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Posted on 28 June 2019

Device Failure due to Electrical and Thermal Conditions

Power Semiconductor Device Failure

 

 

 

 

 

 

 

Electrical and thermal conditions beyond those recommended for safe operation of a power semiconductor device (PSD) can result in device failure. These conditions can be divided into several groups.

 

1. Impermissible rates of current or surge current peak

Weld Penetration on Damaged Power Semiconductor DeviceDamaged power semiconductor device elements are characterized by “large” weld penetration areas located in the area of power current flow. Since the device failure is normally caused by thermal breakdown or (in overlapping of surge current mode) by filament formation of high peak current, the weld penetration areas usually appear in the most overheated areas of the element as well as in the areas with poor heat dissipation.

When thermal filament formation occurs in overlapping of standby high peak current mode, the temperature in the thyristor or diode base increases rapidly. As a result, intrinsic carrier density increases as well, so that the density in the most heated area of the semiconductor element almost equals the concentration of injected carriers. As resistance in this area decreases, current that flows through it increases. This triggers a reaction motion which leads to current constriction into a local thermic path, which in turn destroys the semiconductor element. Pitting of the cathode area occurs during the smelting of one of the layers of the semiconductor element (aluminum and silicon compound that has the lowest melting point). Consequently, one of the semiconductor junctions gets damaged, usually the one used to block voltage directly. Total destruction of the semiconductor element may also occur.

 

2. Low anode current of thyristor

Circuit Diagram with Auxiliary and Main Thyristor Elements Some thyristors are semiconductors that consist of auxiliary and main thyristors.  An anode circuit current Ia is divided into two currents: anode current of the auxiliary thyristor (IAaux), which is also the amplifying electrode current of main thyristor (Iae main) and anode current of the main thyristor (Imain). If the anode current is low, current in the amplifying electrode circuit of the main thyristor cannot reach the level where the main thyristor opens completely over the amplifying electrode perimeter. A conduction area will be formed at a local point where high local power causes overheating and destroys the semiconductor element. If anode current cannot switch the main thyristor at all, even at the local point, then during long running of thyristors at such conditions there is the possibility of overheating of the amplifying electrode area of the main thyristor, which in turns leads to the destruction of the semiconductor element.

During circuit design, anode currents of the main power currents and of the snubber circuits should be taken into consideration. Such effects should be  studied thoroughly for fast thyristors, which have to be subdivided into amplifying electrodes of the main element and result in high gate current in the main thyristor. Problems associated with amplification are complicated by the fact that, structurally, amplifying areas have one-way heat dissipation. Heat dissipation is carried out from the anode side. On the cathode side, there is a wafer with a center mouth that shortens the amplifying and cathode area. The possibility of device failure therefore grows as the temperature of the device increases.

 

3. Undue Processes in Thyristor Amplifying Circuit

Thyristor Damage due to Poor Amplifying Signal
A poor amplifying signal or switching of the thyristor by an interfering signal may lead to device failure since this initiates switching of the thyristor only in a local point close to the auxiliary amplifying electrode instead of all over the perimeter of amplifying electrodes, as in the case of a standard control pulse.The thyristor conducts anode current exactly in this area and at the same time the direct blocking voltage starts decreasing. Thus, at this point, the peak power driven out of the thyristor has a maximum value and is localized in one or several locations. This may lead to local overheating and destruction of the semiconductor element.

 

Thyristor Damage due to Undue SwitchingThe signal in the amplifying circuit overrunning the safe operating areas - for example, high peak control current or voltage and reversed polarity current in the amplifying circuit - can lead to device failure with a characteristic local area of damage similar to that mentioned above. A high voltage in the amplifying circuit and the resulting unlimited current flow through the amplifying circuit leads to local overheating and destruction of the semiconductor element.

 

4. Simultaneous presence of amplifying and reverse anode voltage signals

Auxiliary Thyristor Weld Penetration Spot In such cases, at the expense of transistance, reverse anode current (leakage current) increases rapidly and greatly exceeds the admissible values for the thyristor.  As a result, substantial power is accumulated close to the amplifying electrode which may lead to the device's destruction. The higher the amplifying signal, the higher the leakage current, and as a result the driven out power. The following should also be taken into consideration – higher temperature leads to higher leakage current and a higher possibility of parametric failure of the power semiconductor device. Such conditions are not recommended for application, but it is possible in some cases if the given circuit is well studied and operating conditions for the device are chosen which would guarantee its reliable and proper functioning.

 

5. Excessive rate of anode current rise or low control current rate of rise (di/dt effect)

Thyristor Switching by Standard Control PulseIn the case of a standard control pulse, the thyristor switches all over the amplifying electrode perimeter of the auxiliary thyristor and the longitudinal propagation of on-state has the end rate. This implies that at high rates of rise of anode current and limitation of spreading speed in the on-state of the thyristor, local current density close to the amplifying electrode of the auxiliary element can exceed its limit value, which can cause overheating and destruction of the semiconductor element. The same can happen when the rate of rise of control current is too low.

Thyristor Failure by Undue SwitchingSuch a mechanism may occur upon switching the main thyristor. In this case the area of destruction is close to the amplifying electrode of the main thyristor. Device failure usually occurs at pulse-frequency conditions, i.e. when exceeding the limit of the recurrent value of admissible di/dt and when it exceeds the limit for a single value of admissible di/dt.

 

6. Device Failure due to Undue Anode Switch-over of Thyristors without Control Signal

The most characteristic factors leading to device failures in this case include:

  • Switching over due to critical rate of rise of off-state voltage (dv/dt effect)

The destruction spot is located within the cathode area of the main and auxiliary thyristors. Generally, this spot is located close to the amplifying electrode of the auxiliary thyristor or of the main thyristor since these areas are the most sensitive to switching over initiated by capacitance current.

  • Switching over when applying direct voltage in the end of the charge diffusion process when switching off during a time less than the turn-off time (tq)

 

Improper Switching Over of a Thyristor The area most likely to be affected is the cathode area of the main thyristor. Such damage is typical in cases where the concentration of electron-hole plasma in the base layers of the device is not enough to initiate the switching over process over most of the element's surface. This is a local process in one or several spots which have the highest carrier lifetime, or which are characterized by the lowest performance of distributed cathode diversion.

  • Failure at overvoltage in direct and reverse directions

Periphery Damage to a Power Semiconductor DeviceA strong electric field causes avalanche breakdown as the electric field on the surface increases. In this case, device failure occurs at the periphery of the semiconductor element of the thyristor or diode.

 

 

 

 

 

For more information, please read:

Device Failure due to Incorrect Mounting

Served Out Power Semiconductor Devices

 

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