ower MOSFet Power MOSFETS are marketed by different manufacturers with differences in internal geometry and with different names such as MegaMOS, HEXFET, SIPMOS, and TMOS. They have unique features that make them potentially attractive for switching applications. They are essentially voltage-driven rather than current-driven devices, unlike bipolar transistors The gate of a MOSFET is isolated electrically from the source by a layer of silicon oxide. The gate draws only a minute leakage current of the order of nanoamperes. Hence the gate drive circuit is simple and power loss in the gate control circuit is practically negligible. Although in steady state the gate draws virtually no current, this is not so under transient conditions. The gate-to-source and gate-to-drain capacitances have to be charged and discharged appropriately to obtain the desired switching speed, and the drive circuit must have a sufficiently low output impedance to supply the required charging and discharging currents. The circuit symbol of a power MOSFET is shown in Fig. 30.5. Power MOSFETs are majority carrier devices, and there is no Drain minority carrier storage time. Hence they have exceptionally fas rise and fall times. They are essentially resistive devices when turned on, while bipolar transistors present a more or less co It VcEst) over the normal operating range. Power dissipation in MOSFETs is Id-Rpston), and in bipolars it is ICVcEsat). At low currents, therefore, a power MOSFET may have a lower conduc- Gate tion loss than a comparable bipolar device, but at higher cur ents, the conduction loss will exceed that of bipolars. Also, the ncreases An important feature of a power MOSFET is the absence of a secondary breakdown effect, which is present in a bipolar transistor, and as a result, it has an extremely rugged switching performance. In MOSFETS, Rps on) increases with temperature, Source and thus the current is automatically diverted away from the hot FIGURE 30.5 Power MoSFEt circuit symbol. pot.The drain body junction appears as an antiparallel diode (Source: B. K. Bose, Modern Power electronics between source and drain. Thus power MOSFETs will not sup- Evaluation, Technology and Applications, p 7@ port voltage in the reverse direction. Although this inverse diode 1992 IEEE is relatively fast, it is slow by comparison with the MOSFET. Recent devices have the diode recovery time as low as 100 ns. Since MOSFETs cannot be protected by fuses, n electronic protection technique has to be used. With the advancement in MOS technology, ruggedized MOSFETs are replacing the conventional MOSFETS. The need to ruggedize power MOSFETs is related to device reliability. If a MOSFET is operating within its specification range at all times, its chances for failing catastrophically are minimal. However, if its absolute naximum rating is exceeded, failure probability increases dramatically. Under actual operating conditions, a MOSFET may be subjected to transients- either externally from the power bus supplying the circuit or from the circuit itself due, for example, to inductive kicks going beyond the absolute maximum ratings. Such conditions are likely in almost every application, and in most cases are beyond a designers control. Rugged devices are made to be more tolerant for over-voltage transients. Ruggedness is the ability of a MOSFet to operate in an environment of dynamic electrical stresses, without activating any of the parasitic bipolar junction transistors. The rugged device can withstand higher levels of diode recovery dv/dt and static dv/dt. Insulated-Gate Bipolar Transistor (IGBT The IGBT has the high input impedance and high-speed characteristics of a MOSFET with the conductivity characteristic (low saturation voltage)of a bipolar transistor. The IGBT is turned on by applying a positive voltage between the gate and emitter and, as in the MOSFet, it is turned off by making the gate signal zero or slightly negative. The IGBT has a much lower voltage drop than a MOSFET of similar ratings. The structure f an IGBT is more like a thyristor and MOSFET. For a given IGBT, there is a critical value of collector current c 2000 by CRC Press LLC© 2000 by CRC Press LLC Power MOSFET Power MOSFETs are marketed by different manufacturers with differences in internal geometry and with different names such as MegaMOS, HEXFET, SIPMOS, and TMOS. They have unique features that make them potentially attractive for switching applications. They are essentially voltage-driven rather than current-driven devices, unlike bipolar transistors. The gate of a MOSFET is isolated electrically from the source by a layer of silicon oxide. The gate draws only a minute leakage current of the order of nanoamperes. Hence the gate drive circuit is simple and power loss in the gate control circuit is practically negligible. Although in steady state the gate draws virtually no current, this is not so under transient conditions. The gate-to-source and gate-to-drain capacitances have to be charged and discharged appropriately to obtain the desired switching speed, and the drive circuit must have a sufficiently low output impedance to supply the required charging and discharging currents. The circuit symbol of a power MOSFET is shown in Fig. 30.5. Power MOSFETs are majority carrier devices, and there is no minority carrier storage time. Hence they have exceptionally fast rise and fall times. They are essentially resistive devices when turned on, while bipolar transistors present a more or less con￾stant VCE(sat) over the normal operating range. Power dissipation in MOSFETs is Id2 RDS(on), and in bipolars it is ICVCE(sat). At low currents, therefore, a power MOSFET may have a lower conduc￾tion loss than a comparable bipolar device, but at higher cur￾rents, the conduction loss will exceed that of bipolars. Also, the RDS(on) increases with temperature. An important feature of a power MOSFET is the absence of a secondary breakdown effect, which is present in a bipolar transistor, and as a result, it has an extremely rugged switching performance. In MOSFETs, RDS(on) increases with temperature, and thus the current is automatically diverted away from the hot spot. The drain body junction appears as an antiparallel diode between source and drain. Thus power MOSFETs will not sup￾port voltage in the reverse direction. Although this inverse diode is relatively fast, it is slow by comparison with the MOSFET. Recent devices have the diode recovery time as low as 100 ns. Since MOSFETs cannot be protected by fuses, an electronic protection technique has to be used. With the advancement in MOS technology, ruggedized MOSFETs are replacing the conventional MOSFETs. The need to ruggedize power MOSFETs is related to device reliability. If a MOSFET is operating within its specification range at all times, its chances for failing catastrophically are minimal. However, if its absolute maximum rating is exceeded, failure probability increases dramatically. Under actual operating conditions, a MOSFET may be subjected to transients — either externally from the power bus supplying the circuit or from the circuit itself due, for example, to inductive kicks going beyond the absolute maximum ratings. Such conditions are likely in almost every application, and in most cases are beyond a designer’s control. Rugged devices are made to be more tolerant for over-voltage transients. Ruggedness is the ability of a MOSFET to operate in an environment of dynamic electrical stresses, without activating any of the parasitic bipolar junction transistors. The rugged device can withstand higher levels of diode recovery dv/dt and static dv/dt. Insulated-Gate Bipolar Transistor (IGBT) The IGBT has the high input impedance and high-speed characteristics of a MOSFET with the conductivity characteristic (low saturation voltage) of a bipolar transistor. The IGBT is turned on by applying a positive voltage between the gate and emitter and, as in the MOSFET, it is turned off by making the gate signal zero or slightly negative. The IGBT has a much lower voltage drop than a MOSFET of similar ratings. The structure of an IGBT is more like a thyristor and MOSFET. For a given IGBT, there is a critical value of collector current FIGURE 30.5 Power MOSFET circuit symbol. (Source: B.K. Bose, Modern Power Electronics: Evaluation, Technology, and Applications, p. 7. © 1992 IEEE.)