Embedded Queries - 3 (Reservoir Capacitor)

What is a reservoir capacitor? (or) What is the need for large capacitance at the output of the  power supply regulators?

Generally, in the fast switching or high frequency designs, we see that the frequency might rise suddenly based on the application need or another example is an additional load is switched on during a product operation. When we talk about frequency rise or load, it is basically the current that need to be sourced to them. The entire responsibility of sourcing the current is on power supply. So, in these cases, when we have these transitions on the power supply loads, the general observation must be that there must be a dip in the voltage but we don't see it. It is the high capacitance at the output of regulator, that helps to store energy during the normal operation and help tackle the sudden load changes without voltage drop. 

Just adding few more pointers on this topic,

1. A high capacitance value at the output of regulator helps to

• Eliminate low frequency noise
• Act as reservoir

2. A low capacitance value at the output of regulator helps to

• Eliminate high frequency noise
• These are mostly ceramic which has low ESR and helps to reduce the power loss, output stability, noise

The term reservoir capacitor is used commonly in the smoothing circuit used in AC-DC conversion. But the same terminology applies (not used generally) to high capacitance used at the output of the regulators. Reservoir capacitors are generally electrolytic capacitors.