Wednesday, February 6, 2008

Comparison: Ceramic and Film High Voltage Capacitors

Both ceramic and film capacitors are widely used in high voltage applications. Ceramic high voltage capacitors have the following advantages, as compared to film:
  • Lower equivalent series inductance (ESL) and equivalent series resistance (ESR)
  • Smaller size for the same value (capacitance & voltage)
  • Better temperature coefficient available, as well as a choice of temperature coefficient characteristics
Disadvantages of ceramic capacitors include:
  • Higher voltage coefficient, resulting in substantially lower capacitance as the applied voltage is increased. (This characteristic is often overlooked, and its effects are often more severe than you would think.)
  • Lower maximum available capacitance rating
  • Lower maximum available voltage rating
  • Susceptibility to piezoelectric effects
  • Typically smaller lead-to-lead distance


Monday, August 13, 2007

Technique improves voltage breakdown characteristics

Vishay has introduced a surface-mount multi layer ceramic chip capacitor that has average voltage breakdown twice that of standard commercial-grade products. They call the approach HVArc Guard® . It's a patent pending technology that essentially increases the effective creepage distance along the part's surface. Voltage breakdowns are twice that of some competitor
products. Sounds pretty interesting!

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Wednesday, July 25, 2007

Precision High Voltage Resistors

When selecting and subsequently evaluating precision HV resistors, it is important to consider how rugged the parts are, with respect to surges and rapid voltage transitions. Both conditions can occur in typical applications.

Precision HV resistors are often used in high voltage divider applications. Thus, you are relying on the absolute resistance to remain constant over long term conditions.

If there is significance capacitance across the divider, then it is possible to have a large surge current if there is a momentary fault within the capacitor. THis is not unusaul with high voltage film capacitors, as they are self-repairing (if the internal fault is minor). High surge current can cause localized heating, resulting in permanent resistance change (as well as a temporary change).

If there is a rapid dV/dt voltage change, such as when an external arc occurs, it is possible for the resistor to suffer a permanent change in resistance value. In addition, depending on parasitic capacitances, voltage during a transient will most likely not distribute evenly over the total divider resistance. Thus, the localized dV/dt may be very high, and the absolute voltage across some portions of the resistor(s) can be higher than its ratings.

Thus, it can be worthwhile to run surge and dV/dt tests on precision resistors that are being considered for your applications.

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Thursday, July 5, 2007

High Voltage Capacitors

What makes a capacitor identifies as "high voltage" is a rating above 2kV. That's because there are many manufacturers that build capacitors rated up to 1600V or 2000V. (This is in accordance with my definition of high voltage, "High voltage starts at the point where designers have to consider additional technical issues, and where there are significantly fewer component suppliers to choose from. " Typical dielectrics used for high voltage capacitors include ceramic, polyester and mica. Issues that become more important at higher voltages that affect performance and reliability include capacitor construction (and how it affects the electric field), voltage coefficient, creepage distance along the surface, surge current capability (during an arc) and compatibility with potting compounds. See this list of high voltage capacitor manufacturers.

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