Power semiconductor device overload can be caused by excessive current or by excessive voltage. In both cases, possible destruction of the component is caused by (local) overheating of the silicon. Destruction of the elements can be prevented by appropriately limiting the duration of the overload.
Limiting Reverse Voltage using the Avalanche Effect and Zener Effect
When the maximum blocking voltage is exceeded, steep increase in leakage current may occur due to the avalanche effect or the Zener effect.
The avalanche effect (also known as avalanche breakdown) occurs when free charge carriers (electrons) are accelerated through the electric field in the space charge zone such that their energy is sufficient to cause impact ionization and form new free electron-hole pairs in its wake. If the space charge region is wide enough, the new free particles may also be accelerated so much so that they trigger impact ionization and generate even more electrons and holes. This increase in available charge carriers results in a steep increase in reverse current.
The Zener effect on the other hand occurs when an extremely high electric field strength breaks the bond between two atoms which results in a new electron-hole pair. This happens at many points in the depletion zone, causing the reverse current to rise steeply. This is referred to zener effect.
The avalanche effect generally occurs at electrical field strengths on the order of 3 × 10 5 V/cm and requires a depletion layer that is large enough to accomodate the effect. The zener effect occurs at much higher field strengths, generally on the order of 1 × 10 6 V/cm. High field strengths can only be found in very highly doped silicon.
Figure 1. Characteristics of a Zener Diode
The zener and avalanche effects inevitably lead to steep rises of reverse voltage. For this reason, both effects are used to limit voltages (Zener Diodes) or to cap peak voltages (avalanche diodes). The zener effect has a negative temperature coefficients which means that zener voltage decreases as temperature increases. The temperature coefficient of the avalanche effect is positive. A breakdown voltage of approximately 5V, the temperature coefficient is 0. This is the point of transition from the zener effect to the avalanche effect. All Zener diodes whose breakdown voltage are greater than 5 V are in a real sense avalanche diodes.
For the avalanche test, an avalanche diode (a diode which has guaranteed capacity upon avalanche breakdown) is loaded for a short period of time (10 μs) in the reverse direction with very high reverse current within the given rated current range .
The resulting thermal load (for a 1000 V diode, 1kW per 1 A rated current) must be spread out evenly within the component. The device is considered to be admissible if it survives this test.
Figure 2. Avalanche test on a 100 A diode
A component can survive the avalanche test only if the avalanche breakdown is spread throughout the entire active surface of the chip. If a local avalanche breakdown occurs at one or many small points in the component, the device is destroyed due to local overheating. The avalanche resistance of a component is determined by its structure, especially the design of the edge region of the reverse-biased p-n junction. An avalanche test can also be performed on an IGBT, albeit only at relatively low temperatures (approximately 10 mA for a 100 A IGBT). This tests the stability of the reverse behavior, especially the quality of passivation of the surface of the element.
Figure 3. Characteristic curve for an avalanche test IGBT
A pulse test, also known as LEM, test is a short term exposure to 2 to 3 times the rated current whereby the IGBT eventually switches off. Weak IGBTs are usually overwhelmed thermally and vaporize, which leads to the destruction of the IGBT.
Active clamping, using a clamping diode, is applied in order to limit overvoltage (caused by inductance Lk) that occurs upon switch off. If the voltage VCE in the IGBT exceeds the avalanche voltage of the clamping diode, the IGBT gate current becomes sufficient for the IGBT to conduct (active characteristic region). In this way, the IGBT voltage is limited to the avalanche voltage of the clamping diode. The stored energy associated with the inductance before switch off is converted into heat within the IGBT.
Figure 4. Active clamping with a clamping Diode
The clamping diode avalanche voltage must also be smaller than the reverse voltage of the IGBT that requires protection. The IGBT must not be fully turned on. It should only be brought to the conducting state so as to reduce overvoltage. During clamping, the voltage of the IGBT is equal to the avalanche voltage of the clamping diode. The two avalanche diodes between the gate and the emitter (avalanche voltage of about 15 - 20 V) limit the VGE to a nondestructive level.
Surge Current in Diodes (IFSM) and Thyristors (ITSM)
For diodes and thyristors, surge current is the measure used to deduce current overload capacity. Surge current refers to the maximum, instantaneous input current drawn by an electrical device when first turned on. Surge current is usually about 10 times greater than the rated current.
The peak load integral value, or i² t value, serves as a reference variable for selecting necessary components for short-circuit protection. The I² t value corresponds the shaded area in Figure 6.
Figure 5. Peak Load Integral
Short Circuit Behavior of IGBTs
For diodes and thyristors, current increases upon short circuiting and continues to do so until the component is destroyed (no limitation by the component). In MOSFETS and IGBTs however, the component itself limits short circuit current. The current does not rise above 6 to 8 times the nominal current so that the component absorbs voltage and returns from the saturation region. Due to the amount of losses in the component (simultaneous high voltage and high current), the short circuit can only run for up to 10 μs (for some only up to 6 μs). The short circuit current increases strongly as the gate voltage increases. The autonomous and automatic limitation of short circuit current by MOSFETs and IGBTs gives them an important advantage over other semiconductors.
Figure 6. Short-Circuit behaviour of an IGBT
Short Circuit Safe Operating Area
Upon switching off the (short circuit) current, the internal parasitic inductance of a module due to the rapid change in di/dt of the current must be taken into account, since these produce additional voltages and cause strain on the chip the external terminals of the module. On top of that, these changes are immeasurable. This must be taken into account when determing the short circuit safe operating area of a component.
For more information, please read: