Charge storage capacitance

Updated March 2, 2007

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New for March 2007! 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 and average current to keep the cap charged up. Charge storage is often integrated within transmit/receive modules, or located nearby. Charge storage capacitors are nearly always tantalum electrolytics, because high-density capacitance is needed. These caps have a polarity that you need to observe!

Let's start with the most basic high-school 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 (micro-Farad 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 capactors, be sure to get caps that have voltage rating of double what the system runs at if you want to sleep well!