CHAPTER 10: PASSIVE COMPONENTS - Analog Devices

[Pages:30]PASSIVE COMPONENTS

CHAPTER 10: PASSIVE COMPONENTS

INTRODUCTION

SECTION 10.1: CAPACITORS

BASICS DIELECTRIC TYPES TOLERANCE, TEMPERATURE, AND OTHER EFFECTS PARASITICS DIELLECTRIC ABSORPTION CAPACITOR PARASITICS AND DISSIPATION FACTOR ASSEMBLE CRITICAL COMPONENTS LAST

SECTION 10.2: RESISTORS AND POTENTIOMETERS

BASICS RESISTOR PARASITICS THERMOELECTRIC EFFECTS VOLTAGE SENSITIVITY, FAILURE MECHANISMS, AND AGING RESISTOR EXCESS NOISE POTENTIOMETERS

SECTION 10.3: INDUCTORS

BASICS FERRITES REFERENCES

10.1 10.3 10.3 10.3 10.9 10.10 10.11 10.13 10.14 10.15 10.15 10.17 10.18 10.20 10.20 10.22 10.23 10.23 10.25 10.27

BASIC LINEAR DESIGN

PASSIVE COMPONENTS INTRODUCTION

CHAPTER 10: PASSIVE COMPONENTS

Introduction

When designing precision analog circuits, it is critical that users avoid the pitfall of poor passive component choice. In fact, the wrong passive component can derail even the best op amp or data converter application. This section includes discussion of some basic traps of choosing passive components. So, you've spent good money for a precision op amp or data converter, only to find that, when plugged into your board, the device doesn't meet spec. Perhaps the circuit suffers from drift, poor frequency response, and oscillations--or simply doesn't achieve expected accuracy. Well, before you blame the device, you should closely examine your passive components-- including capacitors, resistors, potentiometers, and yes, even the printed circuit boards. In these areas, subtle effects of tolerance, temperature, parasitics, aging, and user assembly procedures can unwittingly sink your circuit. And all too often these effects go unspecified (or underspecified) by passive component manufacturers. In general, if you use data converters having 12 bits or more of resolution, or high precision op amps, pay very close attention to passive components. Consider the case of a 12-bit DAC, where ? LSB corresponds to 0.012% of full scale, or only 122 ppm. A host of passive component phenomena can accumulate errors far exceeding this! But, buying the most expensive passive components won't necessarily solve your problems either. Often, a correct 25-cent capacitor yields a better-performing, more cost-effective design than a premium-grade (expensive) part. With a few basics, understanding and analyzing passive components may prove rewarding, albeit not easy.

10.1

BASIC LINEAR DESIGN

Notes:

10.2

PASSIVE COMPONENTS CAPACITORS

SECTION 10.1: CAPACITORS

Basics

A capacitor is a passive electronic component that stores energy in the form of an electrostatic field. In its simplest form, a capacitor consists of two conducting plates separated by an insulating material called the dielectric. The capacitance is directly proportional to the surface areas of the plates, and is inversely proportional to the separation between the plates. Capacitance also depends on the dielectric constant of the substance separating the plates.

Capacitive reactance is defined as: XC = 1/C = 1/2fC

Eq. 10-1

where XC is the capacitive reactance, is the angular frequency, f is the frequency in Hertz, and C is the capacitance.

Capacitive reactance is the negative imaginary component of impedance.

The complex impedance of an inductor is then given by: Z =1/ jC = 1/j2fC

Eq. 10-2

where j is the imaginary number.

j =-1

Eq. 10-3

Dielectric types

There are many different types of capacitors, and an understanding of their individual characteristics is absolutely mandatory to the design of practical circuits. A thumbnail sketch of capacitor characteristics is shown in the chart of Figure 10.1. Background and tutorial information on capacitors can be found in Reference 2 and many vendor catalogs.

With any dielectric, a major potential filter loss element is ESR (equivalent series resistance), the net parasitic resistance of the capacitor. ESR provides an ultimate limit to filter performance, and requires more than casual consideration, because it can vary both with frequency and temperature in some types. Another capacitor loss element is ESL (equivalent series inductance). ESL determines the frequency where the net impedance of the capacitor switches from a capacitive to inductive characteristic. This varies from as low as 10 kHz in some electrolytics to as high as 100 MHz or more in chip ceramic types. Both ESR and ESL are minimized when a leadless package is used, and all capacitor types discussed here are available in surface mount packages, which are preferable for high speed uses.

10.3

BASIC LINEAR DESIGN

The electrolytic family provides an excellent, cost effective low-frequency filter component, because of the wide range of values, a high capacitance-to-volume ratio, and a broad range of working voltages. It includes general-purpose aluminum electrolytic types, available in working voltages from below 10 V up to about 500 V, and in size from 1 F to several thousand F (with proportional case sizes). All electrolytic capacitors are polarized, and thus cannot withstand more than a volt or so of reverse bias without damage. They have relatively high leakage currents (this can be tens of A, but is strongly dependent upon specific family design, electrical size and voltage rating versus applied voltage). However, this is not likely to be a major factor for basic filtering applications.

Also included in the electrolytic family are tantalum types, which are generally limited to voltages of 100 V or less, with capacitance of 500 F or less. In a given size, tantalums exhibit higher capacitance-to-volume ratios than do the general purpose electrolytics, and have both a higher frequency range and lower ESR. They are generally more expensive than standard electrolytics, and must be carefully applied with respect to surge and ripple currents.

A subset of aluminum electrolytic capacitors is the switching type, which is designed and specified for handling high pulse currents at frequencies up to several hundred kHz with low losses. This type of capacitor competes directly with the tantalum type in high frequency filtering applications, and has the advantage of a much broader range of available values.

More recently, high performance aluminum electrolytic capacitors using an organic semiconductor electrolyte have appeared. These OS-CON families of capacitors feature appreciably lower ESR and higher frequency range than do the other electrolytic types, with an additional feature of low low-temperature ESR degradation.

Film capacitors are available in very broad ranges of values and an array of dielectrics, including polyester, polycarbonate, polypropylene, and polystyrene. Because of the low dielectric constant of these films, their volumetric efficiency is quite low, and a 10 F/50 V polyester capacitor (for example) is actually a handful. Metalized (as opposed to foil) electrodes does help to reduce size, but even the highest dielectric constant units among film types (polyester, polycarbonate) are still larger than any electrolytic, even using the thinnest films with the lowest voltage ratings (50 V). Where film types excel is in their low dielectric losses, a factor which may not necessarily be a practical advantage for filtering switchers. For example, ESR in film capacitors can be as low as 10 m or less, and the behavior of films generally is very high in terms of Q. In fact, this can cause problems of spurious resonance in filters, requiring damping components.

Typically using a wound layer-type construction, film capacitors can be inductive, which can limit their effectiveness for high frequency filtering. Obviously, only noninductively made film caps are useful for switching regulator filters. One specific style which is noninductive is the stacked-film type, where the capacitor plates are cut as small overlapping linear sheet sections from a much larger wound drum of dielectric/plate

10.4

PASSIVE COMPONENTS CAPACITORS

TYPE TYPICAL DA ADVANTAGES

DISADVANTAGES

Polystyrene

0.001% to 0.02%

Polypropylene

0.001% to 0.02%

Teflon

0.003% to 0.02%

Polycarbonate 0.1%

Polyester

0.3% to 0.5%

NP0 Ceramic 0.2% >0.003%

Aluminum Electrolyte

Very high

Tantalum Electrolyte

Very high

Inexpensive Low DA Good stability (~120ppm/?C)

Damaged by temperatures >+85?C Large High inductance Vendors limited

Inexpensive Low DA Stable (~200ppm/?C) Wide range of values

Damaged by temperatures >+105?C Large High inductance

Low DA available Good stability Operational above +125?C Wide range of values

Expensive Large High inductance

Good stability Low cost Wide temperature range Wide range of values

Large DA limits to 8-bit applications High inductance

Moderate stability Low cost Wide temperature range Low inductance (stacked film)

Large DA limits to 8-bit applications High inductance (conventional)

Small case size Inexpensive, many vendors Good stability (30ppm/?C) 1% values available Low inductance (chip)

DA generally low (may not be specified) Low maximum values ( 10nF)

Low inductance (chip) Wide range of values

Poor stability Poor DA High voltage coefficient

Low loss at HF Low inductance Good stability 1% values available

Quite large Low maximum values ( 10nF) Expensive

Large values High currents High voltages Small size

High leakage Usually polarized Poor stability, accuracy Inductive

Small size Large values Medium inductance

High leakage Usually polarized Expensive Poor stability, accuracy

Fig. 10.1: Capacitor Comparison Chart 10.5

BASIC LINEAR DESIGN

material. This technique offers the low inductance attractiveness of a plate sheet style capacitor with conventional leads. Obviously, minimal lead length should be used for best high frequency effectiveness. Very high current polycarbonate film types are also available, specifically designed for switching power supplies, with a variety of low inductance terminations to minimize ESL.

Dependent upon their electrical and physical size, film capacitors can be useful at frequencies to well above 10 MHz. At the very high frequencies, stacked film types only should be considered. Some manufacturers are also supplying film types in leadless surface-mount packages, which eliminates the lead length inductance.

Ceramic is often the capacitor material of choice above a few MHz, due to its compact size and low loss. But the characteristics of ceramic dielectrics varies widely. Some types are better than others for various applications, especially power supply decoupling. Ceramic dielectric capacitors are available in values up to several F in the high-K dielectric formulations of X7R and Z5U, at voltage ratings up to 200 V. NP0 (also called COG) types use a lower dielectric constant formulation, and have nominally zero TC, plus a low voltage coefficient (unlike the less stable high-K types). The NP0 types are limited in available values to 0.1 F or less, with 0.01 F representing a more practical upper limit.

Multilayer ceramic "chip caps" are increasingly popular for bypassing and filtering at 10 MHz or more, because their very low inductance design allows near optimum RF bypassing. In smaller values, ceramic chip caps have an operating frequency range to 1 GHz. For these and other capacitors for high frequency applications, a useful value can be ensured by selecting a value which has a self-resonant frequency above the highest frequency of interest.

All capacitors have some finite ESR. In some cases, the ESR may actually be helpful in reducing resonance peaks in filters, by supplying "free" damping. For example, in general purpose, tantalum and switching type electrolytics, a broad series resonance region is noted in an impedance versus frequency plot. This occurs where |Z| falls to a minimum level, which is nominally equal to the capacitor's ESR at that frequency. In an example below, this low Q resonance is noted to encompass quite a wide frequency range, several octaves in fact. Contrasted to the very high Q sharp resonances of film and ceramic caps, this low Q behavior can be useful in controlling resonant peaks.

In most electrolytic capacitors, ESR degrades noticeably at low temperature, by as much as a factor of 4 to 6 times at ?55?C versus the room temperature value. For circuits where a high level of ESR is critical, this can lead to problems. Some specific electrolytic types do address this problem, for example within the HFQ switching types, the ?10?C ESR at 100 kHz is no more than 2? that at room temperature. The OSCON electrolytics have an ESR versus temperature characteristic which is relatively flat.

Figure 10.2 illustrates the high frequency impedance characteristics of a number of electrolytic capacitor types, using nominal 100 F/20 V samples. In these plots, the impedance, |Z|, vs. frequency over the 20 Hz to 200 kHz range is displayed using a high resolution 4-terminal setup. Shown in this display are performance samples for a 10.6

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