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1.1

Partial Discharge In Power Electronics

Ian Cotton, Ningyan Wang

The University of Manchester

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

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

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

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

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

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

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

1.9

Voltage–Current Plots Of Unfilled & Filled Gels

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

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

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%)

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

1.14

Unfilled And Filled Substrate-Gel Samples

1.15

Probability Of Partial Discharge

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

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