Tutorial: Power Supply Capacitors and Inductors

This tutorial installment is: Power Supply Capacitors and Inductors. This topic answers the following questions:

• What is the purpose of capacitors and inductors in power electronics?
• What are the energy storage and terminal time properties of capacitors and inductors?
• What are the power supply applications of capacitors and inductors?

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Power Supply Capacitors and Inductors – Introduction

In order to function properly, power supplies need components to store and deliver energy. There are two types of energy storage devices used in power supplies: capacitors and inductors.

Unlike resistors, ideal inductors and capacitors only store energy, but never dissipate energy. Therefore over one complete steady state switching cycle, the average power of the device is zero. However, all capacitors and inductors are non-ideal, and there may be dissipation effects to consider.

Inductors and capacitors either act as energy reservoirs, or temporarily store buckets of energy for transfer to the output. When acting as a reservoir, the change in inductor current or capacitor voltage change is considered small for analysis purposes. When transferring buckets of energy, the inductor current or capacitor voltage change is large and may be of resonant character.

Power Supply Capacitors

A capacitor stores energy in the electric field between two parallel conducting plates. The energy stored is proportional to the square of the voltage across it:

An important property of capacitors is that they resist the change of voltage across the plates and therefore act as reservoirs of available charge. The current through an ideal capacitor can change instantaneously, but the voltage never can, always maintaining voltage continuity. The current through a capacitor is equal to:

Non-ideal power supply capacitors have equivalent series resistance and leakage current. Common types for power supply capacitors are aluminum electrolytic, tantalum, multilayer ceramic, film. Aluminum and tantalum types are polarity sensitive. They also have an RMS current rating which often determines the amount of capacitance in an application since there must be enough capacitors to handle the RMS current. Another important fact for capacitors is the lifetime dependence upon temperature. For each ten degrees C of lesser ambient temperature, the estimated lifetime doubles. A little known fact about multilayer ceramic capacitors is that the capacitance exhibits strong dependence upon temperature.

Power Inductors

A power inductor is the electrical dual of the power supply capacitor, and stores energy in the magnetic flux established by the geometry of the inductor winding, core material and the current through the winding. The energy stored in the inductor is proportional to the square of the current through it:

The voltage across an ideal inductor can change instantaneously, but the current never can, always maintaining current continuity. The voltage across an inductor is equal to:

A non-ideal inductor has copper and core loss due to the winding resistance and core resistance. These non-idealities need to be considered in any design. Common types for power electronics are toroidal shapes employing various magnetic powder formulations and various shapes of different ferrite core materials. These materials are chosen according to the performance and cost requirements. Have a current rating, beyond which the permeability reduces greatly causing the core to saturate.

Linear Supply Applications of Capacitors and Inductors

Power supply capacitors enable the smoothing of rectifier outputs through energy storage. A smoothing capacitor bank is often referred to as the bulk capacitance. The energy stored in the bulk capacitance becomes the input to the regulator pass element. Linear power supplies also employ a capacitor at the output of the regulator. The purpose of the output capacitor is to provide control loop stability and holdup energy storage in the event of a momentary loss of input power. Linear power supplies must filter out line frequency noise, and the capacitors and inductors are therefore large. Linear supplies typically do not use power inductors in most applications.

Switching Supply Applications of Capacitors and Inductors

Power supply capacitors are also used by switching power supplies as the bulk capacitor and at the output for control stability and holdup. Capacitors at these locations, when also coupled with inductors, can also be configured as low pass LC filters for ripple voltage reduction on the output, and ripple current reduction on the input, and for averaging the switching output voltage. In switching power supplies, the energy stored in capacitors and inductors is managed by a high frequency power switch. Switching power supplies operate at high frequency and the capacitors and inductors are therefore small. Capacitors in resonant supplies can also large voltage swings as they resonantly transfer energy packets from the input to the output.

Next Topic

The next tutorial installment is: Power Semiconductor Switches, Ideal Switches. This next topic will answer the following questions:

• What is the basic function of a power semiconductor switch in a switching supply?
• What are the most basic properties of a power semiconductor switch?

If you need assistance with power electronics design, call or email us today for help with your requirements. You can also go to our power electronics consultant website for more information about our services for business clients. Thank you for reading this tutorial article entitled “Power Supply Capacitors and Inductors”

Next topic: Power Semiconductor Switches, Ideal Switches
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