When the IGBT is turned off, the collector current Ic rapidly decreases to 0, and the rapidly changing di/dt flows through the stray inductance in the system, resulting in an induced voltage ΔV. ΔV is superimposed on the bus voltage, subjecting the IGBT to higher than usual voltage stress. Even if this voltage spike is short-lived, it may cause permanent damage to the IGBT.
The di/dt is related to the characteristics of the IGBT chip and the current of the device when it is turned off. When the device is turned off in a short-circuit or overcurrent condition, the collector voltage overshoot can be excessively large, possibly exceeding the rated value, thereby damaging the IGBT.
So how to suppress the voltage spike during turn-off is a topic worthy of discussion.
The formula for calculating the overshoot voltage from the collector:
We can see that there are two ways to reduce voltage overshoot:
1. Reduce system stray inductance
2. Reduce the current, thereby reducing the current rate of change di/dt
3. Drive slower, thereby reducing the current rate of change di/dt
To reduce system clutter is a system-level problem, and we will discuss this separately.
However, reducing the current change rate di/dt will increase the turn-off loss. How to solve this contradiction?
This article mainly wants to discuss some methods to reduce the current change rate and suppress the voltage overshoot from the perspective of driver design.
To reduce the current rate of change, the first solution that many people think of is to increase the gate resistance, but this method is not always useful, especially for the FS+trench stop technique. A slight increase in the gate resistance may even increase the di/dt, and the di/dt will only decrease when the gate resistance increases very large. Blindly increasing the gate turn-off resistance will significantly increase the turn-off loss, so this method is not desirable.
The turn-off waveform of IGBT4 changes the gate resistance, and the collector overvoltage does not change significantly
So is there any way to reduce the di/dt other than increasing the gate resistance? From a driver perspective, there are three approaches:
1 Two-level shutdown
The idea of two-level turn-off is to slow down the turn-off speed and di/dt during the turn-off process, thereby reducing the turn-off overvoltage to a reasonable value. When the IGBT is turned off, the gate voltage is not directly reduced to 0V or a negative voltage, but in a very short time, the gate voltage first drops to UTLTO, which is lower than the normal turn-on voltage, but higher than Miller Platform voltage. Then reduce from UTLTO to 0V or negative voltage. Generally speaking, UTLTO can choose the voltage between 9~14V, and the voltage and duration of UTLTO can be adjusted.
The two-level shutdown function can be integrated in the IGBT driver chip, such as 1ED020I12-FT. The voltage and duration of the two-level turn-off is usually achieved with a capacitor CTLTO or a combination of capacitors and resistors. When the capacitor is charged to a specific value, it will trigger the output signal UOUT of the driver. If the input signal Uin is shorter than the set tTLTO, the input signal will usually be suppressed, and the output signal will remain unchanged.
The figure below shows the turn-off short-circuit current comparison with and without the TLTO function. Figure a turns off the short circuit without using the TLTO technique, while Figure b shows the waveform with TLTO turn off. It can be clearly seen that the strong oscillations in the gate voltage and the emitter-collector voltage are significantly reduced, and more importantly, the resulting overvoltage is reduced. In this example, a peak voltage of 1125V appears in Figure a. In the measurement method shown in Figure b, the voltage is only 733V (in each case the DC bus voltage is 400V, and a 400A/1.2kV IGBT is used). Find components in stock on the only sample mall
(a) No TLTO function
(b) has TLTO function
The two-level shutdown function can be integrated in the driver chip. The traditional IGBT driver IC with integrated two-level turn-off function is shown in the figure below. The TLSET pin is connected to a Schottky diode and a capacitor. The Schottky diode is used to set the voltage of the two-level turn-off; and the capacitor is used to set the time of the two-level turn-off.
Infineon’s latest X3 Enhanced driver chip, 1ED38X1MX12M, does not require external capacitors and resistors, and only through digital configuration, the level and duration of two-level turn-off can be set, which simplifies circuit design and BOM.
1ED38X1MX12M two-level shutdown timing diagram
1ED38X1MX12M Two-level judgment parameter setting gear
2 Soft off
Soft turn-off ensures safe turn-off under short-circuit conditions. If the driver detects a short circuit, instead of pulling the gate voltage of the IGBT down to 0V or a negative voltage with a standard turn-off resistor, the soft turn-off function uses a relatively high impedance to release the gate current, which delays the gate The discharge of the polar capacitor makes the IGBT turn-off slower. Once the gate voltage drops to a certain value (eg 2V), the high impedance is shorted by a low impedance, which ensures fast and complete discharge of the gate-emitter capacitance. The principle of soft shutdown is shown in the figure below.
The IGBTs with soft-off function are Infineon 1ED34X1MX12M and 1ED38X1MX12M. These two chips configure the soft-off current by analog and digital respectively, and there are up to 16 soft-off current gears to choose from.
Soft-off current regulation gear of 1ED3431
3 Active Clamp
Also known as collector-emitter clamp, the following is a typical implementation of active clamp:
The principle of active clamping is: in the process of turning off, the voltage spikes are generated between IGBT CEs due to di/dt. As long as the potential at the collector exceeds the avalanche voltage of the diode VD1, the unidirectional TVS diode VD1 will conduct and pass current. The current I1 flows through VD1, VD2, RG and VT2, if the voltage drop across the gate resistor Rg is higher than the threshold voltage Vth of the IGBT, the IGBT is turned on again, thereby reducing the di/dt during turn-off. Therefore, in order to increase the gate voltage, sufficient current must be generated.
If the IGBT external gate voltage is 1ohm, the gate voltage is -15V, and the threshold voltage is 6V, in order to turn on the IGBT again, the active clamp current must be greater than 21A, so the TVS diode VD1 and blocking diode VD2 must meet 21A demand for pulsed current. In addition, the TVS tube must be a high-voltage diode, and the commonly used series model is 1.5KExxx
However, this kind of circuit also has shortcomings, such as the breakdown voltage is closely related to temperature, and the blocking diode has a large junction capacitance. When the IGBT is switched, the displacement current will be additionally increased by du/dt.
Another more concise method is to feed the signal before driving the push-pull circuit, as shown below:
Current I2 passes through blocking diode VD5, resistor R2 and MOSFET VT8. Resistor R2 is much higher than RG, so as long as a portion of current I1 flows, enough voltage is generated to turn VT5 on and turn off VT6. Once VT5 is turned on, I1 no longer passes through the gate resistor RG, but charges the input capacitor CGE. All of this has the following benefits for the circuit:
1. Since the current through the diode is low, cheaper TVS SMD diodes can be used.
2. The space required is only determined by creepage distances and clearances.
3. The circuit reacts very quickly.
The above are several commonly used methods of collector voltage spike suppression. Among them, the active clamp requires high-voltage diodes, and the additional cost is high; the two-level shutdown can be integrated in the driver chip. The traditional solution only needs to add diodes and capacitors to the driver chip. The latest Infineon 1ED X3 digital version chip, The parameter adjustment of two-level shutdown can be realized without external devices. In addition, the 1ED X3 analog/digital chip also integrates the soft-off function, which can realize 16-speed soft-off current adjustment without external devices.
Reviewing Editor Huang Haoyu