Working Characteristics of IGBT

IGBT features include both static and dynamic:

Static characteristics: the static characteristics of IGBT mainly include volt-ampere characteristics, transfer characteristics, and switching characteristics.

The volt-ampere characteristic of IGBT is the relation curve between drain current and gate voltage when the gate source voltage Ugs is taken as a parameter. The output drain current ratio is controlled by the gate source voltage Ugs, the higher the UGS, the greater the ID. It has similar output characteristics to GTR and can be divided into 3 parts: Saturation Region 1, Amplification Region 2, and breakdown characteristics. Under the cut-off state, the forward voltage of the IGBT is assumed by the J2 junction, and the reverse voltage is assumed by the J1 junction. If there is no N + buffer, the forward and reverse blocking voltage can achieve the same level, after adding N + buffer, the reverse switching voltage can only reach a few tens of volts level, thus limiting some applications of IGBT.

The transfer characteristic of IGBT is the relation curve between output drain current ID and gate source voltage Ugs. It has the same transfer characteristics as MOSFET, when the gate source voltage is less than the open voltage Ugs (th), the IGBT is in the off state. In most of the range of drain current after IGBT conduction, the relationship between ID and UGS is linear. The maximum gate voltage is limited by the maximum drain current, and the optimum value is about 15V.

The switching characteristics of IGBT refer to the relationship between drain current and drain source voltage. When IGBT is in the on state, its B value is extremely low because its PNP transistor is a wide base transistor. Although the equivalent circuit is Darlington, the current flowing through the MOSFET accounts for the majority of the total current in the IGBT.

At this point, the on-state voltage Uds (on) can be expressed as Uds (on) = Uj1 + UDR + IdRoh, in which UJ1 is the forward voltage of JI junction with a value of 0.7-iv, UDR is the voltage drop on the RDR of the spreading resistor, and ROH is the channel resistor. The on-state current Ids can be expressed as Ids = (1 + Bpnp) Imos, in which Imos is the current flowing through the MOSFET.

Due to the conductivity modulation effect in the N+region, the on-state voltage drop of IGBT is small, and the on-state voltage drop of IGBT with a withstand voltage of 1000V is 2-3V. When the IGBT is in the off state, only a small leakage current exists.

Dynamic characteristics: During the turn-on process of IGBT, most of the time it operates as a MOSFET. However, in the later stage of the decrease in drain-source voltage Uds, the PNP transistor increases its delay time from the amplification region to saturation. Td (on) is the opening delay time, and tri is the current rise time. The opening time ton of the drain current commonly given in practical applications is the sum of td (on) tri. The decrease time of the leakage source voltage is composed of tfe1 and tfe2

During the shutdown process of IGBT, the waveform of the drain current changes into two segments. Because after MOSFET shutdown, the storage charge of the PNP transistor is difficult to quickly eliminate, resulting in a longer tail time of drain current. td (off) is the shutdown delay time, and trv is the rise time of voltage Uds (f). The decreased time Tf of the drain current often given in practical applications consists of two segments t (f1) and t (f2) in Figure 2-59, and the turn off time t (off) of the drain current is td (off)+trv+t (f) (2-16). In this equation, the sum of td (off) and trv is also known as the storage time.