Power semiconductor cold knowledge 2: loss during IGBT short circuit

Power semiconductor cold knowledge 2: loss during IGBT short circuit

[Introduction]IGBTs are mainly used in motor drives and various types of converters. The short-circuit resistance of IGBTs is one of the guarantees for the reliable operation and safety of the system. way to achieve.

IGBT is allowed to be short-circuited. It has such confidence. The data sheet of EconoDUAL™3 FF600R12ME4 600A 1200V IGBT4 describes the short-circuit capability. When the driving Voltage does not exceed 15V, the typical value of short-circuit current is 2400A. As long as it is successfully turned off within 10us The short-circuit current is broken, and the device will not be damaged.

Power semiconductor cold knowledge 2: loss during IGBT short circuit

The short-circuit withstand capability of the IGBT buys time for the short-circuit protection, and the drive protection circuit can turn off the short-circuit current calmly and safely.

Short circuit capability is not free

The short-circuit capability of a device is not free, the cost is device loss. The short-circuit capability can be described by the short-circuit withstand time. Increasing the short-circuit withstand time may require sacrificing the saturation voltage drop, which in turn is related to the turn-off loss. Because the saturation voltage drop is high, it is sometimes necessary to sacrifice the turn-off loss to reduce.

One method is to make the MOS channel in the IGBT wider and increase the width-to-length ratio W/L of the MOS channel, which can reduce the saturation voltage drop during turn-on, but this will increase the short-circuit current and shorten the short-circuit withstand time. .

Power semiconductor cold knowledge 2: loss during IGBT short circuit

Based on this mechanism, IGBT technology is developing:

Some applications do not short-circuit the device, such as Boost circuits, etc. In this case, devices that do not guarantee short-circuit withstand time can be used, such as Infineon TRENCHSTOP™5 series, which do not support short-circuit conditions, but can support extremely low conduction losses or extremely high switching frequencies.

Thanks to the advancement of application technology and the improvement of drive protection circuits, the time required for the system to identify a short circuit and turn off the IGBT is getting shorter and shorter, thus allowing us to design an IGBT with a shorter short circuit time. For example, the short-circuit time of Infineon’s IGBT7 is [email protected], and the EDT2 chip is [email protected] The short-circuit withstand time is exchanged for the low-loss performance of the chip.

High current is not necessarily short circuit

In some applications discussed above, the device will not be short-circuited, but the system output will not be short-circuited. The system output short-circuit will generate a large current on the device. Appropriate overload protection must be considered in the design to control the shutdown current of the device in the reverse direction. In the safe operating area, it is generally twice the nominal current for IGBT modules.

High current is not necessarily short circuit. In order to clarify this problem, we need to analyze the output characteristics of IGBT Ic=f(Vce). The picture shows the output characteristics of FF900R12ME7_B11,900A 1200V IGBT7, which gives 2 times the nominal current within, the voltage between collector and emitter when driven at different gate voltages. The maximum current in the chart is 1800A, which is the guaranteed turn-off current value for a 900A IGBT module.

Power semiconductor cold knowledge 2: loss during IGBT short circuit

In order to discuss the problem, I expand the output characteristic Ic=f(Vce), expand it to 9 times the nominal current, 7-8 times the saturation voltage drop, so that various IGBT high current conditions to be explained are on the figure.

Power semiconductor cold knowledge 2: loss during IGBT short circuit

1. Normal working condition – reverse working safety area RBSOA:

The green part in the figure is a small part of the reverse operation safe area RBSOA. In this area, as long as the maximum operating junction temperature does not exceed the maximum operating junction temperature, the current that can be reliably turned off in each cycle is provided in a saturated state. During the off process, the Vce voltage rises, but cannot exceed the withstand voltage value of the device.

2. Short circuit area

The green frame is the short-circuit area with gate voltage of 13-15V, which can be read in the figure at Vge=15V. The short-circuit current is automatically limited to 5 times the nominal current by the device. At this time, the device exits saturation and the Vce voltage is fast. When it rises, the drive circuit detects that Vce rises to several times the saturation voltage drop, and short-circuit protection can be performed, and the device is safe.

3. Derating short circuit area

During a short circuit, due to the dv/dt of the collector, a small voltage will be induced at the gate through the CGC, which will raise the Vge voltage. At this time, the IGBT enters the derating short circuit area, the short circuit current increases, and the short circuit withstand time is shortened.

4. Prohibit switch area

The device current exceeds 2 times the nominal current, but the device does not exit saturation. At this time, the voltage on the device is lower than that of the short circuit. It seems that the device is more comfortable than the short circuit, but no, it is not allowed to turn off at this time. The device is out of saturation to allow the IGBT to be turned off.

The high-current working range is the green area. The example of the short-circuit area is the part framed by red and green boxes. At this time, the current is large, and the device has exited the saturation area, and the reverse operation is safe. The current cannot exceed the specified value of RBSOA, and it is turned off At the beginning the device is in saturation, which are two regions of discontinuity.

Principle explanation reference: How to understand the desaturation phenomenon of IGBT and the safe working area

High current and short circuit which is more terrible?

In order to illustrate the problem, we make a calculation problem:

High Current

Give the IGBT an inductive load, the red inductive current rises linearly from zero, and reaches 2 times the IGBT nominal current within 100 milliseconds, the blue is the IGBT saturation voltage, Vce=V0+Ic*r, the voltage is on the basis of Vo. Sexual rise.

Power semiconductor cold knowledge 2: loss during IGBT short circuit

Power semiconductor cold knowledge 2: loss during IGBT short circuit

short circuit

The IGBT is connected to the 900V DC bus. In the initial state before the short circuit, the current is close to 2 times the nominal current. At this time, a short circuit occurs, and the current rapidly rises to 6 times the nominal current. The short circuit detection circuit is successfully closed at 10us. When the IGBT is turned off, the bus voltage before turning off is 900V. Within 10us, the short-circuit power is 6 times the nominal current multiplied by 900V. If 600A 1200V is used as an example, the short-circuit instantaneous power is 3.24MW!!!

Power semiconductor cold knowledge 2: loss during IGBT short circuit

Power semiconductor cold knowledge 2: loss during IGBT short circuit

Calm down and look at the result of the integration. The energy loss of IGBT within 100ms is 0.3Ws*In/A, while the energy of short circuit in 10us is 0.054Ws*In/A. In this example, the loss in short circuit is only 18% of the IGBT 100ms inductance condition. However, due to the very large instantaneous current and power during short circuit, the junction temperature will greatly exceed the allowable working junction temperature of the chip, and the mechanical stress on the physical connection of the device is also very large, which is a severe working condition. Participate in the reference article: Power semiconductor Cold Knowledge: IGBT Short-Circuit Junction Temperature Times.

in conclusion

As long as the short circuit protection circuit of the IGBT and the system overload protection are properly designed, there is no need to tremble in the short circuit.

The Links:   CM20TF-12H LTM12C285