What is IGBT
IGBT (Insulated Gate Bipolar Transistor), by the BJT (bipolar triode) and MOS (insulated gate field effect tube) composite fully controlled voltage-driven power electronic devices, both the high input impedance of the MOSFET and the low on-state voltage drop of the GTR has the advantages of both sides of the GTR saturation voltage reduction, current-carrying density, but the drive current is larger; MOSFET drive power is very small, but the on-state voltage drop is large. GTR saturation voltage reduction, high current-carrying density, but the drive current is larger; MOSFET drive power is very small, fast switching speed, but the on-state voltage drop is large, small current-carrying density.IGBT synthesizes the advantages of the above two devices, drive power is small and the saturation voltage is lowered. IGBT is very suitable for application in DC voltage of 600V and above converter system such as AC motors, inverters, switching power supply, lighting circuits, traction drive and other fields.
N-channel enhancement insulated gate bipolar transistor structure, the N+ region is called the source region and the electrode attached to it is called the source.The N+ region is called the drain region. The control region of the device is the gate region and the electrode attached to it is called the gate. The channel is formed immediately adjacent to the boundary of the gate region. The P-type region between the drain and source (including the P+ and P-region), in which the channel is formed, is called the subchannel region. On the other side of the leakage area of the P + area is called leakage injection area (Drain injector), which is a unique IGBT functional area, with the leakage area and subchannel area together to form a PNP bipolar transistor, play the role of the emitter, the leakage injected holes, conductive modulation, in order to reduce the device's on-state voltage. The electrode attached to the drain injection region is called the drain.
The switching function of the IGBT is to form a channel by adding a positive gate voltage to provide base current to the PNP transistor, so that the IGBT is turned on. Conversely, add the reverse gate voltage to eliminate the channel, cut off the base current, so that the IGBT off.IGBT driving method and MOSFET is basically the same, only need to control the input pole N a channel MOSFET, so has a high input impedance characteristics. When the channel of the MOSFET is formed, from the P + base injected into the N layer of holes (lessons), the N layer of conductance modulation, reducing the resistance of the N layer, so that the IGBT in the high voltage, but also has a low on-state voltage.
The operating characteristics of IGBTs include both static and dynamic categories：
Static Characteristics
The static characteristics of IGBT are mainly volt-ampere characteristics, transfer characteristics and switching characteristics.
The volt-ampere characteristic of IGBT refers to the relationship curve between drain current and gate voltage when the gate voltage Ugs is used as a covariate. The output drain current ratio is controlled by the gate voltage Ugs, the higher Ugs is, the larger Id is. It is similar to the output characteristics of the GTR. Can also be divided into saturation region 1, amplification region 2 and breakdown characteristics of 3 parts. In the cut-off state of the IGBT, the forward voltage is borne by the J2 junction, the reverse voltage is borne by the J1 junction. If there is no N + buffer, the forward and reverse blocking voltage can do the same level, after adding N + buffer, reverse shutdown voltage can only reach a few tens of volts level, thus limiting the scope of some applications of IGBT.
The transfer characteristic of IGBT is the relationship curve between the output drain current Id and the gate source voltage Ugs. It is the same as the transfer characteristic of the MOSFET, when the gate source voltage is less than the turn-on voltage Ugs(th), the IGBT is in the off state. Id is linearly related to Ugs for most of the drain current range after the IGBT is turned on. The maximum gate source voltage is limited by the maximum drain current, and its optimum value is generally taken as about 15V.
The switching characteristic of IGBT refers to the relationship between drain current and drain voltage. when IGBT is in the on-state, because its PNP transistor is a wide-base transistor, so its B value is very low. Although the equivalent circuit is a Darlington structure, the current flowing through the MOSFET becomes the main part of the total current of the IGBT.At this point, the on-state voltage Uds(on) can be expressed by the following equation
Uds (on) = Uj1 + Udr + IdRoh
Where Uj1 - the forward voltage of the JI junction, the value of 0.7 ~ 1V; Udr - the voltage drop across the expansion resistor Rdr; Roh - channel resistance.
The on-state current Ids can be expressed by the following equation:
Ids=(1+Bpnp)Imos
Ids=(1+Bpnp)Imos where Imos - the current flowing through the MOSFET.
Due to the conductance modulation effect in the N+ region, the on-state voltage drop of the IGBT is small, and the on-state voltage drop of an IGBT with a voltage of 1000 V is 2-3 V. When the IGBT is in the off-state, there is only a small leakage current.
Dynamic Characterization
The IGBT operates as a MOSFET most of the time during the turn-on process, only in the late stage of the drain-source voltage Uds drop process, the PNP transistor is saturated from the amplification region, and a delay time is added. td (on) is the turn-on delay time, and tri is the current rise time. In practice, the drain current turn-on time ton is often given as the sum of td (on) tri. The drain voltage fall time is composed of tfe1 and tfe2.
The triggering and shutdown of an IGBT requires both positive and negative voltages to be applied between the gate and the base, and the gate voltage can be generated by different driver circuits. When selecting these driver circuits, it is important to do so based on the following parameters: device turn-off bias requirements, gate charge requirements, solid-state resistance requirements, and power supply conditions. Because of the high gate-emitter impedance of IGBTs, they can be triggered using MOSFET drive technology, but because the input capacitance of IGBTs is larger than that of MOSFETs, the turn-off bias of IGBTs should be higher than that provided by many MOSFET drive circuits.
The waveform of the drain current of the IGBT changes to two segments during the turn-off process. This is because the stored charge of the PNP transistor is difficult to eliminate quickly after the MOSFET is turned off, resulting in a longer tail time for the drain current. td(off) is the turn-off delay time, and trv is the rise time of the voltage Uds(f). Practical applications often give the fall time Tf of the drain current consists of two segments t(f1) and t(f2) in the figure, while the turn-off time of the drain current
t(off)=td(off)+trv ten t(f)
where the sum of td(off) and trv is also known as the storage time.
The switching speed of IGBT is lower than that of MOSFET, but significantly higher than that of GTR.IGBT does not need negative gate voltage to reduce the turn-off time when it is turned off, but the turn-off time increases with the increase of the shunt resistance of the gate and the emitter.The turn-on voltage of the IGBT is about 3-4V, which is comparable to that of the MOSFET.The saturation drop of the IGBT when it is turned on is lower than that of the MOSFET and close to that of the GTR, and the saturation drop increases with the increase of the gate voltage and decreases with the increase of the gate voltage. The saturation voltage drop decreases with the increase of gate voltage.
IGBT power module adopts IC drive, various drive protection circuits, high-performance IGBT chips, new packaging technology, and develops from compound power module PIM to intelligent power module IPM, power electronic building block PEBB, power module IPEM. PIM develops to high-voltage and high-current, and its product level is 1200-1800A/1800 -3300V, IPM in addition to frequency conversion speed control, 600A/2000V IPM has been used in electric locomotive VVVF inverter. Planar low inductance packaging technology is a high-current IGBT module for active devices PEBB, used for missile launchers on ships. IPEM uses co-fired ceramic multi-chip module technology to assemble PEBB, greatly reducing circuit wiring inductance, improve system efficiency, has been developed successfully in the second generation of IPEM, in which all the passive components are buried in a buried layer way buried in the substrate. Intelligent, modular IGBT development hotspot.
Now, high-current, high-voltage IGBT has been modularized, its drive circuit in addition to the above by the discrete components, has now produced an integrated IGBT special drive circuit. Its performance is better, the reliability of the machine is higher and smaller.
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