Operating Conditions for RF Power Capacitors
Ratings
The electrical performance is determined by the voltage, current and reactive power ratings. For the ideal case of a pure sinusoidal voltage waveform these parameters are related by the following equations:
Where Wr = reactive power (kVA)
Ur = a.c. voltage (kVpk)
I = current (amps r.m.s)
f = frequency (MHz)
C = capacitance (pF)
Vr is the peak a.c. + d.c. voltage (Ur + dc) kVpk for which the capacitor is designed.
Rated current Ir is the r.m.s. current for which the capacitor is designed.
Rated reactive power Wr is such that the capacitor temperature rise shall not exceed 45ÁC when operated, in still air, at an ambient temperature of 30ÁC (or as otherwise specified in the datasheet).
Frequency
When a capacitor is subjected to an RF load, heat is generated in the dielectric due to internal losses and in the electrode system due to resistance heating.
The performance is characterised by a typical frequency load curve as shown below:
At frequencies below f1 (Zone 1) the capacitor can be operated at it's maximum ac rating Ur.
At frequencies above f2 (Zone 3) the capacitor can be operated at it's maximum current rating Ir.
At frequencies between f1 and f2 (Zone 2) the capacitor can be operated at it's maximum power rating Wr.
(Units are as those given above)
Operating Conditions
In certain circumstances such as intermittent operation the reactive power and current ratings can be exceeded but in no case should the maximum body temperature of the capacitor be exceeded normally 95ÁC or as otherwise stated in the data sheet.
For guidance short term power ratings and current ratings of 1.5 x are permissible. The voltage ratings must not be exceeded.
If the capacitors are operated in an ambient temperature higher than 30ÁC then the reactive power rating must be reduced as shown in the graph.
The user should ensure that the permissible operating conditions for the capacitor are not exceeded.
Care should be taken in the mechanical mounting to ensure that mechanical / thermal stresses are minimised. Vertical mounting recommended.
The use of forced cooling methods allows higher reactive powers to be applied provided that the maximum temperature of the capacitor is not exceeded. Methods of cooling commonly used are:
- Forced air cooling
- Water cooled connections
- Oil immersion
