Switching power supply requires capacitor energy storage

What is a switching power supply?

Switching power supplies are used in almost every end-equipment that needs a long battery life, low heat genera-tion, or to meet ENERGY STAR® guidelines. When designing a switching power supply, it is difficult to decide which output capacitor type to use.

What type of capacitor should a switching power supply use?

When designing a switching power supply, it is difficult to decide which output capacitor type to use. Electrolytic capacitors have high equivalent series resis-tance (ESR), making power loss high and transient response too poor for use with tough load-response requirements.

What is hold up time in a switching power supply?

In switching power supplies, energy is stored in the bulk (input electrolytic) capacitor providing a useable hold up time to protect against transient power outages. Hold-up time is a function of the energy storage capability of the power supply and the specific loading of the power supply.

What type of capacitor should be used for energy storage?

Ideally, the output capacitor would be very large for energy storage and have very low impedance at the loop crossover and switching frequencies. Polymer and tantalum capacitors come in large values with low ESR, but they are expensive and the ESR is still not as low as a ceramic capacitor.

When do you need an external circuit for a power supply?

When a power supply requires the capability of continued operation for a short period of time following a momentary input power interruption, an external circuit providing additional capacitance can be easily designed.

Why do we use 4 capacitors?

Using four capacitors also provides margin for the capacitor breakdown voltage when used with high value of Vin. In addition, for this circuit a resistor (R_limit) is used to limit in-rush current during hold-up capacitor charging. A typical value for R_limit is around 50 ohms.