Q Short circuit overcurrent protection 11 Short circuit withstand capability

In the event of a short circuit, first the IGBT's collector current will rise, once it has reached a certain level, the C-E voltage will spike. Depending on the device's characteristics, during the short-circuit, the collector current can be kept at or below a certain level, however the IGBT will still continue to be subjected to a heavy load (high voltage and high current).

Therefore, this condition must be removed as soon as possible. The amount of time allowed between the start of a short circuit until the current is cut off, is limited by the IGBT's short circuit withstand capability.

The short-circuit withstand capability, as illustrated in Fig. 5-1. It is determined by the amount of time it takes from the start of the short-circuit current until the module is destroyed. The withstand capability of the N series IGBTs is as follows:

Short-circuit withstand capability: 10^s minimum

< Conditions >

1200V series: Ed(Vcc)=800V

• RG: Standard RG value (from the specifications)

In general, the higher the supply voltage (Ed) or temperature (Tj) rises, the lower the short-circuit withstand capability.

Breakdown point

Breakdown point

What Causes Short Circuit
Short-circuit withstand capability (Pw)

Fig. 5-1 Measuring circuit and waveform

1.2 Short-circuit modes and causes

Table 5-1 lists the short-circuit modes and causes that occur in inverters.

Table 5-1 Short circuit mode and cause

Table 5-1 Short circuit mode and cause

How Cause Short Circuit

1.3 Short-circuit (overcurrent) detection 1) Detection in the circuit

As stated previously, in the event of a short-circuit, the IGBT must be disabled as soon as possible. Therefore, the time from overcurrent detection to the complete turn-off in each circuit must be as short as possible.

Since the IGBT turns off very quickly, if the overcurrent is shut off using an ordinary drive signal, the collector-emitter voltage will rise due to the inductive kick, and the IGBT may be destroyed by overvoltage (RBSOA destructions). Therefore, it is recommended that when cutting off the overcurrent that the IGBT be turned off gently (Soft turn-off).

Figure 5-2 shows the insertion methods for overcurrent detectors, and Table 5-2 lists the features of the various methods along with their detection possibilities. After determining what kind of protection is necessary, select the most appropriate form of detection.

Figure 5-2 shows the insertion methods for overcurrent detectors, and Table 5-2 lists the features of the various methods along with their detection possibilities. After determining what kind of protection is necessary, select the most appropriate form of detection.

Transformer Short Circuit Current Table
Table 5-2 Overcurrent detector insertion positions and function

Detector insertion position

Features

Detection function

Insertion in line with smoothing capacitor

Fig. 5-2/®

• AC current transformer available

• Low detection precision

• Arm short-circuit

• Short in output circuit

• Series arm short-circuit

• Ground fault

Insertion at inverter input Fig. 5-2/©

• Necessary to use DC current transformer

• Low detection precision

• Arm short-circuit

• Short in output circuit

• Series arm short-circuit

• Ground fault

Insertion at inverter output Fig. 5-2/®

• AC current transformer available for high frequency output equipment

• High detection precision

• Short in output circuit

• Ground fault

Insertion in line with switches Fig. 5-2/®

• Necessary to use DC current transformer

• High detection precision

• Arm short-circuit

• Short in output circuit

• Series arm short-circuit

• Ground fault

2) Detecting using VcE(sat)

This method can protect against all of the short-circuit types listed in Table 5-1. Since all operations from overcurrent detection to protection are done on the drive circuit side, this offers the fastest protection possible. A short-circuit protection schematic, based in VCE(sat) detection, is shown in Fig. 5-3.

Igbt Protection Circuit

This circuit uses D1 to constantly monitor the collector-emitter voltage, so if during operation the IGBT's collector-emitter voltage rises above the limit at D2, then a short-circuit condition will be detected and T1 will be switched on while T2 and T3 are switched off. At this time, the accumulated charge at the gate is slowly released through the RGE, so a large voltage spike is prevented when the IGBT is turned off.

Fuji Electric's gate driver hybrid ICS (model EXB840, 841) have the same kind of protective circuit built in, thereby simplifying the drive circuit design. For more details, refer to Chapter 7 "Drive Circuit Design". Fig. 5-4 shows an IGBT waveform during short circuit protection.

Tek Run: 25.0MS/S Sample IHBE

Tek Run: 25.0MS/S Sample IHBE

cm 200 V aiE 500mV M 2.00ns Ch3 S 4.0 V 24 lul 2003

2MBI300UD-120

Ed=600V, Vge=+15V, -5V (EXB841), Rg=3.3Q, Tj=125°C VcE=200V/div, Ic=250A, VGE=10V/div, t=2|js/div Fig. 5-4 Waveforms during short circuit protection

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Responses

  • goldilocks
    How to cause a short circuit?
    8 years ago
  • sabrina wagner
    How to test igbt using multimeter?
    7 years ago
  • Jonne
    How to protect igbt from short circuit?
    3 years ago

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