6.5 Summary

The Insulated Gate Bipolar Transistor, or IGBT, can be thought of as a combination of a power BJT and a power E-MOSFET. As such, it combines the low on-state conduction losses of the BJT with the relatively easy drive requirements of the EMOSFET. The IGBT is available in two variants; the PT, or punch through, and the NPT, or non-punch through types. The PT type includes an N+ buffer layer in its construction and this endows the device with faster switching speed and lower onstate losses.

The IGBT’s characteristic curves tend to echo that of the E-MOSFET. Conduction does not begin until the gate-emitter voltage exceeds a threshold voltage, 𝑉𝐺𝐸(π‘‘β„Ž) . From there, the current-voltage characteristic follows a square-law trajectory, and at sufficiently high current levels it can be approximated as a straight line. The IGBT exhibits a negative temperature coefficient of transconductance, like the MOSFET, making it less prone to thermal runaway and current hogging issues. A family of collector curves (i.e., 𝑉𝐢𝐸 vs. 𝐼𝐢 ) shares attributes with BJT collector curves and MOSFET drain curves. The curves echo the same overall shape, starting with a section where current rises rapidly compared to voltage, and then leveling out into a constant current region. The initial region of rapid change is somewhat drawn out as it is in the MOSFET. Also, the entire set of curves is displaced positively by about a volt, rather than current increasing immediately from the origin.

In general, the IGBT offers higher voltage, current and power capability than the power E-MOSFET although it lags behind in switching speed. Further, switching times for the on- and off-state are asymmetrical. Compared to the power BJT, the IGBT tends to be more expensive. Consequently, power E-MOSFETs tend to be favored at low and moderate power levels when high switching speeds are needed and BJTs tend to be preferred when cost is a major component in more modest designs. As such, IGBTs find use across a range of applications including power inverters, uninterruptible power supplies, induction heaters, solar power systems, motor controllers and so forth.

Review Questions

  1. What are the advantages and disadvantages of IGBTs compared to power BJTs?
  2. What are the advantages and disadvantages of IGBTs compared to power E-MOSFETs?
  3. What are the differences between NPT and PT IGBTs
  4. Compare the simple IGBT model to that of a Sziklai pair.
  5. Explain the basic operation of an induction heater and how an IGBT is used to control the generation of heat.
  6. Explain how pulse width modulation might be used to control the speed of a DC motor via an IGBT.
  7. Explain how an IGBT can be used to translate a DC power source into an AC power source.

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