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), which also has MOSFET The advantages of high input impedance and low on-Voltage drop of GTR. The saturation voltage of GTR is reduced, the current-carrying density is high, 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. IGBT combines the advantages of the above two devices, with low driving power and reduced saturation voltage. It is very suitable to be used in converter systems with a DC voltage of 600V and above, such as AC motors, frequency converters, switching power supplies, lighting circuits, traction drives and other fields.
In layman’s terms: IGBT is a high-power power electronic device. It is a non-on-off switch. IGBT has no function of amplifying voltage. It can be regarded as a wire when it is turned on, and it is regarded as an open circuit when it is turned off. The three main characteristics are high voltage, high current, and high speed.
- IGBT module
IGBT is the abbreviation of Insulated Gate Bipolar Transistor (Insulated Gate Bipolar Transistor). IGBT is a device composed of MOSFET and bipolar transistor. Its input is MOSFET and output is PNP transistor. It combines these two The advantages of the device are not only the advantages of low driving power and fast switching speed of MOSFET devices, but also the advantages of reduced saturation voltage and large capacity of bipolar devices. Its frequency characteristics are between MOSFET and power transistors, and it can work normally at several times. Within the frequency range of ten kHz, it has been used more and more widely in modern power electronic technology, and it has occupied a leading position in high-frequency and medium-power applications.
The equivalent circuit of IGBT is shown as in Fig. 1. It can be seen from Figure 1 that if a positive driving voltage is applied between the gate and emitter of the IGBT, the MOSFET is turned on, so that the collector and base of the PNP transistor are in a low resistance state and the transistor is turned on; if the IGBT If the voltage between the gate and the emitter is 0V, the MOS is cut off, cutting off the supply of the base current of the PNP transistor, making the transistor cut 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 of the uA level flows, and basically does not consume power.
- The choice of IGBT module
The voltage specification of the IGBT module is closely related to the input power supply of the device used, that is, the test power supply voltage. The relationship between them is shown in the table below. When the collector current of the IGBT module increases during use, the resulting rated loss also increases. At the same time, the switching loss increases, which intensifies the heating of the original. Therefore, the rated current should be greater than the load current when selecting the IGBT module. Especially when it is used as a high-frequency switch, due to the increase of switching loss and heat generation, it should be used when selecting it.
- Precautions in use
Since the IGBT module is a MOSFET structure, the gate of the IGBT is electrically isolated from the emitter by a layer of oxide film. Because this oxide film is very thin, its breakdown voltage generally reaches 20-30V. Therefore, gate breakdown due to static electricity is one of the common causes of IGBT failure. Therefore, pay attention to the following points in use:
When using the module, try not to touch the driver terminal part with your hands. When you must touch the module terminal, first discharge the static electricity on the human body or clothes with a large resistance ground before touching;
When using conductive materials to connect the drive terminals of the module, please do not connect the module before the wiring is connected;
Try to operate with the bottom plate well grounded.
In applications, although it is guaranteed that the gate drive voltage does not exceed the maximum rated voltage of the gate, the parasitic inductance of the gate connection and the capacitive coupling between the gate and the collector will also produce an oscillating voltage that damages the oxide layer. For this reason, twisted-pair wires are usually used to transmit drive signals to reduce parasitic inductance. Connecting a small resistor in series with the gate connection can also suppress the oscillating voltage.
In addition, when there is an open circuit between the gate and the emitter, if a voltage is applied between the collector and the emitter, as the collector potential changes, due to the leakage current flowing through the collector, the gate potential rises and the collector Then there is current flowing. At this time, if there is a high voltage between the collector and the emitter, it may cause the IGBT to heat up and damage it.
In the case of using IGBT, when the gate loop is abnormal or the gate loop is damaged (the gate is in an open state), if a voltage is applied to the main loop, the IGBT will be damaged. A resistance of about 10KΩ is connected in series between the pole and the emitter.
When installing or replacing the IGBT module, the state of the contact surface between the IGBT module and the heat sink and the degree of tightening should be paid great attention to. 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 IGBT module will generate heat and malfunction when the heat dissipation of the heat sink is poor. Therefore, the cooling fan should be checked regularly. Generally, a temperature sensor is installed on the heat sink close to the IGBT module. When the temperature is too high, it will alarm or stop the IGBT module.
Three, IGBT drive circuit
The function of the IGBT drive circuit is mainly to amplify the power output by the single chip microcomputer to achieve the purpose of driving the IGBT power device. On the premise of ensuring the reliable, stable and safe operation of IGBT devices, the drive circuit plays a vital role.
The equivalent circuit and conformity of IGBT are shown in Figure 1. IGBT is controlled by the positive and negative voltages of the gate. When a positive grid voltage is applied, the tube is turned on; when a negative grid voltage is applied, the tube is turned off.
IGBT has similar volt-ampere characteristics to bipolar power transistors. As the control voltage UGE increases, the characteristic curve shifts upward. The IGBT in the switching power supply makes it work alternately in two states of saturation and cut-off through the change of UGE level.
(1) Provide proper forward and reverse voltage to enable IGBT to be turned on and off reliably. When the positive bias voltage increases, the IGBT on-state voltage drop and turn-on loss will 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 detrimental to its safety. It is better to use UGEν15V. Negative bias voltage can prevent the IGBT from being turned on by mistake due to excessive surge current during turn-off. Generally, UGE=-5V is appropriate.
(2) The switching time of IGBT should be considered comprehensively. Fast turn-on and turn-off are beneficial to increase the operating frequency and reduce switching losses. However, under large inductive loads, the switching frequency of the IGBT should not be too large, because high-speed switching and switching will produce high peak voltages, and may cause the IGBT itself or other components to breakdown.
(3) After the IGBT is turned on, the drive circuit should provide sufficient voltage and current amplitude so that the IGBT will not exit saturation and be damaged under normal operation and overload conditions.
(4) The resistance RG in the IGBT drive circuit has a greater impact on the working performance. The larger RG is beneficial to suppress the current rise rate and voltage rise rate of the IGBT, but it will increase the switching time and switching loss of the IGBT; RG is small , It will cause the current rise rate to increase, causing the IGBT to be turned on or damaged by mistake. The specific data of RG is related to the structure of the drive circuit and the capacity of the IGBT, generally ranging from a few ohms to several tens of ohms. The RG value of a small-capacity IGBT is larger.
(5) The drive circuit should have strong anti-interference ability and protection function for IG2BT. The control, drive and protection circuits of IGBT should match its high-speed switching characteristics. In addition, G-E cannot be opened without proper anti-static measures.
Fourth, the structure of IGBT
IGBT is a three-terminal device, which has a gate G, a collector c and an emitter E. The structure of IGBT, simplified equivalent circuit and electrical graphic symbols are shown in the figure.
As shown in the figure, it is a cross-sectional schematic diagram of the internal structure of an N-channel IGBT (N-IGBT) combined with an N-channel VDMOSFFT and GTR. IGBT has one more layer of P+ implantation area than VDMOSFET, forming a large-area PN Junction J1. Since the P+ injection region emits minority carriers to the N base region when the IGBT is turned on, the conductivity of the drift region is modulated, and the IGBT has a strong current flow capability. The N+ layer between the P+ injection region and the N- drift region is called a buffer zone. The presence or absence of a buffer determines the different characteristics of the IGBT. IGBTs with N* buffer are called asymmetric IGBTs, also called punch-through IGBTs. It has the advantages of small forward voltage drop, short dog-off time, and small tail current when it is turned off, but its reverse blocking ability is relatively weak. IGBTs without N-buffer are called symmetrical IGBTs, also called non-punch-through IGBTs. 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 driven by a MOSFET, and the other part is a thick base PNP transistor.
Five, the working principle of IBGT
Simply put, an IGBT is equivalent to a thick base PNP transistor driven by a MOSFET. Its simplified equivalent circuit is shown in Figure 2-42(b), where RN is the modulation resistance in the base area of the PNP transistor. It can be clearly seen from this equivalent circuit that IGBT is a composite device of Darlington structure composed of transistors and MOSFETs. The transistor in the picture is a PNP transistor, and the MOSFET is an N-channel field effect transistor, so the IGBT of this structure is called an N-channel IIGBT, and its symbol is N-IGBT. Similarly, there are P-channel IGBTs, namely P-IGBTs.
The electrical graphic symbol of IGBT is shown in Figure 2-42(c). IGBT is a kind of field control device. Its turn-on and turn-off are determined by the voltage UGE between the gate and the emitter. When the gate-to-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 type transistor provides the base current to turn on the IGBT. At this time, the holes (minority carriers) injected from the P+ region to the N- region modulate the conductance of the N- region, reducing the resistance RN of the N- region, making the IGBT highly resistant. The low voltage IGBT also has a small on-state voltage drop. When no signal or reverse voltage is applied between the gate and the emitter, 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 IGBT is basically the same as that of MOSFET.
① When UCE is negative: J3 junction is in reverse bias state, and the device is in reverse blocking state.
②When uCE is positive: UCUTH, an N channel is formed under the insulated gate, and conduction occurs in the N-region due to the interaction of carriers Modulate to make the device forward.
The structure of the IGBT silicon chip 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-through-IGBT technology does not add this part), and one MOSFET drives two bipolar devices. (There are two polarity devices). The application of the substrate creates a J junction between the P and N+ regions of the tube body. When the positive gate bias causes the P base area to be reversed under the gate, an N channel is formed, and an electron flow appears at the same time, and a current is generated exactly in the manner of a power MOSFET. If the voltage generated by this electron flow is in the range of 0.7V, J1 will be forward biased, some holes will be injected into the N- zone, and the resistivity between N- and N+ will be adjusted, which reduces the power conductance. The total loss of the pass and initiates the second charge flow. The final result is the temporary emergence of two different current topologies within the semiconductor hierarchy: an electron flow (MOSFET current); a hole current (bipolar). When UCE is greater than the turn-on voltage UCE(th), a channel is formed in the MOSFET to provide base current for 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 when the IGBT enters the on-state. This voltage decreases as UCS rises.
When a negative bias is applied to the gate or the gate voltage is lower than the threshold, the channel is prohibited 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 will gradually decrease. This is because after the start of commutation, there are still a few carriers (less than) in the N layer. The reduction of this residual current value (wake current) depends entirely on the charge density at turn-off, and the density is related to several factors, such as the amount and topology of dopants, layer thickness and temperature. The attenuation of minority carriers gives the collector current a characteristic wake waveform. Collector current will cause increased power consumption and cross-conduction problems, especially in equipment using freewheeling diodes, the problem is more obvious.
In view of the fact that the wake is related to the recombination of minority carriers, the current value of the wake should be closely related to the chip’s Tc, IC: and uCE, and has a close relationship with the hole mobility. Therefore, depending on the temperature reached, it is feasible to reduce this undesirable effect of the current acting on the terminal equipment design. 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 will be controlled by the reverse bias, and the depletion layer will expand to the N-region. If you reduce the thickness of this layer too much, you will not be able to obtain an effective blocking ability, so this mechanism is very important. In addition, if you increase the size of this area too much, it will continuously increase the pressure drop.
5) Positive blocking
When the gate and emitter are shorted and a positive voltage is applied to the collector terminal, the J junction is controlled by the reverse voltage. At this time, the depletion layer of the N drift zone still bears the externally applied voltage.
The IGBT has a parasitic PNPN thyristor between the collector and the emitter. Under special conditions, this parasitic device will turn on. This phenomenon will increase the amount of current between the collector and the emitter, reduce the ability to control the equivalent MOSFET, and usually cause device breakdown. The thyristor turn-on phenomenon is called IGBT latch-up. Specifically, the causes of such defects are different, but they are closely related to the state of the device.