Capacitors
Tutorial
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:
(1) Forced air cooling
(2) Water cooled connections
(3) Oil immersion
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