Lifetime of HCPP Film Capacitor
for Marx Modulator
Tao Tang, Mark Kemp, Craig Burkhart
Power Conversion Department
Outline
• Background of ILC Marx Modulator
• High Crystalline Poly-Propylene (HCPP) self
healing film capacitor
• Capacitor life time model (DC)
• Voltage related lifetime change
• Thermal related lifetime change
• AC capacitor lifetime scaling
• Image of failed capacitor films
• Future work
Marx Modulator
•
ILC Marx Modulator
–
–
–
–
–
–
•
Pulse Voltage:
Voltage Regulation:
Pulse Current:
Pulse Length [flat-top]:
Repetition Rate:
Total # of RF Stations:
120 kV
±0.5%
140 A
1.6 ms
5 Hz
576
Current statues
– Installed in ESB: summer 2009
– Operating with klystron: Sept. 2009
– Life time testing now : modulator ran 43 days
last year (April 2010- April 2011)
•
Capacitance degradation
– RF power start to droop after a few months
running
– Degradation depends on depth of discharge
– Residual voltage on capacitors (as high as
1kV)
– 105hr theoretical life time103 hr
HCPP Film Capacitor
• High Crystalline PolyPropylene (HCPP) film
capacitor
– High energy density
– Used in industry
applications
Capacitor
electrode
• DC: storage capacitor, filter
capacitor
• Deep discharge: defibrillator
• Marx application
– Compact system  High
energy density
– In new operation region
• Discharge 20%-40%
• Like DC operation(?)
Film(4.8µm)
metallization
2-4Å
Solder point
Capacitor Life Time Model (DC)
• Lifetime scaling relationship model widely
used in literature for film capacitors
– Suggested by manufacture
– Definite end-of-life >5% capacitance
change
r
hs
• Design and testing for P2 capacitors
Scaling Relationship
experimental data
2000
Accelerated Lifetime (hr)
–
• Other fail mechanism after >5% change
 
Formula:
LsVsexp coef .
La  exp 2
Va
• DC voltage & Temperature
2500
– Optimize capacitor design
– Testing at elevated voltage stress
(standard method used in industry) 20%
droop
– Conclusion: 264V/um for >10^5 hr life
time
– P1 capacitor(40% droop): 194V/um
1500
1000
500
0
390
400
410
420
430
440
Accelerated Film Field Stress(V/µm)
450
460
Voltage Related Life Time Change
• Voltage related shorten lifetime
– Residual voltage induced uneven voltage
distribution
• Improvement
– Replace end-of-life capacitors
– Add balancing elements: balancing resistors
• Results: capacitance decrease at same rate
Thermal Related Life Time Change
• Average power
– Very small
– P1 Capacitor thermal design is very conservative
– Change repetition rate did not help
• Instantaneous heating
– Manufacture defect on edge connection of large
capacitors is sensitive to instantaneous power
– Testing of small capacitors  same degradation
rate
Capacitor Life Time Model(AC)
• AC capacitor fail model
– Anodic oxidation of Al
metallization(Corona related)
– AC voltage (ie. Depth of
discharge) related
– DC field can not initiate or
sustain the corrosion process
• Relation of life time and depth
of discharge
– ESB operation data (I^2 t also
changed accordingly)
– Controlled environment (same
I^2 t) at B015
Percent Capacitance Change per Million
Shoots (%/mil. shot)
1
0.1
0.01
0.001
15
20
25
30
35
Droop (delta V/ Vmax)
40
45
Image of the Film
• Scanned image of films
– Only on one polarity of the
film (cathode or anode)
– Almost perfect round
• Optical microscope image
– Can not see pin hole
• SEM
– Film charging
– Material analysis: can not
find Al in metalized area
Future Work
• Solution for current issue
– Decrease depth of discharge to 20%
– Shorter pulse width (short term)
– Double capacitance (long term)
• Understanding the failure mechanism
– Find the relation of AC voltage to life time (increase
with AC voltage or sharp change after certain
threshold)
– Find the threshold field strength for this failing
mechanism