Decoupling Capacitors

Boulder, CO (September 2024) - This experiment explored the impact of current draw on a Power Distribution Network (PDN) including switching noise and rail collapse. Results indicate that placing the decoupling capacitor close to the source relative to the component reduces switching noise. Additionally, using a larger capacitor does not improve the switching noise beyond a certain point. However, the larger capacitor does reduce the rail collapse when the component is switched on.

The setup (Figure 1) included creating a 9-volt power rail with a DC power supply to power the Arduino and the MOSFET source. Figure 2 shows the voltage drop across the 10Ω resistor used to determine the loop current of 275mA. The MOSFET gate was set up to be able to manually switch between the slow edge from the operational amplifier which has a rise time of approximately 3 microseconds (Figure 3) and the fast edge from the Arduino pin which has a rise time of approximately 50 nanoseconds (Figure 4). Comparing the respective PDN switching noise without a decoupling capacitor shows that the fast edge generates significantly more switching noise than the slow edge. 

Based on Equation 1, which provides the instantaneous voltage drop across an inductor, this behavior is expected; an increase in the rise time will result in a lower voltage drop. This indicates that decoupling capacitors are more important for signals with faster edges. With the 4V drop from Figure 5, the 50ns rise time from Figure 3, and the measure 275mA calculated from the results of Figure 2, the loop inductance is 727nH.

The voltage drop on the PDN for the fast edge is shown in Figure 3 where the voltage drops to nearly half of its initial value. Adding a decoupling capacitor close to the MOSFET reduces this voltage drop significantly. This result is the same whether using a 1μF or 1,000μF capacitor because either is large enough to provide enough charge to overcome the instantaneous drop in voltage. Equation 2 can be used to determine the minimum capacitor size, C, needed to limit the instantaneous voltage drop, dV, for a signal with a rise time of dt and a current of i.

Moving the capacitor further from the MOSFET results in increased noise when compared to placing it close, however, it still provides an improvement over not having a decoupling capacitor. This results from the increased inductance from the breadboard rail between the MOSFET and the capacitor. An equivalent circuit shows this relationship in Figure XXXX.

Finally, the rail collapse for the different capacitors was analyzed as shown in Figures 7 and 8 for both the slow and fast edges. Although the increase in capacitance does not impact switching noise in this case, it does impact rail collapse. The larger capacitor can maintain the voltage better, but both capacitors keep the voltage drop within a few millivolts of the initial value. 

The key takeaways from this experiment are that signals with fast rise times are more impacted by switching noise and placing decoupling capacitors close to the relevant component greatly reduces the switching noise.  Equation 1 can be used to estimate the loop inductance and Equation 2 can be used to estimate a minimum capacitor size required to meet a voltage drop requirement.