Reliability and Design Considerations for Long Life Using Mica Capacitors in High-Voltage Apps
- Wednesday, 07 February 2007
Over the last 40 years, a series of misconceptions regarding mica capacitor applications has led novice users to consistently over- derate wound or rolled mica/ epoxy dielectric capacitors.
Mica, K2A13(Si04)3, a complex aluminum silicate in dielectric form, has been successfully used for many years as an integral part of high- voltage (2KVDC to 50KVDC) capacitor manufacturing — particularly in the 50pF to 5μF value range. Mica has unrivaled physical and electrical properties in comparison to other capacitor dielectrics, especially ceramic. Mica is extremely stable. Capacitance will change only -2% at -54°C and to +3% at +125°C. Mica is an excellent insulator, and is resistant to high temperature, thermal shock, mechanical shock, and vibration.
Reliability is directly related to electrical field stresses through the mica/epoxy dielectric structure. An assessment and derating, in volts per mil (0.001 inch or 0.025mm) can be discerned from the industry standard, 1250 to 1330 volts per mil, operating, for greater than 100,000 hours mean time before failure (MTBF) applications (See figure).
An example of this misconception is as follows: Given an application for a capacitor of 50nF at 10KVDC working/operating voltage, a capacitor can be designed within the approximate dimensions of 3.5" ´ 3.5" ´ 0.25" (volume=3.0625 cu.in.) with a dielectric thickness of 8.0 mils. While operating at 10KVDC, this capacitor, which is at an electrical field stress of 1250 volts per mil (10KVDC/8 mils = 1250 volts per mil), can be designed and expected to work for >100,000 hours without failure, assuming that correct epoxy impregnation processes have been utilized. A circuit designer usually has either in-house or customer-driven mandates to derate his product, sometimes by a factor of 60%.
Consider then the recalculation required to re-assess the volume necessary to fill this derated application: 10KVDC/0.6 = 16.67KVDC rating in order to meet this mandate. A design to produce this revised capacitor will increase the size of the device to 5.0" ´ 5.0" ´ 0.36", with a volume of 9.0 cu. in., and will require an increase in the thickness of the dielectric to 13.5 mils. Again, while operating at 10KVDC, this capacitor will be functioning at an electrical field stress of only 741 volts per mil (10KVDC/13.5 = 741 volts per mil stress). Not obvious to the uninitiated, the voltage squared function (CV2/2 stated in joules) has produced a capacitor that has a volume/energy multiplier of 2.939 times that of the previous non-derated capacitor. This unnecessary over-derating causes useful volume to be wasted without any benefit of electrical performance.
A mica dielectric capacitor should be specified by actual working voltage; that is, the actual voltage at which the circuit will function during its operational life, not at a derated voltage. The mica capacitor also can be specified in not-to-exceed n volts per mil terms. Both methods of specification will allow for optimum design volume. Reynolds recommends a range of 1250 to 1330 volts per mil for greater than 100,000 hours of operation.
Mica surpasses both ceramic and film-type capacitors when temperature, capacitance stability, and deep current discharge stability are a concern. Ceramic capacitors can operate at high temperature, but exhibit undesirable characteristics that may allow less capacitance than the circuit requires. High current discharges are also an enemy of ceramic capacitors. Many film-type capacitors start to lose their effectiveness in applications above +85°C.
Wound mica capacitors have seen operation in military and commercial applications for over 40 years. They are utilized in power supply filtering, energy storage with high current discharge, oil field well logging, voltage multiplier, university and national laboratory, and baggage x-ray systems.