IGBT (Insulated Gate Bipolar Transistor), insulated gate bipolar transistor, is a composite fully controlled voltage-driven power semiconductor device composed of BJT (bipolar transistor) and MOS (insulated gate field effect transistor), and has MOSFET The advantages of both the high input impedance and the low turn-on voltage drop of the GTR. The GTR saturation voltage is reduced, the current carrying density is large, but the driving current is large; the MOSFET driving power is small, the switching speed is fast, but the conduction voltage drop is large, and the current carrying density is small. The IGBT combines the advantages of the above two devices, with low driving power and reduced saturation voltage. It is very suitable for converter systems with DC voltages of 600V and above, such as AC motors, inverters, switching power supplies, lighting circuits, traction drives, etc.
Generally speaking, IGBT is a kind of high-power power electronic device. It is a non-on-off switch. IGBT does not have the function of amplifying voltage. It can be regarded as a wire when it is turned on and an open circuit when it is disconnected. The three characteristics are high voltage, high current and high speed.
IGBT is the abbreviation of Insulated Gate Bipolar Transistor. IGBT is a kind of device composed of MOSFET and bipolar transistor. Its input is extremely MOSFET, and the output is extremely PNP transistor. It combines these two kinds. The advantages of the device not only have the advantages of small driving power and fast switching speed of the MOSFET device, but also have the advantages of low saturation voltage and large capacity of the bipolar device. The frequency characteristics are between the MOSFET and the power transistor, and can work normally. In the frequency range of 10 kHz, it has been widely used in modern power electronics technology, and it has occupied a dominant position in high-frequency large and medium power applications.
The equivalent circuit of the IGBT is shown in Figure 1. It can be seen from Fig. 1 that if a positive driving voltage is applied between the gate and the emitter of the IGBT, the MOSFET is turned on, so that the collector and the base of the PNP transistor are in a low-resistance state, so that the transistor is turned on; When the voltage between the gate and the emitter is 0V, the MOS is turned off, and the supply of the base current of the PNP transistor is cut off, so that the transistor is turned off. The IGBT is also a voltage-controlled device like the MOSFET. A DC voltage of more than ten V is applied between its gate and emitter, and only the leakage current in the uA stage flows, and substantially no power is consumed.
The voltage specification of the IGBT module is closely related to the input power of the device used, that is, the voltage of the test power supply. Their relationship is shown in the table below. In use, when the collector current of the IGBT module increases, the resulting rated loss also becomes large. At the same time, the switching loss increases, which increases the heat of the original. Therefore, the rated current should be greater than the load current when the IGBT module is selected. Especially when used as a high-frequency switch, the heating loss is aggravated due to the increase of the switching loss, and should be used when it is selected.
Since the IGBT module is a MOSFET structure, the gate of the IGBT is electrically isolated from the emitter through an oxide film. Since the oxide film is very thin, its breakdown voltage generally reaches 20 to 30V. Therefore, gate breakdown due to static electricity is one of the common causes of IGBT failure. Therefore, pay attention to the following points during use:
When using the module, try not to touch the drive terminal part by hand. When it is necessary to touch the module terminal, first discharge the static electricity on the human body or clothing with a large resistance grounding, and then touch.
When connecting the module drive terminals with conductive materials, do not connect the modules until the wiring is not connected;
In applications, although it is ensured that the gate drive voltage does not exceed the maximum rated voltage of the gate, the parasitic inductance of the gate wiring and the capacitive coupling between the gate and the collector also generate an oscillating voltage that damages the oxide layer. To this end, twisted pairs are often used to transmit drive signals to reduce parasitic inductance. The oscillating voltage can also be suppressed by connecting a small resistor in series with the gate wiring.
In addition, when an open circuit is applied between the gate and the emitter, if a voltage is applied between the collector and the emitter, the gate potential increases due to the leakage current of the collector due to the change of the collector potential, and the collector Then there is a current flowing. At this time, if there is a high voltage between the collector and the emitter, there is a possibility that the IGBT generates heat and is damaged.
In the case of using an IGBT, when the gate circuit is abnormal or the gate circuit is damaged (the gate is in an open state), if a voltage is applied to the main circuit, the IGBT is damaged. To prevent such a fault, it should be in the gate. A 10KΩ resistor is connected in series between the pole and the emitter.
When installing or replacing an IGBT module, the contact surface condition and tightening degree of the IGBT module and the heat sink should be taken seriously. In order to reduce the contact thermal resistance, it is best to apply thermal grease between the heat sink and the IGBT module. Generally, a heat dissipation fan is installed at the bottom of the heat sink. When the heat dissipation fan is damaged, the heat dissipation of the heat dissipation fin may cause the IGBT module to generate heat and malfunction. Therefore, the cooling fan should be inspected regularly. Generally, a temperature sensor is installed on the heat sink near the IGBT module. When the temperature is too high, the IGBT module will be alarmed or stopped.
The function of the IGBT drive circuit is mainly to amplify the power of the pulse output of the single chip to achieve the purpose of driving the IGBT power device. The drive circuit plays a vital role in ensuring reliable, stable and safe operation of the IGBT device.
The equivalent circuit of the IGBT and the conformity are shown in Figure 1. The IGBT is controlled by the gate positive and negative voltage. When a positive gate voltage is applied, the tube conducts; when a negative gate voltage is applied, the tube is turned off.
The IGBT has a volt-ampere characteristic similar to that of a bipolar power transistor, and as the control voltage UGE increases, the characteristic curve shifts upward. The IGBT in the switching power supply changes its UGE level to alternate between saturation and cutoff.
(1) Provide appropriate forward and reverse voltages to enable the IGBT to be turned on and off reliably. When the positive bias voltage increases, the IGBT on-state voltage drop and turn-on loss decrease, but if the UGE is too large, the IC will increase with the increase of UGE when the load is short-circuited, which is unfavorable for safety. It is better to use UGEν15V in use. The negative bias voltage can prevent the IGBT from being mis-conducted due to excessive surge current during shutdown. Generally, UGE=-5V is preferred.
(2) The switching time of the IGBT should be considered comprehensively. Fast turn-on and turn-off helps increase operating frequency and reduce switching losses. However, under large inductive loads, the turn-on frequency of the IGBT should not be too large, because high-speed breaking and turn-off will generate high peak voltage and may cause breakdown of the IGBT itself or other components.
(3) After the IGBT is turned on, the drive circuit should provide sufficient voltage and current amplitude to prevent the IGBT from being damaged due to normal saturation and overload.
(4) The resistor RG in the IGBT drive circuit has a large influence on the working performance, and the RG is large, which is beneficial to suppress the current rising rate and the voltage rising rate of the IGBT, but increases the switching time and switching loss of the IGBT; Will cause the current rise rate to increase, causing the IGBT to be mis-conducted or damaged. The specific data of RG is related to the structure of the driving circuit and the capacity of the IGBT, generally in the range of several ohms to tens of ohms, and the IGBT value of the small-capacity IGBT is large.
(5) The drive circuit should have strong anti-interference ability and protection function for IG2BT. The control, drive and protection circuits of the IGBT should be matched with its high-speed switching characteristics. In addition, G-E can not be opened without proper anti-static measures.
Fourth, the structure of the IGBT
The IGBT is a three-terminal device that has a gate G, a collector c, and an emitter E. The structure of the IGBT, simplified equivalent circuit and electrical graphic symbols are shown in the figure.
The figure shows a schematic cross-sectional view of the internal structure of an N-channel IGBT (N-IGBT) combined with an N-channel VDMOS FFT and a GTR. The IGBT has one more P+ implant region than the VDMOSFET, forming a large-area PN junction J1. Since the IGBT is turned on, the P+ implant region emits a minority to the N-base region, so that the drift region conductivity is modulated, and the IGBT can have a strong current-passing capability. The N+ layer between the P+ implant region and the N-drift region is called a buffer. The presence or absence of a buffer determines the IGBT's different characteristics. IGBTs with N* buffers are called asymmetric IGBTs, also known as punch-through IGBTs. It has the advantages of small forward voltage drop, short dog break time, and small tail current when shutting down, but its reverse blocking ability is relatively weak. An IGBT without an N-buffer is called a symmetrical IGBT, also called a non-punch-through IGBT. It has strong forward and reverse blocking capability, but its other characteristics are not as good as asymmetric IGBTs.
The simplified equivalent circuit shown in Figure 2-42 (b) shows that the IGBT is a Darlington structure composed of GTR and MOSFET. Part of the structure is MOSFET drive, and the other part is thick base PNP transistor.
Simply put, the IGBT is equivalent to a thick base PNP transistor driven by a MOSFET. Its simplified equivalent circuit is shown in Figure 2-42(b). The RN in the figure is the modulation resistor in the base of the PNP transistor. It is clear from the equivalent circuit that the IGBT is a Darlington-structured composite device composed of a transistor and a MOSFET. The transistor in the figure is a PNP transistor, and the MOSFET is an N-channel field effect transistor. Therefore, the IGBT of this structure is called an N-channel IIGBT, and its symbol is an N-IGBT. Similarly there is a P-channel IGBT, ie a P-IGBT.
The electrical graphic symbol of the IGBT is shown in Figure 2-42(c). The IGBT is a field-controlled device whose turn-on and turn-off are determined by the gate-emitter voltage UGE. When the gate-emitter voltage UCE is positive and greater than the turn-on voltage UCE(th), a channel is formed in the MOSFET and is PNP. The transistor provides a base current to turn on the IGBT. At this time, N-holes (minor carriers) are injected from the P+ region to conduct conductance modulation on the N-region, and the resistance RN of the N-region is reduced to make the resistance high. The pressed IGBT also has a small on-state voltage drop. When no signal is applied between the gate emitters and a reverse voltage is applied, the channel in the MOSFET disappears, the base current of the PNP transistor is cut off, and the IGBT is turned off. It can be seen that the driving principle of the IGBT is basically the same as that of the MOSFET.
1 When UCE is negative: J3 junction is in reverse bias state, and the device is in reverse blocking state.
2 When uCE is positive: UC< UTH, the channel cannot be formed, the device is in a forward blocking state; UG>UTH, N-channel is formed under the insulating gate, and conductance is generated in the N-region due to carrier interaction Modulation to make the device forward.
The structure of the IGBT silicon wafer is very similar to that of the power MOSFET. The main difference is that JGBT adds a P+ substrate and an N+ buffer layer (NPT-non-punch-IGBT technology does not add this part), one of the MOSFETs drives two bipolar devices. (There are two polar devices). The application of the substrate creates a J, junction between the P and N+ regions of the tube. When the positive gate bias causes the P-base region to be inverted below the gate, an N-channel is formed, simultaneously with a flow of electrons, and a current is generated in full accordance with the power MOSFET. If the voltage generated by this electron current is in the range of 0.7V, then J1 will be in forward bias, some holes will be injected into the N-region, and the resistivity between N- and N+ will be adjusted, which reduces the power conductivity. The total loss is passed and a second charge flow is initiated. The end result is two temporary current topologies in the semiconductor hierarchy: one electron current (MOSFET current) and one hole current (bipolar). When UCE is greater than the turn-on voltage UCE(th), a channel is formed in the MOSFET to provide a base current to the transistor and the IGBT is turned on.
2) Conduction pressure drop
The conductance modulation effect reduces the resistance RN and the on-state voltage drop is small. The so-called on-state voltage drop refers to the tube voltage drop UDS of the IGBT entering the conduction state, and this voltage decreases as the UCS rises.
When a negative bias is applied to the gate or the gate voltage is below the threshold, the channel is disabled and no holes are injected into the N-region. In any case, if the current of the MOSFET drops rapidly during the switching phase, the collector current gradually decreases. This is because after the start of commutation, there are still a small number of carriers (less than) in the N layer. This reduction in residual current value (wake) depends entirely on the density of the charge at turn-off, which in turn is related to several factors, such as the amount and topology of the dopant, the layer thickness and the temperature. The attenuation of the minority carriers causes the collector current to have a characteristic wake waveform. The collector current will cause increased power consumption and cross-conduction problems, especially on devices that use freewheeling diodes.
In view of the fact that the wake is related to the reorganization of the minority, the current value of the wake should be closely related to the chip's Tc, IC: and uCE, and is closely related to the hole mobility. Therefore, depending on the temperature reached, it is feasible to reduce the undesirable effects of this current on the design of the terminal device. When a back pressure or no signal is applied between the gate and the emitter, the channel in the MOSFET disappears, the base current of the transistor is cut off, and the IGBT is turned off.
4) Reverse blocking
When a reverse voltage is applied to the collector, J is subjected to reverse bias control and the depletion layer is extended to the N-region. This mechanism is important because it can't achieve an effective blocking ability because it reduces the thickness of this layer too much. In addition, if the size of this area is excessively increased, the pressure drop is continuously increased.
5) Positive blocking
When the gate and emitter are shorted and a positive voltage is applied to the collector terminal, J, the junction is controlled by the reverse voltage. At this time, the externally applied voltage is still received by the depletion layer of the N-drift zone.
ICBT has a parasitic PNPN thyristor between the collector and the emitter. Under special conditions, this parasitic device will turn on. This phenomenon causes an increase in the amount of current between the collector and the emitter, a decrease in the controllability of the equivalent MOSFET, and usually causes a breakdown of the device. The thyristor conduction phenomenon is called an IGBT latch. Specifically, the causes of such defects vary, but are closely related to the state of the device.