How to suppress IGBT collector overvoltage spike

木犯001号

How to suppress IGBT collector overvoltage spike [Copy link]

When the IGBT is turned off, the collector current Ic decreases rapidly to 0, and the rapidly changing di/dt flows through the system stray inductance, generating an induced voltage ΔV. ΔV is superimposed on the bus voltage, causing the IGBT to be subjected to higher-than-usual voltage stress. Even if this voltage spike is short-lived, it may cause permanent damage to the IGBT.

di/dt is related to the characteristics of the IGBT chip and the device current when it is turned off. When the device is turned off in a short circuit or overcurrent state, the collector voltage overshoot will be extremely large and may exceed the rated value, thereby damaging the IGBT.

Therefore, how to suppress the voltage spike during shutdown is a topic worth discussing.

Calculation formula from collector overshoot voltage:

V=Ls*di/dt

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 change rate di/dt

3. Drive slower to reduce the current change rate di/dt

Reducing system noise is a system-level issue, which we will discuss in a separate topic.

However, reducing the current change rate di/dt will increase the turn-off loss. How to resolve this contradiction?

This article mainly wants to explore some methods to reduce the current change rate and thus suppress voltage overshoot from the perspective of drive design.

To reduce the current change rate, the first method that many people think of is to increase the gate resistance, but this method is not always useful, especially for FS+trench stop technology. Slightly increasing the gate resistance may even increase di/dt. Only when the gate resistance increases to a very large value can di/dt be reduced. Increasing the gate turn-off resistance blindly will significantly increase the turn-off loss, so this method is not advisable.

The turn-off waveform of IGBT4 changes the gate resistance, but the collector overvoltage does not change significantly.

So besides increasing the gate resistance, is there any other way to reduce di/dt? From a driver perspective, there are three ways:

1 Two-level shutdown

The idea of two-level shutdown is to slow down the shutdown speed and reduce di/dt during the shutdown process, thereby reducing the shutdown 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 conduction voltage, but higher than the voltage of the Miller platform. Then it is reduced from UTLTO to 0V or a negative voltage. Generally speaking, UTLTO can choose a voltage between 9 and 14V, and the voltage and duration of UTLTO are adjustable.

The two-level shutdown function can be integrated into the IGBT driver chip, such as 1ED020I12-FT. The voltage and duration of the two-level shutdown are usually implemented with a capacitor CTLTO or a combination of a capacitor and a resistor. When the capacitor is charged to a specific value, the output signal UOUT of the driver is triggered. If the input signal Uin is shorter than the set tTLTO, the input signal is usually suppressed, and the output signal remains unchanged.

The following figure shows the turn-off short-circuit current comparison with and without the TLTO function. Figure a shows the short-circuit turned off without TLTO technology, while Figure b shows the waveform with TLTO turned off. It can be clearly seen that the strong oscillations in the gate voltage and emitter-collector voltage are significantly reduced and, more importantly, the generated 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).

(a) No TLTO function

(b) With TLTO function

The two-level shutdown function can be integrated into the driver chip. The traditional IGBT driver IC with integrated two-level shutdown 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 shutdown; and the capacitor is used to set the time of the two-level shutdown.

1ED020I12_BT/FT

Infineon's latest X3 Enhanced driver chip, 1ED38X1MX12M, does not require external capacitors and resistors. The level and duration of the two-level shutdown can be set through digital configuration, which can simplify circuit design and BOM.

1ED38X1MX12M

1ED38X1MX12M two-level shutdown timing diagram

1ED38X1MX12M two-level judgment parameter setting gear

木犯001号

2 Soft shutdown

Soft shutdown ensures safe shutdown in short-circuit conditions. If the driver detects a short circuit, the soft shutdown function does not use a standard shutdown resistor to pull the gate voltage of the IGBT down to 0V or a negative voltage, but uses a relatively high impedance to release the gate current. This impedance can delay the discharge of the gate capacitance and slow down the IGBT shutdown. Once the gate voltage drops to a certain value (for example, 2V), the high impedance is short-circuited by a low impedance, which ensures that the gate-emitter capacitance is discharged quickly and completely. The principle of soft shutdown is shown in the figure below.

The IGBT drivers with soft shutdown function include Infineon 1ED34X1MX12M and 1ED38X1MX12M. These two chips configure the soft shutdown current through analog and digital methods respectively, and both have up to 16 soft shutdown current gears to choose from.

1ED34X1MX12M

1ED38X1MX12M

1ED3431 soft shutdown current adjustment gear

木犯001号

3 Active clamping

Also known as collector-emitter clamping, the following is a typical implementation of active clamping:

The principle of active clamping is: during the shutdown process, a voltage spike is generated between IGBT CE due to di/dt. As long as the potential at the collector exceeds the avalanche voltage of diode VD1, the unidirectional TVS diode VD1 will be turned on and pass current. Current I1 flows through VD1, VD2, RG and VT2. If the voltage drop generated on the gate resistor Rg is higher than the threshold voltage Vth of the IGBT, the IGBT will turn on again, thereby reducing the di/dt during the shutdown process. 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 the blocking diode VD2 must meet the 21A pulse current requirement. In addition, the TVS tube must be a high-voltage diode, and the commonly used series model is 1.5KExxx

However, this circuit also has disadvantages, such as the breakdown voltage is closely related to temperature, and the blocking diode junction capacitance is large. When the IGBT is switched, the displacement current will be additionally increased by du/dt.

Another more concise method is to feed the signal back to the driver before the push-pull circuit, as shown below:

The current I2 passes through the blocking diode VD5, the resistor R2 and the MOSFET VT8. The resistor R2 has a much higher resistance than RG, so as long as part of the current I1 flows out, it can generate enough voltage to open VT5 and close 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. Because the current through the diode is very low, cheaper TVS SMD diodes can be used.

2. The required space is determined only by creepage distance and clearance distance.

3. The circuit reacts very quickly.

The above are several commonly used methods for suppressing collector voltage spikes. Among them, active clamping requires a high-voltage diode, which has a high additional cost; two-level shutdown can be integrated in the driver chip. The traditional solution only requires adding a diode and a capacitor to the driver chip, while the latest Infineon 1ED X3 digital version chip can achieve parameter adjustment of two-level shutdown without external devices. In addition, the 1ED X3 analog/digital chip also integrates a soft shutdown function, which can achieve 16 levels of soft shutdown current adjustment without external devices.

吾妻思萌

Can TVS absorb it?