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Switching times for a MOSFET-bipolar switch are faster than the IGBT (Insulated Gate Bipolar Transistor), which saves size, weight and cost in high power applications.
Many high power, high current switching applications use the IGBT and conventional power MOSFET. However, the conventional bipolar transistor combined with a MOSFET input can provide improved performance in power conversion and motion systems. Probably less is known about the IGBT and how it compares with other devices, so a comparison requires some explanation of IGBT characteristics.
Conventional fall time specifications do not apply to IGBTs because they exhibit a slow turnoff tail defined as t2 , which is usually below 10% of peak current. This affects the IGBT's efficiency and produces a misleading falltime.
Minority carrier recombination in the base region of the IGBT's bipolar PNP structure controls the turnoff tail. As shown in Figure 1, this device has a four-layer parasitic thyristor in which the p+ layer is the drain. This base region is not available externally to pull minority carriers out through a reverse bias base drive technique as done in speeding up bipolar transistors. Instead, t2 ranges from 500 to 2000nsec depending on the rate at which minority carriers in the base region combine. This recombination of minority carriers in the base region is a function of device design and process technology.
A major concern with the IGBT is latch-up that occurs when the p-n junction capacitance and base-emitter shunt resistance experience very high current densities. Latch-up results during forced turnoff into an inductive load, which forward biases the IGBT's parasitic NPN. The problem gets worse when the temperature rises.
Some IGBT problems can be overcome with judicious circuit and thermal design. For example, a snubber reduces dv/dt and current density, reducing the tendencv for latch-up. Also, better heat sinking reduces the IGBT's junction temperature and reduces latch-up tendencies. However, these approaches increase both circuit complexity and cost.
A better device than the IGBT for high current applications is a cascaded MOS-bipolar device as shown in Figure 2. It has a flyback diode for protection against inductive loads and a gate drive circuit for the MOSFET that protects the device and speeds turn-on time.
Table 1 compares the characteristics of the MOS-bipolar device with several IGBTS.
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