Analysis and solution of sparking fault in IGBT connection copper bar of compact driver

With the high-speed development of modern industrial automation, the demand for miniaturization of the high-current driver is higher and higher, which makes the space inside the driver more and more compact. According to the traditional installation form, in the production process, there will be screw deviation or loose during the use of the problem. Then, this can lead to final product quality problems such as fire. To solve the mechanical connection problems, based on the Datsan of the ignition problem, this paper puts forward some solutions, such as operation, Torque Detection, and Mark re-checking. A method is proposed to improve the reliability and save the cost of the drive product is recommended.


Driver introduction
The servo driver is a control mechanism that enables industrial robots to perform specified actions, consisting of a control module, an operation panel module, a main power module, a power drive module, a detection module, and a braking module.As a necessary component of industrial robots, it can perform high-precision closed-loop control of speed, velocity, vector, etc., as shown in Figure 1.The driver is mainly composed of a main control board, a drive board, an absorbing capacitor, an IGBT, a contactor, a soft starting resistor, a current sensor, a copper bar, a laminated busbar, a support capacitor, and a fan.The structure is shown in Figure 2.After a long time of development and production tests, it is found that in a compact place, the installation effect will directly affect the overall performance and safety of the driver.Taking the single fire problem as shown in the enlarged part of Figure 2, the fault tree is used to analyze the fault, determine the fault point and solve the problem.The problem phenomenon is that inside the driver, the U-phase IGBT output terminal is centered on the gradual radiant hot melt to the outside.There are signs of high-temperature shrinkage in the nearby control line bushings, such as the fan, the heat shrink bushings at the copper bar connection are melted, and the bottom shell of the current sensor above the copper bar is melted.The two absorption capacitors in the upper and lower tubes of the U-phase IGBT are melted.The melting phenomenon is more obvious the closer the U-phase IGBT output terminal is, as shown in Figure 3.

Fault tree analysis
From the principle analysis, there are two reasons for the ignition at the ignition point: abnormal operation of the control circuit and abnormal operation of the power circuit.The fault tree analysis diagram is shown in Figure 4.

Bottom event X1: abnormal operation of control software
If the control software is abnormal, this phenomenon will occur in batches.It can be determined that the X1 event is not the bottom event that caused the fault.

Bottom event X2: abnormal operation of the main control board
After the malfunction, the main control board was inspected, and its surface condition was intact.Subsequently, a special test was conducted on the main control board, and the test results were normal.
From the above, it can be determined that the X2 event is not the bottom event that caused the fault.

Bottom event X3: abnormal operation of the driver board
After the malfunction, the drive board was inspected, and the surface condition of the drive board was intact.Subsequently, a special test was conducted on the driver board, and it was found to be functioning normally.Based on the inspection and testing results, it can be determined that the X3 event is not the bottom event that caused the fault.

Bottom event X4: abnormal interconnection cable
There is one interconnection cable.After inspection, there are no burning marks on the surface of the interconnection cable.After testing, the performance of the interconnection cable is normal.It can be determined that the X4 event is not the bottom event that caused the fault.

Bottom event X5: abnormal soft start resistance
There are two soft start resistors.Upon inspection, there are no burning marks on the surface of the soft start resistor.After testing, the soft start resistance value is normal, and the performance is normal.It can be determined that the X5 event is not the bottom event that caused the fault.

Bottom event X6: abnormal dc contactor
There is one DC contactor.After inspection, there are no burning marks on the surface of the DC contactor.After testing, the performance of the DC contactor is normal.It can be determined that the X6 event is not the bottom event that caused the fault.

Bottom event X7: abnormal insulation layer of stacked busbar
There are some burning marks on the edge of the U-phase insulation layer of the busbar, and there are burning marks in the middle part.There are no obvious melting marks on the body, and the insulation characteristics of the stacked busbar are normal after measurement.It can be determined that the X7 event is not the bottom event that caused the fault.

Bottom event X8: abnormal current sensor
Appearance and performance checks are conducted on the U-phase current sensor.After inspection, there are burning marks on the adjacent surface between the current sensor and the U-phase IGBT output terminal, and the upper surface of the sensor is normal.After testing, the performance of the current sensor is normal.It can be determined that the X8 event is not the bottom event that caused the fault.

Bottom event X9: IGBT abnormal operation
There are four IGBTs inside the driver, among which the U-phase IGBT has obvious erosion marks, and the other IGBTs have intact surface conditions.After the internal diode voltage drop test of IGBT and the forward and reverse impedance test between CE, all four IGBT parameters were normal.When the fault occurs, the driver still outputs current normally, and there is no fault.This indicates that the X9 event is not the underlying event that caused the fault.

Bottom event X10: abnormal operation of absorption capacitor
There are a total of 6 absorption capacitors, among which the two absorption capacitors near the U-phase near the output end of the U-phase IGBT are melted with glue.The capacitor body has varying degrees of burning phenomenon.
After external open flame burning and 5-minute high-temperature baking, it was found that the open flame causes the capacitor to burn without expansion while baking causes deformation and expansion of the absorbing capacitor.The external open flame burning is similar to the absorption capacitance state of this fire issue.
From the analysis of troubleshooting tests, it can be seen that the state of the capacitor when it is burned by an external open flame is consistent with the ignition state of the driver.
At the same time, after measurement-by-measurement units, the analysis report on the effectiveness of the components concluded that the two absorption capacitors were burned due to external high temperatures.
It can be determined that the X10 event is not the bottom event that caused the fault.

Bottom event X11: fan abnormal
There are a total of 6 fans for this type of drive, including one near the U-phase IGBT fan.There are burning marks on the packaging edge, and the functional performance of the fan is normal after measurement.If this is the ignition point, it will gradually radiate hot melt outward with the fan as the center, which is not consistent with the description of the phenomenon.It can be determined that the X11 event is not the bottom event that caused the fault.

Bottom event X12: abnormal copper bar
There are obvious burning marks on the U-phase output copper bar.The high-temperature melting absorption capacitance and colloid have formed a relatively complete coating on the U-phase IGBT output terminal.However, after the removal of the copper bar, it was found that the screw installation position on the upper surface of the copper bar is clean and smooth without burning marks.There are large areas of burning marks on the lower surface, indicating that there is a gap between the copper bar and the IGBT connection surface.Thus, combustion substances can enter between the copper bar and the IGBT.There is uneven deformation on the inner edge of the installation hole, which can be inferred that the copper bar and IGBT output terminal were not tightly connected at this position before the gel melted.Flames or electric sparks burned the copper bar and installation hole, causing deformation of the copper bar and installation hole.The surface condition of other copper bars is intact.
The driver copper bar is designed based on current requirements, and after preliminary verification, the current carrying capacity meets the requirements.After measurement, except for the deformation of the U-phase output copper bar, the measured dimensions of other copper bars are normal.In the experiment of reducing the contact area between copper bars and IGBT, considering safety, a current of 100 A was operated, and the temperature was measured with a thermometer.The temperature of the copper bars showed no abnormal heating.ME-2023 Journal of Physics: Conference Series 2741 (2024) 012037 IOP Publishing doi:10.1088/1742-6596/2741/1/0120375

Bottom Event X13: cable exception
The driver cable is designed according to the current requirements, and the selection design is normal.After preliminary verification, the current carrying capacity meets the requirements.It can be determined that the X13 event is not the bottom event that caused the fault.

Bottom event X14: false soldering
There is a welding point between the absorption capacitor and the stacked busbar near the ignition point.If the welding point catches fire from this point, it should gradually radiate hot melt outward from this point as the center, which is not consistent with the phenomenon.After inspection, there is no false soldering problem between the absorption capacitor and the stacked busbar welding point.It can be determined that the X14 event is not the bottom event that caused the fault.

Bottom event X15: fixing screws not tightened
The internal part of the driver gradually radiates hot melt outward from the U-phase IGBT output terminal, where the U-phase IGBT output terminal and the U-phase output copper bar are installed with fixed screws.The space is compact, and there is a possibility that production personnel may not tighten the fixed screws.It cannot be ruled out that the X15 event is the underlying event that caused the fault to occur.

Fault location
After fault tree decomposition and comprehensive analysis of 15 bottom event faults, it is determined that bottom events X1 to X14 are not the bottom events that caused the fault.It is determined that the cause of this fault is bottom event X15: the fixing screw is not tightened.
Due to the lack of tightening of the fixing screws between the U-phase IGBT and the U-phase copper busbar, an electrical gap is generated between the U-phase copper busbar and the U-phase IGBT, which generates high voltage and subsequently leads to ignition at the gap.This leads to the melting of the heat shrink sleeve, U-phase current sensor, and laminated busbar at the U-phase copper busbar, as well as the ignition of the absorption capacitor, fan, and wire.

Test verification
1) Test 1 is a high-temperature combustion test of the absorption capacitor: a) Using a warm gun, the absorption capacitor is baked at 600℃ for 5 minutes, with the capacitor expansion deformation and no combustion; b) An open flame is used to grill the absorption capacitor.After about half a minute, the capacitor burns and fires, showing a short-circuit state, confirming that the absorption capacitor is passive ignition. [7]) Test 2: In order to consider the safety of personnel, 48 V electricity is used in the laboratory to verify the simulated air breakdown fault when the positive and negative electrodes are close to the test.It is proved that when the positive and negative electrodes are almost in contact, the air breakdown igniting phenomenon occurs.Therefore, under the condition of 600 V voltage, if the gap between the positive and negative electrodes was small enough, an igniting phenomenon would inevitably occur, as shown in Figure 5. [8][9]

Mechanism analysis
Through troubleshooting, positioning, and mechanism analysis, the problem was that the worker did not tighten the IGBT screw when fixing the U-phase output.Under the condition of generating electrical gaps, the voltage value between the U-phase output copper bar and the IGBT terminal is calculated as follows: = + in the formula, U is the DC bus voltage, L is the parasitic inductance of the U-phase output circuit, and U is the voltage value between the U-phase output copper bar and the IGBT terminal. [10]he opening frequency of IGBT is 7 kHz.At this time, the U-phase output copper bar and IGBT terminal are in an open and closed state, with the voltage switching between 0 V and high voltage.According to the types of gas dielectric breakdown discharge, this working condition belongs to a typical DC voltage breakdown phenomenon.

Measures and plans
In response to this abnormal ignition phenomenon, the development of measures needs to clarify the following three issues: 1) The IGBT manufacturer recommends tightening torque.The manufacturer recommends a tightening torque of 2.5-6 N.m.Customers can choose their own tightening torque based on usage conditions.
2)Statistical analysis of IGBT tightening torque data in the early stage.It is checked whether the fixed state of IGBTs produced is in the same standard as the previous stage.4 drivers, each with 4 IGBTs, are tested.Each IGBT has 3 screws.A total of 48 screws are used for the test.The test data are shown in Table 1 And the original fixed screw of IGBT was in the form of a nail, elastic pad, and flat pad in one.After testing, the tightening torque of the elastic pad was about 3 N.m.The actual implementation of 5.5-6.25 N.m can ensure sufficient deformation of the elastic pad and ensure the anti-loosening effect.
3)According to the "Product Specification" of this type of driver, the corresponding vibration stress test plan is developed.A prototype is used (one with four IGBTs, each IGBT3 screws), a total of 12 IGBT screws (4 with 5 N.m force fastening, 6 with 6 N.m force fastening, 2 with 7 N.m force fastening) to do vibration test to verify the tightness of screws during tightening torque.According to the requirements of the vibration stress test, the test verifies that the screws are not loose and the function of the driver is normal.
Conclusion: The tightening force of more than 5 N.m can ensure the normal operation of the driver in the vibration environment without loosening.To improve standards and reliability, IGBT fastening screws can be fixed with a 6 N.m fastening force.

Conclusion
Method: 1) The fastening torque requirements for IGBT fixing screws are increased.It needs to make sure that the fastening torque of IGBT connecting screws is 6 N.m; 2) All fixed screws are marked; 3) The inspection of the mark is added in the inspection after the stress screening test; 4) A unified 6 N.m tightening torque is implemented.
By using this method, the ignition failure of the IGBT connection copper bar of the compact driver can be effectively solved with the lowest cost under the premise of ensuring the inherent function performance of the driver.

Figure 1 .
Figure 1.Schematic diagram of the working principle of the servo driver.

Figure 2 .
Figure 2. Servo driver structure and fire point enlargement diagram.

Figure 3 .
Figure 3. Fire point status diagram in the drive.

Figure 5 .
Figure 5. Test diagram of simulated electrical breakdown at 48 V.