There are numerous factors to consider when adding external capacitors to switched-mode power supplies (SMPS). This article will discuss noise, startup, ESR, stability, pre-bias applications, Sense inputs, On/Off (remote enable) controls and other topics.
Should I add external capacitors to switched-mode power supplies (SMPS)?
There are numerous factors to consider when adding external capacitors to switched-mode power supplies (SMPS). This article will discuss noise, startup, ESR, stability, pre-bias applications, Sense inputs, On/Off (remote enable) controls and other topics.
Where are the capacitors located on a power supply?
When we look at almost any power supply application circuit there will be capacitors on the output of the power supply located at the load. One question often asked of power supply vendors is “Why are the output capacitors required on a power supply and how are the capacitors selected?”.
Why are capacitors placed across power supply terminals?
Based upon our discussion it should now be understood that capacitors are often placed across the power supply terminals at the load to reduce the voltage excursions caused by load current transients and the finite bandwidth response of the power supply.
Do external capacitors have a reactive output load?
The specifications also assume that there is no other reactive output load, especially inductance. Be aware that external capacitors include finite amounts of internal, unwanted ESL (Equivalent Series Inductance).
How does external capacitance affect the performance of a DC/DC converter?
Increasing amounts of external capacitance reduce the phase margin of the internal control loop and therefore endangers dynamic performance. In the following application, we see three bypass capacitors, all at different distances from the DC/DC converter.
How to choose a capacitor?
One of the first criteria for selecting the capacitors should probably be how much capacitance is required. When the capacitance required is greater than ones or tens of microfarads, either tantalum or electrolytic capacitors may be the preferred capacitor technology. Capacitors made with these technologies are reasonably compact and affordable.