Difference Between Overcurrent Fault and Short Circuit Fault in IGBT

In the field of power electronics, Insulated Gate Bipolar Transistor (IGBT) is a widely used semiconductor switching device. It combines the advantages of high input impedance from Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) and strong current-carrying capability from Bipolar Junction Transistor (BJT).

However, during its operation, IGBT may encounter various faults, with overcurrent and short circuit being the most common and posing the greatest threats to system stability. This article will analyze in detail the differences between IGBT overcurrent and short circuit faults, enhancing the accuracy and coherence of the explanation through comparisons, examples, and supporting data.

Firstly, let’s understand the basic working principle of IGBT. IGBT controls its conduction or cutoff by applying voltage to the gate. When the gate voltage is above the threshold voltage, IGBT conducts and allows current to pass through; when the gate voltage is below the threshold voltage, IGBT cuts off and prevents current flow. Ideally, IGBT can switch rapidly between full conduction and full cutoff, achieving efficient energy conversion.

Overcurrent fault refers to the current flowing through IGBT exceeding its designed maximum rated current. This situation may arise from a sudden decrease in external load or abnormal increase in power supply voltage. Overcurrent fault does not immediately damage IGBT, but if not addressed promptly, sustained overcurrent can cause device overheating, thereby shortening its lifespan or even leading to damage.

On the other hand, a short circuit fault occurs when a direct electrical connection occurs between the collector and emitter of IGBT, usually caused by external circuit faults or a short circuit on the load side. Short circuit fault results in a high instantaneous current flowing through IGBT, far exceeding its maximum rated current. This intense current peak can immediately damage IGBT and may even lead to fires or other safety incidents.

Overcurrent generally refers to load overload caused by some reason (such as motor blockage) or current divergence and oscillation due to software control issues. Short circuit generally refers to bridge arm straight-through, or bus voltage passing through an unloaded circuit of IGBT (phase-to-phase short circuit or phase-to-ground short circuit).

The protection methods for overcurrent and short circuit are also different. Overcurrent protection typically uses output current sensors as detection components, implementing protection on the control board through hardware or software means. In contrast, short circuit protection can only be detected through hardware and is usually integrated into the IGBT driver circuit, implementing protection by detecting IGBT’s desaturation behavior.

Next, let’s illustrate common short circuit faults using the example of an industrial motor drive inverter, as shown in Figure 1:

Figure 1 Short Circuit Fault in Industrial Motor Drive

① Bridge arm straight-through: Caused by both IGBTs in one bridge arm turning on simultaneously. This situation may result from electromagnetic interference or controller faults, or it could be due to a failure of one IGBT in the bridge arm while the normal IGBT continues switching.

② Phase-to-phase short circuit: May be caused by performance degradation, high temperature, or overvoltage events leading to insulation breakdown between motor windings. It could also result from damaged insulation in transmission cables.

③ Phase-to-ground short circuit: Similarly, this may be caused by performance degradation, high temperature, or overvoltage events leading to insulation breakdown between motor windings and the motor casing. It could also result from damaged transmission cables grounding.

All three types of faults can be considered short circuit faults, but the short circuit loop impedance differs. If the inverter output cable is long, and a short circuit occurs at the end of the cable, the short circuit impedance is relatively large. In such cases, the difference between short circuit and overcurrent may not be significant, and short circuit protection can also be achieved through output current sensors.

It should be noted that the probability of IGBT experiencing a bridge arm straight-through short circuit fault is relatively low. Therefore, in some low-cost applications, IGBT driver circuits may not integrate short circuit protection, only featuring overcurrent protection. In such cases, whether the device can be preserved in the event of a short circuit on the load side mainly depends on the magnitude of the short circuit loop impedance and the response speed of the current sensor.