Development and role of reliable bypass capacitors (I)

Bypass capacitor design is a relatively common design, but how to design the bypass capacitor, how many bypass capacitors are designed, the development of the times is always recurrent. Just like the "Romance of the Three Kingdoms", the world's general trend, the long-term must be combined, the long-term must be divided.

This is an old diode and transistor computer board that performs a simple DTL gate function (diode crystal logic). It was produced in the 1960s or early 1970s. In our power distribution design course, I used this computer board to ask a question: Can you count the number of bypass capacitors on this board? The answer is amazing is zero. Why do we have to place a lot of bypass capacitors on the board today to reduce the noise of our electronics?

Now whether it's a new laptop or a large computer board, we can find hundreds or even thousands of capacitors on it. In order to understand the reason for this significant change in just a few decades, we must study the role of the bypass capacitor and how the bypass capacitor works.

Whether analog or digital, all electronic circuits will eventually generate (and/or) program electrical signals. During the operation of the power supply, the supply current from the power supply is constantly changing, and the required supply current needs to be determined according to the actual operation. Sometimes the circuit requires less current, sometimes more, and the changes in the different requirements also constantly change the current in the power rail. These changing currents flow through the wires and the wiring and layers between the power supply and the electronics on the printed circuit board, producing voltage changes:

Small voltage fluctuations are accepted by the electronics, but each circuit has a maximum and minimum limit to ensure proper operation. We must try to limit the change in the supply voltage, otherwise it will change due to the required current. We must place the accumulator between the series inductance on the circuit and the power path. Capacitance is a good accumulator because, unlike inductors, the voltage across the capacitor (if we ignore its parasitic components) will not change drastically due to sudden current changes.

To determine what we are and how many capacitors, it is more convenient to look at the problem from the frequency domain. Distribution networks (PDNs) can be simulated in many different forms, from simple lumped equivalent circuits to detailed mesh models. For what we are discussing now, I think a simple lumped equivalent circuit is enough.