires a
motor drives, induction heating, renewable energy inverters, ÿ5=þ 10 V gate-emitter voltage transition to ensure reliable
HVDC converters, and static VAR compensators and active output switching.
®lters. By contrast, low-voltage and high-frequency applica- The MOSFET gate and IGBT are similar in many areas
tions that include switching mode power supplies, resonant of operation. For instance, both devices have high input
converters and motion control systems and low-frequency impedance, are voltage-driven, and use less silicon than the

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