Voltage Regulation, Part 3 of 3
This last part presents some discussion aout voltage ripple.
The Output Ripple
The regulator is able to provide the required voltage as it is adjusted using the variable resistor, but sometimes the voltage is not as clean as it is needed. How much this affects the functionality of the circuit depends on the application. In some cases the unclean voltage has no effect, but in others it can wreak havoc and create a scenario that is difficult to troubleshoot.
The undesired component of the voltage which makes it “unclean” is called ripple. This could present itself as an unstable output voltage which fluctuates or as an AC component that is contained within the DC voltage. The AC voltage is usually in the millivolt range, but can be enough to cause odd behavior, again, depending on the application.
Where does ripple come from? We have all been introduced to the basic electronic components, such as the capacitor, resistor and inductor, just to name these three. Each component has a specific purpose within the circuit. The circuit’s designer takes great care in placing these components in such a way so that the circuit performs the desired function. However, sometimes the components affect the circuit in unexpected ways, because they show a certain behavior that in fact belongs to other types of components. For example, an inductor may have capacitance and a capacitor may have some resistance. These unexpected characteristics can produce ripple.
Another possible source of the ripple is the switching action of the regulator. As mentioned before, the regulation function is performed by a circuit that switches ON and OFF. By varying the ratio of ON vs OFF the regulator is able to adjust the output voltage. The high speed switching can also produce ripple on the output voltage.
There are some simple steps that can be taken to reduce the ripple. I should mention that what is mentioned in this section is just touching the surface on what ripple is and the varying techniques to reduce it. This discussion can get complicated fast, so I chose to keep it simple and just provide some basic information.
A common technique to reduce the ripple is to place a capacitor across the output. How much of the ripple is reduced depends on the regulator, on the value of the capacitor and the capacitor type. Electrolytic capacitors are commonly used, but each application may have different needs. The electrolytic capacitor’s positive lead is connected to the positive voltage output and the negative lead is connected to ground.
The first image shows the original ripple voltage found at the regulator’s output. This shows 15.7 mV AC. The second image shows 12.9 mVAC having a 1uF electrolytic capacitor connected across the output. The next figure shows 7.5 mVAC. N this case a 220uF capacitor was used. Finally, the last image shows 4.0mVAC, having a 2200 uF capacitor connected to the output.
Another technique to reduce output ripple is component placing. The norm is to place any filtering capacitors, such as those mentioned above, as close as possible to the regulator’s outputs. In some cases the designer goes as far as soldering the capacitor directly onto the regulator’s own output capacitor. All the regulators that have been mentioned in this article have output capacitors built in.
There are different kinds of regulators to help define a required voltage. This article briefly described four switching regulators, also known as buck converters. Knowing the requirements of the circuit to be powered helps in choosing the best regulator for the job.
All switching regulators have some ripple on their output. The norm is to always place a polarized capacitor at the regulator’s output in addition to a small (usually .1uF) monolithic capacitor near each integrated circuit’s power leads. Minimizing the ripple generated by the regulator may help avoid odd and unexpected behavior within the circuit.
In closing, thanks again to Wright Hobbies Robotics for providing guidance and most of the components used to elaborate this article.