Development and role of reliable bypass capacitors (II)

The blue line on the way represents the impedance of the power supply, which can be a battery or power converter. The impedance is lower at low frequencies, but at higher frequencies, the connected inductance of the wires, wiring, and layers will eventually dominate. The green line represents the impedance of the active device. If we correspond this impedance icon to the board package interface, the green line represents the active device along with the package and the impedance of the PDN capacitor on the package. We call those active devices "silicon silicon," but they can be any type of active circuit: the active device on that old computer board is a germanium transistor.

In summary, the blue and green lines produce a triangular impedance configuration and are flat at the low and high frequency tails. If no bypass capacitor is added to this system, any noise current can cause a lot of noise each time it hits this triangular peak frequency. The thick black line extending from left to right on the frequency axis is used to indicate the frequency range we are interested in and is related to the PDN impedance. The frequency range we are interested in is not necessarily part of a continuous spectrum, especially for composite circuits that have different functions connected to the same supply voltage.

If the frequency range to be studied is not between the frequencies of the impedance peaks formed by the blue and green curves, the circuit will operate normally without the need to increase the bypass capacitor. If the operation of the circuit excites the impedance peak, then we need to increase the bypass capacitor to reduce the impedance and thus reduce the noise. The red dotted line on the icon represents the impedance of a bypass capacitor, which is used to supplement the impedance of the power supply and active devices, and also to make the overall impedance of the wideband lower.

So why do you not need bypass capacitors on older computer boards? Mainly because the switching of the germanium transistor is slow and the clock frequency is very low, that is, the short switching current does not excite the high impedance portion of the PDN impedance.

Today's high-speed electronics, complex circuit functions and high clock frequencies, most of the time we have to care about a wide bandwidth PDN impedance, therefore, we can not allow a very high impedance between the source and load impedance Peak. Therefore, we need a bypass capacitor. If we are fast-forward for decades, we can easily predict that the board will have no bypass capacitors.

Because, when distributed power supplies are small enough that we can place them very close to the load, the interconnect inductance will be lower, and the chip and package capacitance in the active device will be larger, so we will It is back to the era when the circuit board does not need a bypass capacitor.