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1.1
Partial Discharge In Power Electronics
Ian Cotton, Ningyan Wang
The University of Manchester
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1.2
Background• Improved design of the dielectric system in a
power electronic module leads to the ability to increase voltage / increase power density through compaction
• As part of the IeMRC Flagship Project in Power Electronics, work has examined ways to improve the dielectric performance of a typical IGBT module
• Through this work, a 60% increase in the partial discharge inception voltage of a commercial module has been achieved
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1.3
The Dielectric System
• Silicone gel encapsulant (between busbars and in other locations)• Substrate• Edge of substrate metallisation
Baseplate (heat sink)
Copper
AlN (Substrate)
CopperSolder
Transistors or Diode Chips
SiliconeGel
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1.4
Electrostatic Field Analysis• Exact nature of electric fields
within a module varies according to switch state
• Any transient voltage overshoot must also be considered
• Highest electric field in module is usually found at the edge of the metallisation
• This high electric field limits operating voltage of module as it can lead to partial discharge
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1.5
Partial Discharge• A partial discharge is a localised electrical discharge
that does not completely bridge the gap between two conductors– A small spark produced by a locally elevated electric field– Magnitudes from a few pico-coulomb to a few nano-coulomb– Will gradually damage insulation over time and can result in a
full failure
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1.6
PD At Edge Of Metallisation• From initiation of PD to
failure is typically between a few seconds and a few minutes
• When testing volume of samples, a low variation in inception voltages is observed
• A solution was required to eliminate this form of PD
DischargeLocations
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1.7
Ferroelectric Materials• A ferroelectric filler has been used to create a silicone
gel that exhibits a permittivity that increases as a function of the local electric field
• An elevated local permittivity produces a lower local electric field
• The filler used costs around £10/kg (less than the cost of the gel), is easy to mix into the gel and also provides enhanced thermal conductivity
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1.8
Barium Titanate As A Ferroelectric• Barium titanate is an example of a
ferroelectric
• Under an alternating electric field, ionic displacement shifts the relative position of the titanium cation within the oxygen octahedral cage
• In electrical terms, this ionic displacement manifests itself as an increase in permittivity as a function of electric field
• Change in permittivity does not occur under a DC field but neither does significant partial discharge
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1.9
Voltage–Current Plots Of Unfilled & Filled Gels
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1.10
Current Density As A Function Of Field
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00 18.00 20.00Peak electrical field strength (kV/mm)
Pea
k cu
rren
t den
sity
(A/m
2)
E3.14.6 (E)r +=ε
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1.11
Impact Of Filled Gel On Peak Module Field
0
10
20
30
40
50
60
70
80
0 4 8 12 16 20
Voltage(kV, RMS)
Peak
Ele
ctric
Fie
ld(k
V/m
m)
Gel Only Filled Gel Filled Gel With Nonlinear Behaviour
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1.12
Impact Of Filled Gel On Electric Field
• Simple addition of a ferroelectric filler reduces electric field by virtue of enhanced permittivity
• Ferroelectric effect further reduces peak electric field
Applied Voltage (RMS)
Gel Only(kV/mm)
Filled Gel(kV/mm)
Filled Gel (Non-Linear)
(kV/mm)6kV 25.4 22.5 (89%) 18.0 (71%)
12kV 50.8 45.0 (89%) 33.8 (67%)
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1.13
Trials Of Filled Gels• Samples made using commercial substrates that were left
unpopulated
• HV connected to section of substrate metallisation that is joined to collector while rest of substrate metallisation earthed
• Samples housed in metal container that is then filled with silicone gel (both standard and ferroelectric filled)
• Partial discharge test carried to confirm the expected increase in performance
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1.14
Unfilled And Filled Substrate-Gel Samples
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1.15
Probability Of Partial Discharge
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1.16
Trials Of Actual Modules• A commercial 3.3kV module was taken from the assembly line
and filled with standard or ferroelectric doped silicone gel• The results showed a significant increase in performance• No difficulties in manufacturing were found
Unfilled Gel / kV Filled Gel / kV %age increase
Maximum 8.0 13.4 67.5Mean 6.3 10.1 60.3
Minimum 4.8 7.8 62.5
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1.17
Conclusions And Future Work• Partial discharge in power electronic modules is a significant
concern and a barrier to voltage uprating / power density improvements
• The use of a specific ferroelectric filler within silicone gel can result in a significant improvement in performance with no significant impact on manufacturing
• Work is currently underway to further understand the reason why humidity has a major impact
• Testing is also taking place using square-wave voltage sources to confirm the benefit of the ferroelectric effect under these conditions