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Partial Discharge in Pressurized IGBT Model

Introduction

An IGBT power module consists of the substrate, power semiconductors (IGBT and/or diodes), drivers and inter-connections. Ceramic substrates are often used for insulating the semiconductors from the base plate. There is high electric field stress at edges of the semiconductors and the trench between metallization at the top of the ceramic substrate. These stresses result from unipolar switching up to voltage level of 6.5 kV for high voltage IGBTs. At high voltage, local electric field can be high for partial discharge initiation. Embedding the IGBT module in a suitable incompressible liquid will protect the component from mechanical stress and also minimize electrical discharges. Simulation of electric field distribution shows that the highest stress region on the trench is at the triple point region where the metal, substrate and liquid meet. Potential of the conductor under the substrate also influence the electric field at the triple region. As of now, no application specific test methods exist for liquid insulation of power modules while the only test method (IEC 61287) for the insulation performance of power electronic converter is based on ac voltage. The actual voltage conditions in IGBT modules composed of dc and impulse voltage with short rise time. But there is a problem detecting PD from impulse voltage of high dU/dt because of it produce large capacitive current which is detected as PD by the data acquisition system. This also brings about the need to figure out a suitable technique to study PD in dielectric liquids in pressure tolerant power electronic under impulse voltage. The other challenge is that ceramic substrates such as Aluminium Nitride (AlN) that are often used as baseplates in high voltage power electronic modules was claimed to be electroluminescent. It produces light at voltage below PDIV. This makes it unsuitable for optical partial discharge studies as the light produced will be registered as PD by the detector before the inception of PD. PCB which is made from epoxy fibre glass on the other hand is non-electroluminescent and cost effective. That makes it a good material that can be used to model the IGBT baseplate for the study. Since the capacitive current constitutes noise in the measurement of electrical PD, measurement of light emission from the discharge is thought as an alternative technique to explore.

The printed circuit board (PCB) referred to in this report is a glass fiber reinforced epoxy resin (FR4) produced by NCAB group. The production starts with glass cloth wet with liquefied resin. The treated cloth is then semi-cured. The semi-cured material is laid up between two sheets of treated metal foil for lamination process. This is subjected to controlled heat under vacuum to drive the resin to fully cross-linked or cured. The thickness of the laminate is 880 µm. Processing of the laminate under vacuum removed entrapped air, moisture and volatile materials in the laminate. Metallization of the PCB board was achieved by printing copper layer of thickness 400 µm onto the surface of the substrate. The copper was etched to create the specified trench gap of 2.5 mm.

The produced PCB-test object and bespoke pressure test cell for the investigation of pressure dependence of partial discharge is shown in Figs. 1 and 2.

Fig. 1: PCB-board Object Fig. 2: Pressure Test Cell

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Experimental Setup

Experimental test setup has been developed to study the nature of partial discharge in a simulated high voltage IGBT substrate under pressure. A study was first performed on point-plane geometry in the pressure test to qualify the performance of the test cell since PD pattern from point-plane is well documented. The radius of the point was 2 µm and the point-plane gas was 2.5 mm. This was followed by study along the trench between metallization of a PCB-board object embedded in mineral oil in a pressure test cell and different voltage sources was applied. Measurement was done under ac voltage and slow rise bipolar square wave voltage with rise time of about 400 μs from high voltage amplifier, and fast rise unipolar square wave with rise time of about 100 ns from impulse generator. The effect of pressure on partial discharge activity is also evaluated.

Fig. 3: Test Setup with High Speed Camera Fig. 4: Test Setup with Fast transient voltage and pressure

Results and Discussion

Correlation between the partial discharged recorded by electrical and optical measurement system was evaluated with the point-plane geometry. The detected electrical and optical PDs were plotted in the fig. 5. The number and magnitude of electrical and optical PDs displayed good correlation. This support the hypothesis that measuring optical PD in the study of insulation performance under a voltage source where electrical signal is accompanied with unavoidable noise is reliable

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Fig. 5: Electrical and optical PD correlation

Modelling of electric field distribution on the test object was performed using FEM. To obtain the parameters for the geometry of the model, two of the PCB objects were moulded in epoxy and each cut into 2 specimens. Each specimen has 6 edges. The 2D microscopic image is shown in fig. 6. The specimens were viewed in a microscope with up to 1000x magnification and the geometries were measured. The board thickness was measured to be 880 µm, the copper thickness is 400 µm, the copper surface radius is 379 um, the lower tip radius is 2.67 um and the trench gap (distance between the triple points) is 2.186 mm.

Fig. 6: Microscopic image of sliced test object Fig. 7: Electric Field distribution at triple region

7 kV semi-square voltage was applied and maximum electric field of 599 kV/mm was obtained at the triple region when filled with air. Introduction of mineral oil reduced the maximum field at 7 kV to 181 kV/mm. The trench was modelled without the grounded electrode under the board and the field reduced to 87 kV/mm. This is an indication that the ground potential below the board influences the inception of PD.

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Sinusoidal voltage was applied on the test object and the PD pattern is compared with PD pattern obtained from point-plane geometry as shown in figs 8 and 9. PD patterns moving towards 0-crossing for every half-cycle shows the existence of space charges.

Fig. 8: Point-plane PD pattern Fig. 9: Trench PD pattern

A square voltage was then applied on the test object. While the electrical discharge was embedded in noise from capacitive current, weak light produced during the local breakdown was detected by the photomultiplier and simultaneously recorded as Optical PD. Figs. 10 and 11 shows the typical PD pattern for slow rise square voltage with rise time of 400 µs and fast rise square voltage with rise time 100 ns.

Fig. 10: PD pattern of Slow rise square voltage Fig. 11: PD pattern of fast rise square voltage

For slow rise bipolar square voltage, PD inception occurred at 24 kV (peak-peak) while under the fast rise unipolar square wave voltage, the partial discharge inception occurred at 7 kV and 9 kV for negative and positive fast rise square wave voltages respectively. Increase in voltage led to an increase in the number and magnitude of the discharges. Fig 12 shows a graph displaying variation of PD magnitude with respect to voltage under different voltage sources. Fast rise negative unipolar square voltage has lower discharge inception voltage (PDIV) and higher PD magnitude. Although PD magnitude does not seems to have a linear relationship with voltage, but the rate of increase in the PD magnitude with respect to voltage is higher for fast rise square voltages. At 12 kV fast rise negative square wave voltage, large PD with magnitude of 140 pC was observed. This may have result in surface creepage and further increase in voltage may result in the development of streamers that could cross the trench to cause breakdown.

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Fig. 12: Comparison of PD from different voltage source

A: Positive Fast Rise Unipolar Square Voltage B: Negative Fast Rise Unipolar Square Voltage

Fig. 13: PD rate with Voltage and Pressure

A: Positive Fast Rise Unipolar Square Voltage B: Negative Fast Rise Unipolar Square Voltage

Fig. 14: PD Magnitude with Voltage and Pressure

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Fig. 15: Effect of Pressure on PDIV Fig. 16: Effect of Pressure on PD magnitude

Pressure led to a decrease in PD activity. Unipolar fast transient voltage has lower inception voltage and produced PDs of large magnitude. But as observed from the results, the number and magnitude of PDs decrease systematically as the pressure of the liquid in the test cell increased as shown in Figs 13 and 14. The PD inception was influenced by pressure. The PD inception voltage increased with increase in the pressure of the liquid as shown in Fig. 15. The PD magnitude was greatly influenced by pressure. At 12 kV negative fast transient voltage, large PD with magnitude of 140 pC was observed at pressure of 1 bar as shown in Fig. 16. Increasing the pressure to 30 bar reduced the magnitude to 11 pC.

Fig. 17: Simplified PD Model

PD pattern under square voltage was observed to be slew rate dependent. The time range at which PD is contained is a function of rise time. This can be explained using a simple PD model of semi square voltage. Supposing the inception voltage Vi occur at time ti and the voltage reached maximum (Vm) at time tm, then PD will be contained within time range defined as 𝑟 = 𝑡𝑟 − 𝑡𝑖. If 𝑡𝑖

𝑡𝑟= 𝑉𝑖

𝑉𝑚, then 𝑟 =

𝑡𝑟 �1 − 𝑉𝑖𝑉𝑚�. This explains the narrow time range for PD occurrence as the rise time decreases. The PD

magnitude is slew rate dependent as seen from the higher magnitude of PDs under unipolar fast rise square voltage when compared with PD under slow rise square voltage. A local breakdown that produced the partial discharge (PD) occurred when the field over a discharge channel exceeds the withstand ability of the channel. This is accompanied by a transient current which transfers a charge

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from along the discharge channel to make the voltage across the channel becomes zero (or close to zero). The small voltage collapses and charge transfer phenomenon will immediately generate a small transient “voltage dips” (ΔV) on the terminals. Current is supplied into the test object each time discharge occurs in order to compensate the voltage drop at the apparatus. Thus, there exist a small transient current (Δi) that circulates in the capacitive circuit. The voltage step on the terminals (ΔV) gives a current in the external circuit that integrated up gives a charge Qapparent = ∫Δidt.

The slew rate, 𝑑𝑉𝑑𝑡

= 𝑉𝑚−𝑉𝑖𝑡𝑟−𝑡𝑖

= 𝛥𝑉𝛥𝑡

, where Δt is the time lag is equal for all slew rates. Δt can be assumed to be the time within which PD will develop. From the relation, ΔV will become higher as slew rate increases. The increase in the driving voltage behind PD leads to increase in the magnitude of the discharge. Higher slew rate as in the case of fast rise square voltage implied a higher ΔV and the corresponding Δi. This may have contributed to the increased magnitude of the discharges as seen in Fig. 17.

The pressure dependence of the magnitude and number of PDs under different voltages source as shown in Figs 13, 14, 15 and 16 indicates the involvement of gas phase process during the developmental stage of the partial discharge process. This will limit the discharge process and streamers moving across the trench and this result in the suppression of the magnitude and rate of partial discharges.

Effect of Surface Cover on Field Distribution

The surface of electronic devices is often covered with parylene or polyimide to protect the surface. IGBT trench was modelled by covering the surface with polymer and examining the field distribution in the presence of impurities. Parylene C was simulated as the surface cover with relative permittivity of 3.15. Surface cover thickness of 3 µm and 30 µm was simulated. An ellipsoidal (semi axes 0.5 µm and 0.2 µm) and spherical (0.5 µm) shape impurities was simulated and Semi-square voltage was simulated and 7 kV was applied. Fig. 18 shows the mesh of the simulated model with surface cover.

Fig. 18: Model with surface cover of 3 µm thickness (Surface cover in blue)

• The maximum field at the triple region was 181 kV/mm without impurities. The introduction of impurities increased the field to 246 kV/mm (Fig. 19A). The maximum field exists on the surface of the impurities. When the field exceeds the critical breakdown value of the impurity, the high electric field strength can result in a self-sustained discharge. Free electrons are released during this process and could lead to accelerated ionization of the nearby liquid molecules.

• Surface cover of thickness 3 µm was then enabled and the impurities place outside the surface

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cover (Fig. 19B). The maximum field at the triple region reduced to about 143 kV/mm while the impurities have a maximum field of 98 kV/mm.

• This was followed with the simulation of surface cover of thickness 30 µm (Fig. 19C). The maximum field at the triple region slightly reduced further to 140.78 kV/mm and the high field region was completely shielded from impurities. The impurities on the surface of the polymer cover has maximum field of 25 kV/mm.

Fig. 19: Field distribution (A: in the presence of impurities without surface cover, B: 3 µm surface cover with impurities on the surface of the cover, C: 30 µm surface cover with impurities on the surface of the cover)

Summary

• PD activity on the board under ac was seen to concentrated near zero of the half cycle demonstrating presence of space charges

• Absence of space charges under unipolar square wave voltages exposed the sharp edges at the triple region to quick PD inception

• Using ac based test method to qualify power electronic insulation will give a wrong signal and therefore not appropriate for testing high voltage electronic modules that operate on PWM

• Optical PD measurement method is effective in PD study for high dV/dt and can be used to establish a measurement technique to evaluate the performance of insulation power electronic module for subsea application

• The PD magnitude is highly dependent on rise time.

• PDIV (pk-pk) under fast rise square wave is 1/3rd of that seen with sinusoidal voltage

• PD magnitude after inception is higher.

• The discharge settles along the board interface and creepage may create carbonized tracks on the insulation

• Early occurrence of PD under fast rise pulse can result in serious insulation degradation

• Surface cover with parylene decrease the maximum field at the triple region. Surface cover up to 30 µm thickness shields the contaminants from the high field region. This should lead to increase in PD inception voltage since impurities that could trigger PD is prevented from high field region

A B C