Steady state characteristics of an IGBT
Steady State Characteristics of an IGBT:
- There is a qualitatively similar to those of a logic level BJT except that the controlling parameter is not a base current but the gate-emitter voltage.
- When the gate emitter voltage is below the threshold voltage only a very small leakage current flows though the device while the collector – emitter voltage almost equals the supply voltage.
- The device, under this condition is said to be operating in the cut off region. The maximum forward voltage the device can withstand in this mode is determined by the avalanche break down voltage of the body – drain p-n junction.
- Unlike a BJT, however, this break down voltage is independent of the collector current as shown in Fig 7.4(a). IGBTs of Non-punch through design can block a maximum reverse voltage (VRM) equal to VCES in the cut off mode. However, for Punch through IGBTs VRM is negligible (only a few tens of volts) due the presence of the heavily doped n drain buffer layer.
- As the gate emitter voltage increases beyond the threshold voltage the IGBT enters into the active region of operation. In this mode, the collector current ic is determined by the transfer characteristics of the device. This characteristic is qualitatively similar to that of a power MOSFET and is reasonably linear over most of the collector current range. The ratio of ic to (VgE – vgE(th)) is called the forward transconductance (gfs) of the device and is an important parameter in the gate drive circuit design. The collector emitter voltage, on the other hand, is determined by the external load line ABC.
- As the gate emitter voltage is increased further ic also increases and for a given load resistance (RL) vCE decreases. At one point vCE becomes less than vgE – vgE(th). Under this condition the driving MOSFET part of the IGBT (Fig 7.2(c)) enters into the ohmic region and drives the output p-n-p transistor to saturation. Under this condition the device is said to be in the saturation mode. In the saturation mode the voltage drop across the IGBT remains almost constant reducing only slightly with increasing vgE.
- In power electronic applications an IGBT is operated either in the cut off or in the saturation region of the output characteristics. Since vCE decreases with increasing vgE, it is desirable to use the maximum permissible value of vgE in the ON state of the device. vgE(Max) is limited by the maximum collector current that should be permitted to flow in the IGBT as dictated by the “latch-up” condition discussed earlier. Limiting VgE also helps to limit the fault current through the device.
- If a short circuit fault occurs in the load resistance RL the fault load line is given by CF. Limiting vgE to vgE6 restricts the fault current corresponding to the operating point F. Most IGBTs are designed to with stand this fault current for a few microseconds within which the device must be turned off to prevent destruction of the device.
- It is interesting to note that an IGBT does not exhibit a BJT-like second break down failure. Since, in an IGBT most of the collector current flows through the drive MOSFET with positive temperature coefficient the effective temperature coefficient of vCE in an IGBT are slightly positive. This helps to prevent second break down failure of the device and also facilitates paralleling of IGBTs.