A failure mechanism of IGBT module in MMC and improvement

In modular multi-level converter valves, as the blocked mode happens, the failure in IGBT modules often causes a chain explosion reaction, which masks the initial failure mechanism. The root cause of the failure was that the voltage, consisting of the diode forward voltage and the induced voltage on the parasitic inductance, exceeded the reverse blocking capability of the anti-parallel IGBT. The methods against the failure were studied from three aspects: decreasing the diode’s peak forward voltage, increasing the reverse blocking capability of IGBT, and minimizing the parasitic inductance of the package. It was found that the doping concentration’s effect on the diode’s peak forward voltage is not monotonic. It can be explained from the carrier density distribution during forward recovery transient that lower doping concentration reduces the carrier injection, and higher doping concentration reduces the excess carrier concentration in the doping region. Then, the influence of two buffer designs on the reverse blocking capability of IGBT was compared. It was found that a higher reverse blocking voltage would be achieved when a multi-proton implantation buffer replaced the diffused buffer. And the new design has almost no degradation in forward blocking capability.


Introduction
Because of the advantages of high blocking voltage, low on-state loss, high power density, and so on, insulated gate bipolar transistors (IGBTs) are used in rectifiers, inverters, DC transformers, and AC transformers, which are applied in power electronic systems such as smart grid and track traction.
The IGBT modules used in the modular multilevel converters (MMCs) of the high voltage direct current (HVDC) transmission system [1] are composed of many parallel-connected IGBTs and antiparallel diodes.
The peak voltage during IGBT turn-off has gotten more attention [2][3][4] , while the reverse blocking stress of IGBT during converter switch transient was often ignored.Especially in the application of MMC-HVDC, a thicker drift region for IGBT is needed to achieve higher forward blocking capability, which results in poor forward conduction characteristics and higher turn-off loss.A lower doped field stop structure was introduced into punch-through (PT) design, which minimizes the turn-off loss, optimizes the on-state carrier distribution, and ensures sufficient positive blocking capability.However, the reverse blocking capability cannot be guaranteed.
In this paper, the stress applied on the IGBT module during the converter switch transient can be evaluated equivalently using the double pulse test method.The SPICE model could analyze the electrical stress between IGBTs and diodes in the module.The failure mechanism of IGBT was obtained by electric parameter analysis.Based on this, the methods to prevent the failure are studied from three aspects: decreasing the diode's forward voltage overshoot, increasing the reverse blocking capability of IGBT, and reducing the inductance effect on IGBT.

Failure
The double pulse test method usually evaluates the IGBT performance in MMC [5][6] .The circuit is shown in Figure 1.The IGBTs T 1 and T 2 are connected in series, which anti-connect with diodes D 1 and D 2, respectively.Point A is the bonding point of the IGBT module power terminal on DBC.L a1 is the parasitic inductance between the diode anode and point A. L e1 is the parasitic inductance between the IGBT emitter and point A. L e2 is the parasitic inductance between point A and the power terminal of the IGBT module.
During T 2 turn-off, the load current commutates into the diodes D 1, which correspond to certain high-stress states during the blocked mode of MMC.During the double pulse test, the upper bridge IGBT module's failure occurs when the lower bridge IGBT turns off, as shown in Figure 2.

Electrical stress and failure mechanism analysis
The SPICE model of the double pulse circuit is presented in this paper by TCAD.
The electrical stress of the upper IGBT and diode and the lower IGBT is shown in Figure 3.After the lower IGBT turn off, the upper diode enters forward conducting from a reverse blocking state.While then, the diode bears the positive recovery voltage.
As anti-parallel is with the diode, the IGBT needs to withstand the corresponding voltage and the voltage induced on the parasitic inductance.The upper IGBT turn to the reverse blocking state from the forward blocking state.
The following equation can be obtained from the circuit: V Le1 is the voltage induced on inductance L e1 , and V La1 is the voltage induced on the inductance L a1 .When IGBT turns to a reverse blocking state from the forward blocking state, a current iEC flows through L e1 due to the parasitic capacitance charging.As seen in Figure 3, i EC is about a few amperes, and the voltage induced by which inductance L e1 can be ignored.However, the current flowing through the diode is about hundreds of amperes, the voltage induced by which on inductance V La1 cannot be ignored.
Failure will happen if the V EC exceeds the reverse blocking capability of the upper IGBT.

Impact analysis for diode forward voltage
When the diode turns to the forward conducting state from the reverse blocking state, the voltage across the diode can be substantially larger than the on-state voltage, for the drift region does not get modulated due to the finite carrier's diffusion rate, as well as the lower doping concentration in the drift region designed for higher blocking voltage.
From the increase of forward current to current stabilization, the holes and electrons injected by the P+n-and n-n+ junctions diffuse in the drift region to form the current flux and lead to a voltage drop in the junctions and the low-doping drift region.It can be intuitively understood that the voltage drop in the region with low conductance modulation within the drift region constitutes a major part of the forward recovery voltage peak value.
Figure 4 shows that a lower thickness of the drift region by deepening the doping depth of the anode or cathode can reduce the V FM (the peak value of the forward recovery voltage).Certainly, thinning out the thickness directly can also achieve a lower V FM .It can be easily understood using the resistance equation of the drift region during the forward recovery transient, which is given by [7] : where W D is the maximum width of the drift region, μ n is the electron mobility, and n is the electron density.
Then, the effect of the doping concentration on V FM has been studied.The V FM decreases as the doping concentration increases and then increases, which is shown in Figure 5 and Figure 6.
Figure 5.Effect of anode doping concentration on V FM (higher doping concentration from dop1 to dop5) Figure 6.Effect of cathode doping concentration on V FM (higher doping concentration from dop1 to dop4) It was found that the higher doping concentration can increase the carrier concentration in the drift region, which reduces the drift resistance.However, the higher doping concentration will also increase the barrier height in the doping region (the anode region or the cathode region), which leads to the decrease of the excess carrier concentration in this region, increasing the resistance in this region.
The electron density distribution as the anode doping concentration increases is shown in Figure 7.The hole density distribution as the cathode doping concentration increases is shown in Figure 8.Therefore, there is an optimal doping concentration of the anode or cathode to achieve a lower V FM .

Impact analysis for IGBT reverse blocking capability
To meet the requirement of high current density and high forward blocking capability in the MMC application, asymmetric IGBT structures are widely used, such as the Field-Stop structure from Infineon and Soft Punch Through/Soft Punch Through Plus structure from ABB [8][9] .
Figure 9 shows the schematic cross-section of SPT/FS -IGBT.The FS, P-collector, and drift regions determine the reverse voltage capability.The reverse voltage V(x) can be derived by solving Poisson's equation [7] : ( ) where N D and N A are the ionized donor and acceptor concentrations, respectively.ε S is the dielectric constant for the semiconductor, q is the electron charge, and x n \x p is the space-charge region in the ndoped region and the p-doped region.
Two kinds of failure mechanisms can occur in J 1 , one is a punch-through failure, and the other one is avalanche breakdown.The punch-through failure can be easily resolved by increasing the collector doping concentration.However, the avalanche voltage improvement should consider the forward blocking capability and the turn-off characteristic.
By reducing the doping concentration of the FS region's electric field slope, the area under the electric field can be increased, which means that the reverse blocking capability will be improved.As shown in Figure 10, the reverse blocking voltage has been increased from 60 V to more than 300 V.However, the positive blocking characteristics deteriorate, as shown in Figure 11.Replacing the diffused buffer (D-Buffer) with a multi-proton implantation buffer (M-Buffer) [10] greatly enhances the reverse blocking capability, as shown in Figure 12, and has less impact on the forward blocking capability, as shown in Figure 11.The doping profile for the buffer is shown in Figure 13.

The result of the optimized IGBT module
Based on the analysis above, two optimized IGBT module was prepared to assess the optimization.A module has an optimized diode with an anti-paralleled normal IGBT.B module has a higher V FM diode with anti-paralleled optimized IGBT. Figure 14 shows that the A module has a lower V FM , while the B module has a higher V FM , and both pass the test.(The curve of Normal-Module has the maximal V FM of normal design before failure.) Figure 14.Effect on V CE by changing the fs doping concentration

Conclusion
In this paper, the electrical stress of the upper and lower dies in the IGBT module was analyzed by simulation.
The root cause of the failure in the IGBT module when MMC Blocked mode happened was that the voltage, which consists of diode V FM and the voltage on parasitic inductance, exceeding the reverse blocking capability of the anti-parallel IGBT.
The methods against failure are studied from three aspects: reducing the peak value of the diode's forward voltage, improving the reverse blocking ability of IGBT, and optimizing the parasitic parameters in the module circuit.
The conclusions are as follows: 1.The forward peak voltage of the diode can be reduced by reducing the thickness of the drift region or optimizing the doping design in both the anode region and cathode region.
2. The forward peak voltage will increase if the doping concentration in the anode or cathode region is too high or too low.Due to that, a higher doping concentration will raise the barrier and increase resistance in the doping region, and a lower doping concentration will increase resistance in the drift region.
3. Replacing diffused buffer (D-Buffer) with multi-proton implantation buffer (M-Buffer) can obtain higher reverse blocking capability (higher than 400 V, which is enough to meet the application requirement) and has less impact on forward blocking capability.
Implementing the optimization can decrease the probability of IGBT reverse blocking failure and provide support for the trade-off design.

Figure 1 .
Figure 1. Circuit for double pulse test

Figure 2 .
Figure 2. Failure picture of IGBT die in the upper IGBT module

Figure 3 .
Figure 3. Current and voltage curves of upper IGBT and diode

Figure 4 .
Figure 4. Effect of drift region thickness on V FM by changing the anode or cathode depth (larger depth from dep0 to dep1)

Figure 7 .Figure 8 .
Figure 7. Electric density close to the anode region (higher doping concentration from dop1 to dop3)

Figure 9 .
Figure 9. Cross section of IGBT longitudinal structure

Figure 10 .
Figure 10.Effect on V EC by changing the D-Bufferuffer doping concentration