Electrical Power Engineering Department Hijjawi Faculty of Engineering Technology Yarmouk University Irbid, Jordan

Effect of High Frequency Pulses on the Breakdown Voltage and Lifetime of MW Insulation of Flyback Transformer Eyad A. Feilat, Ph.D. US-Jordan Workshop Modern Power Electronics Research and Education December 16-17, 2002, PSUT, Amman, Jordan

Outline   Design Trends in Electrical and Electronics Equipment   Insulation System of Flyback Transformer   Consequences of Miniaturized Design   Statistical Analysis of Failures   Scope of the Research Paper   Accelerated Aging Test System   Experimental Results   Conclusion

Design Trends in Electrical and Electronics Equipment   Reduce Size (Compact Design)   Light Weight   High Reliability (Low Failure Rates)   Reduce Manufacturing Time   Reduce Cost

Design Trends in Electrical and Electronics Equipment   Bobbin-Wound Coils   Fine Gauge Magnet Wires   Thin Layers of Insulation   Encapsulation of HV Coils   Materials with High Thermal Class   High Frequency Switching Technology   DC-DC Converters (Flyback Transformers)   DC-AC Inverters (Adjustable Speed Drives)

DC-DC Converter Flyback Transformer (FBT)  Fine Gauge Magnet Wires (MW)   TV sets and computer monitors.

Pulse Frequencies of FBT

Insulation System of Encapsulated Coil Polyester Housing Layer (Polyethylene Terephthelate) Impregnation Layer (Epoxy) Heavy Build Enamel (Polyurethane) Polyester Bobbin (Polyethylene Terephthelate)  Randomly Wound on Bobbins  Bonded with Baked Coatings  Encapsulated with Epoxy

Magnet Wire (MW)   Insulation Material: polyurethane (PUR)   Over Coat: Polyamide (Nylon) NEMA MW-80C, Class F AWG 41 MW Insulation Thickness = 6.35  m Bare Wire Diameter = 71.1  m

Consequences of Miniaturized Design Random Wound Coils   Beginning and End of the Coil may touch one another   High Level of Voltage Stress between Turns High Frequency Switching   Very Short Pulse Period   Very Short Duty Cycle   High dV/dt   Uneven Voltage Distribution   Steady Degradation of the MW Enamel   High Temperature Rise, typically 100 o -200 o C

Causes of Insulation Failure   Electrical and Thermal Stresses   Partial Discharge Developed in Random Windings   Localized Dielectric Heating   Microvoids and Impurities in the Epoxy Fill Material   Insulation Degradation   Premature Failure

Statistical Analysis of Failures Accelerated Life Tests (Accelerated Aging)   High Electrical Stresses   Elevated Temperatures   Combined Electrical and Thermal Stresses   Various Voltage Waveform and Frequencies

Statistical Analysis of Failures Probability Distribution (Weibull) Life Model (Single Stress, Multistress)

Scope of the Study l l Effect of Rise Time on the Time-to-Failure l l Effect of Duty Cycle on the Time-to-Failure l l Evaluation of the Breakdown Voltage l l Accelerated Life Tests Ô Ô High Temperature (100 o -180 o C ) Ô Ô Pulsating Frequency (15-40 kHz) Ô Ô Positive Polarity

Accelerated Aging System DTS-1500 A

Typical Pulse Waveform T V  D

Experimental Results   Lifetime Studies   Effect of Duty Cycle   Effect of Rise Time

Effect of Duty Cycle V = 950 V f = 15 kHz T = 100 o C  = 200 ns Duty Cycle % Time-to-Breakdown (s)

Effect of Rise Time V = 950 V f = 15 kHz T = 100 o C D = 16%

Experimental Results   Breakdown Voltage Studies   Effect of Temperature   Effect of Frequency

Effect of Temperature on the Breakdown Voltage D = 16%  =200 ns

Effect of Frequency on the Breakdown Voltage D = 16%  =200 ns

Experimental Results   Lifetime Studies   Effect of Pulsating Voltage   Effect of Temperature   Effect of Frequency

Lifetime Characteristics V-t C/C f=15kHz 100 o C 180 o C 155 o C

Lifetime Characteristics V-t C/C E E+4 1.0E+5 1.0E+6 1.0E Voltage (V) Time (s) 15 kHz 25 kHz 40 kHz T=155 o C

Lifetime Characteristics T-t C/C f=15kHz 700 V 800 V 900 V

Lifetime Characteristics T-t C/C V=800 V 15 kHz 25 kHz 40 kHz

Parameters of the Electrical-Thermal Aging Model

Conclusion è è The longer the duty, the shorter is the insulation Lifetime è è The longer the rise time, the longer is the insulation lifetime è è The Breakdown Voltage declines with the increase of both the Frequency and Temperature è è The Accelerated Life Tests show that both the Voltage and Temperature are the two main Factors of Insulation Aging or Degradation

Conclusion Effect of the pulse frequency on the lifetime is indistinct It changes with temperature and voltage stress   Reason:   Change of polarization   Space charge   Dielectric losses   Change of Breakdown Mechanisms