Basic characteristics and main parameters of insulated gate bipolar transistors

Basic characteristics and main parameters of insulated gate bipolar transistors

Basic characteristics of IGBT

(1) Static characteristics

The static characteristics of the insulated gate bipolar transistor(IGBT) include transfer characteristics and output characteristics.

The transfer characteristic of IGBT describes the relationship between the collector current IC and the gate shot voltage UGE, as shown in Figure 1a. This characteristic is similar to the transfer characteristic of a power MOSFET. When the gate-source voltage UGE is less than the threshold voltage UGE(th), the IGBT is in the off state.

Basic characteristics and main parameters of insulated gate bipolar transistors
Figure 1 – Static characteristic curve of IGBT

Figure 1b shows the output characteristic of the IGBT, namely the volt-ampere characteristic. It describes the relationship between the collector current IC and the collector emitter voltage UGE when the gate emitter voltage is the reference variable. The output characteristics of the IGBT are divided into three areas: the forward blocking area, the active area and the saturation area, which correspond to the cut-off area, the amplification area and the saturation area of the GTR respectively. In addition, when uCE<0, the IGBT is in the reverse blocking state. In the power circuit, the IGBT works in the on-off state, switching back and forth between the forward blocking zone and the saturation zone.

(2) Dynamic characteristics

The dynamic characteristic of IGBT is also called switching characteristic, as shown in Figure 2. The turn-on time ton of the IGBT is composed of turn-on delay time td(on) and current rise time tr. Usually the turn-on time is 0.5~1.2μs. The IGBT operates as a MOSFET most of the time during the turn-on process. Only in the late stage of the decrease of the collector-emitter voltage UCE (tfv2), the PNP transistor changes from the amplifying region to the saturation region, thus adding a delay time. Therefore, the collector-emitter voltage UCE waveform is divided into two segments tfv1 and tfv2.

Basic characteristics and main parameters of insulated gate bipolar transistors
Figure 2 – Dynamic characteristics of IGBT

The turn-off time toff of the IGBT is composed of turn-off delay time td(off) and current fall time tf. Within tf, the collector current is divided into two segments tfi1 and tfi2. tfi1 corresponds to the turn-off process of the MOSFET inside the IGBT, and tfi2 corresponds to the turn-off process of the PNP transistor inside the IGBT. After the MOSFET is turned off, the stored charge in the PNP transistor is difficult to eliminate, so the ic drops slowly during this period, resulting in a longer tail time for the collector current. Usually the turn-off time is 0.55-1.5μs.

It should be pointed out that, like the power MOSFET, the switching speed of the IGBT is affected by the internal resistance of its gate drive circuit. The switching process waveform will be affected by the main circuit structure, control mode, buffer circuit and main circuit parasitic parameters, etc., so care should be taken when designing the actual circuit.

The main parameters of IGBT

In addition to the parameters mentioned above, the main parameters of IGBT also include:

(1) Collector-emitter voltage UCES

UCES is the highest voltage that the collector-emitter voltage of the device can withstand when the gate-emitter is short-circuited, and it has a positive temperature coefficient.

(2) Gate-emitter threshold voltage UGE (th)

UGE (th) (is the minimum gate-emitter voltage required for IGBT to turn on. UGE (th) decreases slightly with the increase of temperature, and its value decreases by about 5mV for every 1°C increase in temperature. At +25℃, the value of UGE (th) is generally 2~6V.

(3) Maximum collector current

Including the collector continuous current IC and the maximum collector repetitive peak current ICP, which is the maximum value of multiple rectangular pulses under the conditions of the specified pulse duration and duty cycle. Since IGBTs mostly work in the switching state, IP has more practical significance. When choosing IGBT, the margin should be considered according to the actual situation.

(4) Total power dissipation PCM

The maximum allowable power dissipation at the specified case temperature TC=25°C. With the advancement of chip technology and packaging technology, the maximum junction temperature of IGBT can reach 175°C. For example, the fourth-generation IGBT of Infineon has a higher allowable total power dissipation than the previous generation, that is, the power density is greatly improved.