This article focuses on three IGBT drive circuits. The function of the driving circuit is to power the pulse outputted by the single chip to drive the IGBT to ensure the reliable operation of the IGBT. The driving circuit plays a vital role. The basic requirements for the IGBT driving circuit are as follows:
(1) Provide appropriate forward and reverse output voltages to reliably turn the IGBT on and off.
(2) Provide sufficient transient power or instantaneous current to enable the IGBT to quickly establish a gate-controlled electric field to conduct.
(3) The input and output delay time is as small as possible to improve work efficiency.
(4) High enough input and output electrical isolation to insulate the signal circuit from the gate drive circuit.
(5) Sensitive overcurrent protection.
Drive circuit EXB841/840
The working principle of EXB841 is shown in Fig. 1. When the current of 10 mA of the 14-pin and 15-pin of EXB841 flows through 1us, the IGBT is normally turned on, VCE drops to about 3V, and the voltage of 6-pin is clamped at about 8V. Since the VS1 regulation value is 13V, Therefore, it will not be broken down, V3 will not conduct, the potential of point E is about 20V, and the diode VD is cut off, which does not affect the normal operation of V4 and V5.
When there is no current flowing through pins 14 and 15, V1 and V2 are turned on, V2 is turned on to make V4 turn off, V5 is turned on, IGBT gate charge is rapidly discharged through V5, and pin 3 potential drops to 0V, which is IGBT gate. The negative IGBT with a negative bias of about 5V is used, and the IGBT is reliably turned off, and the rapid rise of VCE causes the pin 6 to "float". The discharge of C2 makes the potential of point B 0V, then V S1 is still not conducting, the subsequent circuit does not operate, and the IGBT is normally turned off.
If an overcurrent occurs, the V CE of the IGBT is too large to make VD2 cut off, so that VS1 breaks down, V3 turns on, C4 discharges through R7, and the potential at point D drops, so that the voltage UGE between the gates of the IGBT is lowered, and the completion is slow. Turn off to achieve protection of the IGBT. The process of overcurrent protection realized by EXB841 shows that the main basis for the EXB841 to determine the overcurrent is the voltage of the 6-pin. The voltage of the 6-pin is not only related to VCE, but also related to the turn-on voltage Vd of the diode VD2.
The typical wiring method is shown in Figure 2. Note the following points when using:
a, IGBT grid-emitter drive circuit round-trip wiring can not be too long (generally should be less than 1m), and should be twisted pair connection to prevent interference.
b. Since the IGBT collector generates a large voltage spike, increasing the IGBT gate series resistance RG is beneficial to its safe operation. However, the gate resistance RG cannot be too large or too small. If the RG is increased, the turn-on and turn-off time is prolonged, so that the turn-on power consumption is increased. Conversely, if the RG is too small, the di/dt is increased, and mis-conduction is likely to occur.
c. Capacitor C is used to absorb the change of supply voltage caused by the impedance of the power supply connection. It is not the power supply filter capacitor of the power supply. The value is generally 47 F.
d, 6 feet over current protection sampling signal connection terminal, connected to the IGBT collector through a fast recovery diode.
e, 14, 15 connected to the drive signal, generally 14 feet connected to the pulse forming part of the ground, 15 feet connected to the positive end of the input signal, the input current of the 15 end should generally be less than 20mA, so add a current limiting resistor before the 15 feet.
f. In order to ensure reliable turn-off and turn-on, a Zener diode is added to the gate emitter.
b, port VL / Reset
This terminal is used to define the input InA and InB with the dense characteristic, so that the input is turned on at 2/3 VL and turned off at I/3 VL. When the PWM signal is TTL level, the terminal is connected as shown in Figure 3-5. When the input InA and InB signals are 15V, the terminal should be connected to the ++15V power supply through a resistor of about 1K. The turn-off voltage should be lov and 5V, respectively. In addition, there is another function in the input UL/Reset terminal: if it is grounded, the error message in the logic drive interface unit 1.DI001 is cleared.
c, gate output
The gate output Gx terminal is connected to the gate of the power semiconductor. When the SCALE driver is powered by 15V, the gate output is 15V. The negative gate voltage is generated internally by the driver. Using the circuit of the structure shown in Figure 3-6 can achieve different speeds of turn-on and turn-off, increasing the flexibility of user use.
d, layout and wiring
The driver should be placed as close as possible to the power semiconductor so that the leads from the driver to the power transistor are as short as possible. Generally, the driver should not be as long as 10 cm longer. At the same time, it is generally required to use a stranded wire for the collector and emitter leads, and an aligning Zener diode (15~18V) can be connected between the gate and the emitter of the IGBT to protect the IGBT from breakdown.
The mode selection terminal MOD of the drive module is externally connected to the +15V power supply, and the input pins RC1 and RC2 are grounded for direct operation mode. The logic control level adopts +15V, and the signal input pins InA and InB are connected together to receive the pulse signal from the single chip microcomputer for synchronous control. 2SD315A SO1 and SO2 two-pin external triode and optocoupler are used to output the working state of two output channels to the single-chip microcomputer. The output structure is open-collector output, which can be applied to various level logic through external pull-up resistor. . A light-emitting diode is placed between pins SO1, SO2 and the power supply and between VisoX and LSX for fault indication. Under normal conditions, both SO1 and SO2 outputs are high. After power-on, D3 and D4 are on first, and after a few seconds, they are off, and D8 and D15 are on.
When a fault signal is detected, the output levels of SO1 and SO2 are pulled down to ground, ie D3 and D4 are lit, while D8 and D15 are flashing. 2SD315A judges whether the loop is short-circuited or over-current by monitoring UCE(sat). When one-way or two-way over-current phenomenon is detected, the detection circuit will return the abnormal state to the drive module, and a fault signal will be generated inside the drive module. It is latched and the latch time is 1 s. During this time, the driver module no longer outputs a signal, but the two sets of IGBTs are turned off in time to protect them. At the same time, the high level of the state output pins SO1 and SO2 is pulled low, the optocoupler TLP521 is turned on, and the two state signals are sent to the single chip through the OR gate 74LS32. In order to prevent a high back electromotive force on the collector of the IGBT due to the shutdown speed being too fast, the circuit structure shown in the figure is used to achieve the difference in turn-on and turn-off speeds. The gate resistance is 3.4Ω at turn-on and 6.8Ω at turn-off. The diode is fast recovery, which reduces the turn-off speed to a safe level.
IGBT short circuit failure mechanism
Several consequences of IGBT load short circuit
(1) Exceeding the thermal limit: The intrinsic temperature limit of the semiconductor is 250 °C. When the junction temperature exceeds the intrinsic temperature, the device will lose its blocking ability. When the IGBT load is short-circuited, the junction temperature rises due to the short-circuit current. At the limit, the gate level protection also fails.
(2) Current holding effect: Under the normal working current, the IGBT has a small sheet resistance Rs and no current holding phenomenon, but in the short circuit state, the voltage drop on Rs is higher than 0.7V due to the large short circuit current. When J1 is positively biased, current is generated, and the gate level loses voltage control.
(3) Shutdown overvoltage: In order to suppress the short-circuit current, when the fault occurs, the control circuit immediately removes the positive gate voltage, turns off the IGBT, and the short-circuit current decreases accordingly. Due to the large short-circuit current, the current drop rate during turn-off is high, and a high voltage is induced in the wiring inductance, especially in the case where the induced voltage on the package lead inductance is hard to suppress, it will make the device An overcurrent has become a shutdown overvoltage and has failed.
IGBT overcurrent protection method
(1) Decompression method: refers to lowering the gate voltage when a fault occurs. Since the short-circuit current ratio is proportional to the plus-gate voltage Ug1, the positive-gate voltage can be lowered in the event of a fault.
(2) Method of cutting off the pulse: Since the Uce voltage rises during overcurrent, we use the method of detecting the collector voltage to judge whether the current is overcurrent, and if it is overcurrent, the trigger pulse is cut off. At the same time, the soft turn-off mode is adopted as much as possible to alleviate the drop rate of the short-circuit current and avoid the damage caused by the overvoltage.
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