IGBT Principle and Protection Technology

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IGBT Principle and Protection Technology [Copy link]

The switching function of IGBT is to form a channel by adding a positive gate voltage, provide base current to the PNP transistor, and turn on the IGBT. Conversely, adding a reverse gate voltage eliminates the channel, and the reverse base current flows, turning off the IGBT. The driving method of IGBT is basically the same as that of MOSFET. It only needs to control the input N-channel MOSFET, so it has a high input impedance characteristic.
After the channel of MOSFET is formed, the holes (minority carriers) injected from the P+ base into the N layer modulate the conductivity of the N layer, reduce the resistance of the N layer, and make the IGBT have a low on-state voltage at high voltage.
The working characteristics of IGBT include static and dynamic:
1. Static characteristics The static characteristics of IGBT mainly include volt-ampere characteristics, transfer characteristics and switching characteristics.
The volt-ampere characteristics of IGBT refer to the relationship curve between the drain current and the gate voltage when the gate-source voltage Ugs is used as a parameter. The output drain current ratio is controlled by the gate-source voltage Ugs. The higher the Ugs, the larger the Id. It is similar to the output characteristics of GTR. It can also be divided into three parts: saturation region 1, amplification region 2 and breakdown characteristics. In the cut-off state of the IGBT, the forward voltage is borne by the J2 junction, and the reverse voltage is borne by the J1 junction. If there is no N+ buffer, the forward and reverse blocking voltages can be at the same level. After adding the N+ buffer, the reverse turn-off voltage can only reach a level of tens of volts, thus limiting some application ranges of the IGBT.
The transfer characteristic of the IGBT refers to 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. In most of the drain current range after the IGBT is turned on, Id and Ugs are linearly related. The maximum gate-source voltage is limited by the maximum drain current, and its optimal value is generally taken as about 15V.
The switching characteristic of the IGBT refers to the relationship between the drain current and the drain-source voltage. When the 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 a Darlington structure, the current flowing through the MOSFET becomes the main part of the total current of the IGBT. At this time, the on-state voltage Uds(on) can be expressed by the following formula:
Uds(on) = Uj1 + Udr + IdRoh
, where Uj1 is the forward voltage of the JI junction;
Udr is the voltage drop on the extended resistor Rdr;
Roh is the channel resistance.
The on-state current Ids can be expressed by the following formula:
Ids=(1+Bpnp)Imos,
where Imos is the current flowing through the MOSFET.
Due to the conductivity modulation effect in the N+ region, the on-state voltage drop of the IGBT is small, and the on-state voltage drop of the IGBT with a withstand voltage of 1000V is 2 to 3V.
When the IGBT is in the off state, only a small leakage current exists.
2. Dynamic characteristics During the turn-on process, IGBT operates as a MOSFET for most of the time. However, in the late stage of the drain-source voltage Uds drop process, the PNP transistor goes from the amplification area to saturation, which adds a delay time. td(on) is the turn-on delay time, and tri is the current rise time. In practical applications, the drain current turn-on time ton is often given as the sum of td (on) and tri. The fall time of the drain-source voltage is composed of tfe1 and tfe2. During the turn-off process of the IGBT, the waveform of the drain current changes into two sections. Because after the MOSFET is turned off, the stored charge of the PNP transistor is difficult to eliminate quickly, resulting in a longer tail time of the drain current. td(off) is the turn-off delay time, and trv is the rise time of the voltage Uds(f). In practical applications, the drain current fall time Tf often given is composed of two sections, t(f1) and t(f2), and the drain current off time
t(off) = td(off) + trv + t(f)
In the formula, the sum of td(off) and trv is also called the storage time.
IGBT module drive and protection technology
2. IGBT gate characteristics
He Zhiwei Yada Power Laboratory, South China University of Technology
IGBT is a composite device of MOSFET and bipolar transistor. It has the characteristics of easy driving of MOSFET and the advantages of large voltage and current capacity of power transistor. Its frequency characteristics are between MOSFET and power transistor, and it can work normally in the frequency range of tens of kHz, so it occupies a dominant position in high-frequency and medium-power applications.
IGBT is a voltage-controlled device. When a DC voltage of more than ten V is applied between its gate and emitter, only a leakage current of μA level flows, and basically no power is consumed. However, there is a large parasitic capacitance (several thousand to tens of thousands of pF) between the gate and emitter of the IGBT. At the rising and falling edges of the driving pulse voltage, several A of charging and discharging current needs to be provided to meet the dynamic requirements of opening and closing. This requires that its driving circuit must also output a certain peak current.
As a high-power composite device, IGBT may be locked and damaged when overcurrent occurs. If the gate voltage is blocked at a normal speed during overcurrent, the excessive current change rate will cause overvoltage. For this reason, soft shutdown technology is required. Therefore, it is very necessary to master the driving and protection characteristics of the IGBT.
1. Gate characteristics
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 can generally only reach 20 to 30V, so gate breakdown is one of the common causes of IGBT failure. In applications, although the gate drive voltage is sometimes guaranteed not to 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 generate an oscillating voltage that damages the oxide layer. For this reason. Twisted wires are usually used to transmit drive signals to reduce parasitic inductance. A small resistor in series in the gate connection can also suppress the oscillation voltage.
Due to the presence of distributed capacitances Cge and Cgc between the gate-emitter and gate-collector of the IGBT, and the presence of distributed inductance Le in the emitter drive circuit, the influence of these distributed parameters makes the actual drive waveform of the IGBT not exactly the same as the ideal drive waveform, and produces factors that are not conducive to the opening and closing of the IGBT. This can be verified by an inductive load circuit with a freewheeling diode (see Figure 1).
At t0, the gate drive voltage begins to rise. At this time, the main factors affecting the rising slope of the gate voltage uge are only Rg and Cge, and the gate voltage rises faster. At t1, the gate threshold value of the IGBT is reached, and the collector current begins to rise. From this point on, there are two reasons that cause the uge waveform to deviate from the original trajectory.
First, the induced voltage on the distributed inductance Le in the emitter circuit increases with the increase of the collector current ic, thereby weakening the gate drive voltage, reducing the rising rate of uge between the gate and emitter, and slowing down the growth of the collector current.
Secondly, another factor that affects the voltage of the gate drive circuit is the Miller effect of the gate-collector capacitance Cgc. At t2, the collector current reaches its maximum value, and then the gate-collector capacitance Cgc begins to discharge, increasing the capacitive current of Cgc in the drive circuit, increasing the voltage drop on the impedance in the drive circuit, and also weakening the gate drive voltage. Obviously, the lower the impedance of the gate drive circuit, the weaker this effect is, and this effect lasts until t3, when uce drops to zero. Its influence also slows down the turn-on process of the IGBT. After t3, ic reaches a steady-state value, and after the factors affecting the gate voltage uge disappear, uge reaches its maximum value at a faster rate of increase.
As can be seen from the waveform in Figure 1, due to the presence of Le and Cgc, the rising rate of uge slows down a lot in the actual operation of IGBT. This effect of hindering the rise of driving voltage is manifested as an obstacle to the rise of collector current and the turn-on process. In order to slow down this effect, the internal resistance of Le and Cgc and the gate drive circuit of the IGBT module should be as small as possible to obtain a faster turn-on speed.
The waveform when the IGBT is turned off is shown in Figure 2. At t0, the gate drive voltage begins to decrease, and at t1, it reaches a level that can just maintain the normal working current of the collector. The IGBT enters the linear working area and uce begins to rise. At this time, the Miller effect of the gate-collector capacitance Cgc dominates the rise of uce. Due to the coupling charging effect of Cgc, uge remains basically unchanged during t1-t2. At t2, uge and ic begin to decrease at a speed determined by the inherent impedance between the gate and the emitter. At t3, uge and ic both drop to zero, and the turn-off ends.
As can be seen from Figure 2, due to the presence of capacitor Cgc, the turn-off process of the IGBT is also prolonged a lot. In order to reduce this effect, on the one hand, an IGBT device with a smaller Cgc should be selected; on the other hand, the internal impedance of the driving circuit should be reduced to increase the charging current flowing into Cgc and accelerate the rising speed of uce.
In practical applications, the amplitude of the uge of the IGBT also affects the saturation conduction voltage drop: as the uge increases, the saturation conduction voltage will decrease. Since the saturation conduction voltage is one of the main reasons for the heating of the IGBT, it must be reduced as much as possible. Usually the uge is 15~18V. If it is too high, it is easy to cause gate breakdown. Generally, 15V is taken. When the IGBT is turned off, adding a certain negative bias voltage to its gate-emitter is beneficial to improve the anti-interference ability of the IGBT, usually 5~10V.
2. The influence of the gate series resistance on the gate drive waveform
The rise and fall rate of the gate drive voltage has a great influence on the IGBT turn-on and turn-off process. The MOS channel of the IGBT is directly controlled by the gate voltage, and the drain current of the MOSFET part controls the gate current of the bipolar part, so that the turn-on characteristics of the IGBT are mainly determined by its MOSFET part, so the turn-on of the IGBT is greatly affected by the gate drive waveform. The turn-off characteristics of IGBT mainly depend on the recombination rate of internal minority carriers. The recombination of minority carriers is affected by the turn-off of MOSFET, so the gate drive also affects the turn-off of IGBT.
In high-frequency applications, the rise and fall rates of the drive voltage should be faster to increase the IGBT switching rate and reduce losses. Under
normal conditions, the faster the IGBT is turned on, the smaller the loss. However, if there is a reverse recovery current of the freewheeling diode and a discharge current of the absorption capacitor during the turn-on process, the faster the turn-on is, the greater the peak current that the IGBT will bear, and the more likely it is to cause damage to the IGBT. At this time, the rise rate of the gate drive voltage should be reduced, that is, the resistance value of the gate series resistor should be increased to suppress the peak value of the current. The cost is a large turn-on loss. Using this technology, the current peak value of the turn-on process can be controlled at any value.
From the above analysis, it can be seen that the gate series resistance and the internal impedance of the drive circuit have a greater impact on the turn-on process of the IGBT, but a smaller impact on the turn-off process. A small series resistance is conducive to accelerating the turn-off rate and reducing the turn-off loss, but too small a series resistance will cause excessive di/dt and generate a large collector voltage spike. Therefore, the series resistance should be comprehensively considered according to the specific design requirements.
The gate resistance also affects the waveform of the driving pulse. If the resistance value is too small, it will cause pulse oscillation. If it is too large, the leading and trailing edges of the pulse waveform will be delayed and slowed down. The gate input capacitance Cge of the IGBT increases with the increase of its rated current capacity. In order to maintain the same leading and trailing edge rate of the driving pulse, a larger leading and trailing edge charging current should be provided for IGBT devices with large current capacity. To this end, the resistance value of the gate series resistance should decrease with the increase of the IGBT current capacity.
3. IGBT protection function
Yan Zhiyuan Wuxi Fule Electronics Co., Ltd.
1. IGBT overcurrent protection
The overcurrent protection circuit of the IGBT can be divided into two categories: one is a low multiple (1.2 to 1.5 times) overload protection; the other is a high multiple (up to 8 to 10 times) short-circuit protection.
For overload protection, there is no need to respond quickly, and centralized protection can be used, that is, to detect the total current of the input end or DC link. When this current exceeds the set value, the comparator flips, blocking the input pulses of all IGBT drivers, and reducing the output current to zero. Once this overload current protection is activated, it must be reset to resume normal operation.
IGBT can withstand short-circuit current for a very short time. The time it can withstand short-circuit current is related to the on-state saturation voltage drop of the IGBT, and it will be extended as the saturation on-state voltage drop increases. For example, the short-circuit time allowed by an IGBT with a saturation voltage drop of less than 2V is less than 5μs, while the short-circuit time allowed by an IGBT with a saturation voltage drop of 3V can reach 15μs, and it can reach more than 30μs at 4-5V. The above relationship exists because as the saturation on-state voltage drop decreases, the impedance of the IGBT also decreases, and the short-circuit current increases at the same time. The power consumption during short circuit increases with the square of the current, causing the short circuit time to decrease rapidly.
The commonly used protection measures are soft shutdown and gate voltage reduction. Soft shutdown refers to directly shutting down the IGBT when overcurrent and short circuit occur. However, soft shutdown has poor anti-interference ability. Once an overcurrent signal is detected, it will shut down, which is prone to misoperation. In order to increase the anti-interference ability of the protection circuit, a delay can be added between the fault signal and the start of the protection circuit. However, the fault current will rise sharply during this delay, greatly increasing the power loss and increasing the di/dt of the device. Therefore, the protection circuit is often activated, but the device is still broken. The
purpose of reducing the gate voltage is to reduce the gate voltage immediately when the device overcurrent is detected, but the device still remains on. After the gate voltage is reduced, a fixed delay is set. The fault current is limited to a smaller value during this delay period, which reduces the power consumption of the device during the fault, prolongs the device's short-circuit resistance time, and can reduce the di/dt when the device is turned off, which is very beneficial to device protection. If the fault signal still exists after the delay, the device will be turned off. If the fault signal disappears, the drive circuit can automatically restore the normal working state, thereby greatly enhancing the anti-interference ability.
2. When the overvoltage during the IGBT switching process
turns off the IGBT, its collector current has a high drop rate, especially in the case of a short-circuit fault. If no soft shutdown measures are taken, its critical current drop rate will reach several kA/μs. The extremely high current drop rate will induce a high overvoltage on the distributed inductance of the main circuit, causing the current and voltage running trajectory of the IGBT to exceed its safe working area and be damaged when it is turned off. Therefore, from the perspective of shutdown, it is hoped that the inductance and current drop rate of the main circuit are as small as possible. However, for the turn-on of the IGBT, the inductance of the collector circuit is conducive to suppressing the reverse recovery current of the freewheeling diode and the peak current caused by the charging and discharging of the capacitor, which can reduce the turn-on loss and withstand a higher turn-on current rise rate. In general, the collector of the IGBT switching circuit does not need a series inductor, and its turn-on loss can be controlled by improving the gate drive conditions.
3. IGBT turn-off buffer absorption circuit
In order to effectively suppress the IGBT turn-off overvoltage and reduce the turn-off loss, it is usually necessary to set a turn-off buffer absorption circuit for the IGBT main circuit. The IGBT turn-off buffer absorption circuit is divided into a charge-discharge type and a discharge prevention type.
There are two types of charge-discharge type: RC absorption and RCD absorption.
The RC absorption circuit will also cause an overshoot voltage due to the voltage drop generated by the charging current of the capacitor C on the resistor R. The RCD circuit overcomes the overshoot voltage by bypassing the charging current on the resistor with a diode.
Figure 4 shows three types of discharge-blocking type absorption circuits. The discharge voltage of the absorption capacitor Cs in the discharge-blocking type snubber circuit is the power supply voltage. Before each shutdown, Cs only feeds back the overshoot energy of the last shutdown voltage to the power supply, reducing the power consumption of the absorption circuit. Because the capacitor voltage starts to rise from the power supply voltage when the IGBT is turned off, its overvoltage absorption capacity is not as good as the RCD type charge-discharge type.
Figure 4 Three types of discharge blocking type absorption circuits
In terms of the ability to absorb overvoltage, the discharge blocking type absorption effect is slightly worse, but the energy loss is smaller.
The requirements for the buffer absorption circuit are:
① Minimize the wiring inductance La of the main circuit;
② The absorption capacitor should use a low-inductance absorption capacitor, and its lead should be as short as possible, preferably directly connected to the terminal of the IGBT;
③ The absorption diode should use a fast turn-on and fast soft recovery diode to avoid the generation of turn-on overvoltage and reverse recovery causing a large oscillation overvoltage.

Schut1n

This is really good, I learned a lot and it is very fulfilling