Charge
storage capacitance
Updated February
7, 2009
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Charge storage is used in pulsed
systems where the power supplies are "power keyed", and
the actual power supply is sufficiently far away from the transmit
amplifier that the amplifier essentially runs off the capacitor
during the pulse and the power supply merely supplies an average
current to keep the capacitor bank charged up. Charge storage is
often integrated within transmit/receive
modules, or located nearby. Charge storage capacitors are nearly
always tantalum electrolytics,
because highdensity capacitance is needed. These caps have a polarity
that you need to observe!
Let's start with the most basic
highschool physical science class equation for charge in a capacitor:
Now we take the derivative of
each side with respect to time (ouch, calculus!)
Now we rearrange the equation
to solve for the "C_{CS}" which is the required
charge storage:
Let's restate the equation in
units that make more sense for microwave modules (microFarad and
microseconds) and replace the derivative format with deltas which
make us feel more comfortable:
OK, let's try out the equation.
What charge storage do you need for a 10 microsecond pulse to power
a 10W power amp running at 8 volts and 5 amps peak? The next question
you have to ask is, what is an acceptable voltage droop? Time for
a Microwaves101 rule of thumb!
An acceptable voltage droop for a power amplifier during pulsed
operation is 5%, which will drop the power by a similar amount (5%,
or about a quarter of a dB). So for a PHEMT amp operating at 8 volts,
you allow a voltage droop of 0.4 volts.
Back to solving for the required
charge storage. The answer is that you'd need 125 micro Farads.
When you are picking out the capacitors, be sure to get caps that
have voltage rating of double what the system runs at if you want
to sleep well! Also, notice that the voltage rating decreases when
you consider operation at high temperature.
