This note discusses the relationship
of current handling, power dissipation,
applied voltage and the maximum rated
voltage of a multi-layer ceramic capacitor.
Either the maximum voltage rating or the
maximum power dissipation of the part
limits the maximum current through a capacitor.
The capacitance value and the operating
frequency determine which of the two becomes
the governing limit. For lower values
of capacitance at a given frequency, or
lower frequencies at a given capacitance,
the voltage limitation is generally reached
before the power dissipation limitation.
The voltage limitation depends on the
capacitors reactive impedance (Xc), where
as the power dissipation depends on the
resistive impedance or equivalent series
resistance (ESR) of the capacitor.
Xc is calculated by Xc=1/[2FC], where
F= frequency in hertz and C = Capacitance
in farads.
The maximum allowable peak current flow
through a capacitor (without exceeding
the capacitors rated voltage) is calculated
by I=Er/Xc, where Er is the capacitors
rated voltage, and I = the peak current
flow in amperes.
The actual current flow through a capacitor
is calculated by I=Ea/Xc, where Ea is
the applied or working voltage.
The following examples illustrate how
these variables effect the voltage/current
limitation for some specific capacitors
at a given frequency.
EXAMPLE 1: A 0.1 pF, 500 Volt capacitor
is to be used at 1000 MHz:
Xc=1/[2(3.14)(1000x106)(0.1x10-12)]=1591
ohms;
Ipeak=500/1591=0.315 Apeak or 0.22 Arms.
If you exceed this current, you will exceed
the voltage rating of the capacitor.
EXAMPLE 2: A 1.0 pF, 500 Volt capacitor
is to be used at 1000 MHz:
Xc=1/[2(3.14)(1000x106)(1.0x10-12)]=159
ohms;
Ipeak=500/159= 3.15 Apeak or 2.2 Arms.
If you exceed this current, you will exceed
the voltage rating of the capacitor.
EXAMPLE 3: A 10 pF, 500 Volt capacitor
is to be used at 1000 MHz:
Xc=1/[2(3.14)(1000x106)(10x10-12)]=15.9
ohms;
Ipeak=500/15.9=31.5 Apeak or 22.2 Arms.
This current level is actually never reached
because at this frequency, values over
approximately 1.5 pF are limited by the
power dissipation of the capacitor.
IMPORTANT NOTE: The power dissipation
limited data assumes an infinite heat
sink at the capacitor terminations. The
heat transferred to the air is ignored.
The 10pF, 500V capacitor at 1000 MHz would
actually be power limited to about 7Apeak
which is about 5 Arms. At this current
level the temperature of the capacitor
would rise to about 125 degrees C. For
reliable operation, it would be practical
to limit the current to 2 Arms with a
decent heat sink, or even 3Arms if there
is a very good heat sink at the terminations.
Last Updated: April 1, 1999
