How passive components are evolving to meet today’s circuit demands

Introduction

When you are evaluating your next smartphone, you naturally focus on parameters like CPU type and memory capacity: hardly surprising, considering how they impact your quality of online experience. Yet the device’s performance – or its ability to function at all – depends just as much on its complement of passive components. And this truth extends out from the phone you were considering to the furthest reaches of today’s technological ecosystem.

The three primary types of passive component are resistors, capacitors and inductors. They have no power gain and cannot amplify, nor can they control current flow. However, they need no external power to operate, while offering a linear response to whatever voltage or current is supplied to them.

This article looks at the choices available to you if you’re seeking to design passives into an electronic or electrical circuit. But these choices are constantly changing as applications become more demanding and new developments appear in the marketplace. As in any other industry throughout history, progress is made through ever-repeating steps; from development of revolutionary new materials or processes, then evolutionary or incremental improvements to existing technologies, and finally, widespread production and normalisation of the new approach.

Accordingly, we review the products currently widely available and their contrasting suitability for various applications – but we also cover more recent evolutionary products to consider, and even some revolutionary developments that indicate what the future could bring.

RESISTORS

Resistors play a crucial role in all electronic circuits for many reasons. They impede the flow of electric current in a circuit, creating a voltage drop. This allows them to regulate current, divide voltage, and set bias points in electronic systems. They come in various resistance values, each colour-coded for easy identification.

Revolution

A review of recent research shows that resistors can play a surprisingly complex role in emerging high-technology applications. Researchers at MIT have created protonic programmable resistors -- the building blocks of analogue deep learning systems -- that can process data a million times faster than the synapses in the human brain. These ultrafast, low-energy resistors could enable analogue deep learning systems that can train new and more powerful neural networks rapidly, which could then be used for novel applications in areas like self-driving cars, fraud detection, and health care.

Evolution

Of the four types of fixed value resistor – which are discussed later – thick film products are particularly popular, and the technology has attracted much innovation.

Thick film resistors are particularly popular in both industrial and consumer applications because of relatively low cost. However, the ever-present need for low power consumption and circuit protection in relevant applications is driving the demand for high-performance thick-film resistors.

Higher current rating, lower TCR: To meet the increasing energy-saving needs of industrial and consumer applications, designers are developing thick-film resistor solutions that offer high power ratings with significantly lower TCR characteristics.

For instance, ROHM recently expanded its LTR series with the LTR100L thick-film shunt resistor that provides industry-leading rated power for high power current detection applications . Leveraging an original approach involving resistor material revision and terminal temperature derating led to a higher-performing thick-film resistor solution. As a result, in addition to favourably competing with existing metal shunt resistors, significant improvements were achieved over conventional thick-film resistors. This allows customers to switch from metal shunt resistors to cheaper thick-film resistors.

Additionally, a lower temperature coefficient of resistance (TCR) enables higher current detection accuracy compared to existing thick-film resistors.

Figure 1 shows the benefits of the device’s higher power rating and superior TCR

Figure 1: Benefits of the LTR100L’s higher power rating and superior TCR

Improved surge characteristics: Panasonic's ERJ-PM8 series anti-surge thick film resistors help reduce the need for high resistance value resistors. They also offer high precision, high voltage, and high resistance with a limited element voltage of 500 V.

Figure 2: Construction detail for Panasonic ERJ PM8 anti-surge thick film chip resistor

Mainstream choices

However, thin film resistors, while offering many exciting advances, aren’t the only game in town. Now we return to a broader discussion of the four types of resistor technology currently available, as listed here:

Carbon Composition Resistor – Made of carbon dust or graphite paste, low wattage values
Film or Cermet Resistor – Made from conductive metal oxide paste, very low wattage values
Wire-wound Resistor – Metallic bodies for heatsink mounting, very high wattage ratings
Semiconductor Resistor – High frequency/precision surface mount thin film technology

Carbon resistors

Carbon resistors are the most common type of composition resistors, are a cheap general purpose type. Their resistive element is manufactured from a mixture of finely ground carbon dust or graphite (similar to pencil lead) with a non-conducting ceramic (clay) powder as a binding agent.

The ratio of carbon dust to ceramic (conductor to insulator) determines the overall resistive value of the mixture; higher carbon content reduces overall resistance. The mixture is moulded into a cylindrical shape with metal wires or leads attached to each end to provide the electrical connection, before being coated with an outer insulating material and colour coded markings to denote its resistive value.

The carbon composite resistor is a low to medium power type with low inductance, making them ideal for high frequency applications, but they are also susceptible to noise and stability issues when hot. The components are generally prefixed with a “CR” notation (e.g., CR10kΩ ) and are available in E6 ( ± 20% tolerance (accuracy) ), E12 ( ± 10% tolerance) and E24 ( ± 5% tolerance) packages with power ratings from 0.250 or 1/4 Watt up to 5 Watts.

The resistors are very cheap to manufacture, and are therefore commonly used in electrical circuits. However, due to their production process, carbon type resistors have very large tolerances, so for more precision and high value resistances, film type resistors are used instead.

The generic term Film Resistor covers metal film, carbon film and metal oxide film resistor types, which are generally made by depositing pure metals, such as nickel, or an oxide film, such as tin-oxide, onto an insulating ceramic rod or substrate.

The resistive value of the resistor is controlled by varying the desired thickness of the deposited film to create either thick-film resistors or thin-film resistors.

Once deposited, a laser is used to cut a high precision spiral helix groove type pattern into the film. Cutting the film increases the conductive or resistive path, like forming a long straight wire into a coil.

This method of manufacture allows for much closer tolerance (1% or less) than possible with simpler carbon composition types. Film type resistors also achieve a much higher maximum ohmic value compared to other types, and values in excess of 10MΩ (10 Million Ohms) are available.

Metal Film Resistors have much better temperature stability than their carbon equivalents, lower noise and are generally better for high frequency or radio frequency applications. Metal Oxide Resistors have better high surge current capability with a much higher temperature rating than the equivalent metal film resistors.

Thick Film Resistors are another type of film resistor, manufactured by depositing a much thicker conductive paste of CERamic and METal, called Cermet, onto an alumina ceramic substrate. Cermet resistors have similar properties to metal film resistors and are generally used for making small surface mount chip type resistors, multi-resistor networks in one package for pcbs, and high frequency resistors. They have good temperature stability, low noise, and good voltage ratings but low surge current properties.

Wirewound resistors

Wirewound resistors are made by winding a thin metal alloy wire (Nichrome) or similar wire onto an insulating ceramic former in a spiral helix similar to the film resistor above.

These resistors are generally only available in very low ohmic, high precision values (from 0.01Ω to 100kΩ), due to the wire gauge and number of turns possible on the former, making them ideal for use in measuring circuits and Wheatstone bridge type applications.

They can also handle much higher electrical currents than other resistors of the same ohmic value with power ratings in excess of 300 Watts. These high power resistors are moulded or pressed into an aluminium heat sink body with fins attached to increase their overall surface area to promote heat loss and cooling, and improve current rating.

Power Wirewound Resistors are variants of wirewound resistors. They are high temperature, high power non-inductive resistor types generally coated with a vitreous or glass epoxy enamel for use in resistance banks or DC motor/servo control and dynamic braking applications. They can even be used as low wattage space or cabinet heaters.

CAPACITORS

Capacitors are passive components that store electric charge. They comprise two conducting plates separated by an insulating dielectric material. When voltage is applied, electric charge accumulates on the plates, with polarity depending on the voltage polarity. The capacitance value determines how much charge can be stored for a given applied voltage .

Capacitor functions include:

Storing and discharging electric charge
Filtering signals
Decoupling power supplies
AC coupling between circuit stages
Tuning and resonance circuits
Snubbing transients

Revolution

Miniaturisation and high power density are essential for all components as products extract higher performance and functionality from smaller form factors. One advanced solution comprises high-density silicon capacitors developed with a semiconductor MOS process. These use the third dimension to substantially increase the capacitor surface and thus its capacitance without increasing the capacitor footprint. This process, which has been perfected by such companies as IPDIA (Murata) in France, offers a glimpse into what might be the next phase in volumetric efficiency of components and begins to augment the ubiquitous MLCC .

Evolution

However, as the next couple of examples show, resilience to harsh environments is frequently sought after, too – and capacitors are no exception.

Metallised polypropylene AC filtering film capacitors : Vishay Roederstein has introduced a new series of metallised polypropylene AC filtering film capacitors optimised for high humidity environments. MKP1847C AC filtering devices withstand demanding temperature humidity bias (THB) testing – 40 degrees C and 93 percent relative humidity for 56 days at rated voltage – without altering their electrical characteristics.

These capacitors are designed to ensure stable capacitance and ESR values over a long service life under harsh environmental conditions during operation. Compared to previous-generation devices, the MKP1847C AC filtering series capacitors offer higher humidity robustness at a lower cost while maintaining the same footprint. The devices are targeted at input and output filtering in UPS systems, renewable energy inverter grid interfaces and welding equipment.

Figure 3: VISHAY MKP1847C510355K2 Power Film Capacitor, Metallized PP, Radial Box - 4 Pin, 1 µF, ± 10%, AC Filter, Through Hole

High-temperature hybrid aluminum capacitors: Panasonic’s new EEH-ZU Series conductive polymer hybrid aluminum capacitors are said to be capable of operating at high temperatures with conductive polymer capacitor performance and aluminum electrolytic capacitor safety in a surface-mount package .

These new capacitors are rated for a 135 degrees C operating temperature and feature a 4,000-hour endurance rating. These hybrid capacitors are said to be able to withstand a voltage range of 25 to 63 VDC, have 100-560 microfarads and are available in vibration-proof variants upon request. AEC-Q200 qualification ensures quality and reliability. These parts are well-suited where high temperature and high current capability are demanded by the application.

Figure 4: PANASONIC EEHZU1V471P Hybrid Aluminium Electrolytic Capacitor, 470 µF, ± 20%, 35 V, Radial Can - SMD, 0.009 ohm

Mainstream devices and choices

Capacitors currently in popular use can be classified into the following five types:

Ceramic Capacitors
Electrolytic Capacitors
Supercapacitors
Film capacitors
Mica Capacitors

Ceramic capacitors

Ceramic capacitors use ceramic materials like titanium dioxide or barium titanate as the dielectric. They are usually compact, inexpensive, and have excellent high-frequency characteristics.

Other benefits include small capacitance values from 1 pF to 1 μF, high stability and low losses, an ability to withstand high voltages up to 100 kV, low ESR and inductance, high reliability and long lifespan. They operate from -55°C to 200°C, with tolerances up to ±1%.

Types of ceramic capacitors:

Multilayer ceramic capacitors (MLCC) – Made by stacking alternate ceramic dielectric and metal electrode layers to achieve high capacitance density. Most common SMT capacitor type.
Ceramic disc capacitors – Leaded capacitors with capacitance from 1 pF to 0.1 μF. They are used for high-frequency coupling and bypassing.
Ceramic power capacitors – Can handle large currents and voltages up to 10 kV. They are used for power applications and snubbers.

Ceramic capacitors are ideal for decoupling, high-frequency filtering, timing, and precision circuits. Their high stability and reliability make them suitable for demanding applications.

Electrolytic capacitors

Electrolytic capacitors use a thin insulating oxide layer as the dielectric, etched on an aluminium or tantalum anode foil. They are polarised and have much larger capacitance than ceramics.

They have medium to high capacitance values, from 1 μF to 1 F, and withstand voltages from 2V up to 600V. They exhibit higher ESR and leakage than ceramics. They are sensitive to polarity, heat, and mechanical damage, with a lower tolerance of ±20%.

Types of electrolytic capacitors:

Aluminium electrolytic is the most common type in through-hole and SMT packages.

Tantalum electrolytic – More stable, reliable, and expensive than aluminum. Used in space-critical applications.

Niobium electrolytic – Replacement for tantalum with higher capacitance density.

Conductive polymer aluminum – More stable and reliable than standard aluminum electrolytic.

Electrolytic capacitors are ideal for smoothing, buffering, and bulk decoupling applications where large capacitance values are needed. Their major downside is a shorter lifespan and degraded performance over time.

Supercapacitors

Supercapacitors, also called ultracapacitors or electric double-layer capacitors, provide very high capacitance in compact sizes. They use porous electrode materials and electrolytes to store charge electrostatically.

Key attributes:

Exceptionally high capacitance, up to thousands of farads
Low internal resistance allows high-power delivery
Operate from 2.3V to 5.5V
High-efficiency charge/discharge cycles
Withstand over 1 million recharge cycles
Work across a broad temperature range
High self-discharge rate when idle

Supercapacitors are ideal for energy storage and burst power delivery in memory backup systems, solar devices, and electric vehicles. Their high power density suits them for applications needing repeated charge/discharge cycles.

Film capacitors

Film capacitors use thin plastic film as the dielectric, such as polyester, polypropylene, polycarbonate, and polystyrene. They are available in leaded and SMT packages.

Characteristics:

Low inductance and ESR
Tolerances from ±1% to ±20%
Operate from 55°C to 125°C
Withstand voltages from 50V to 1,600V
Capacitance ranges from 1 pm to 10 μF
Suitable for frequency filtering and coupling

Film capacitors are reliable AC coupling and filtering capacitors. Their self-healing properties increase lifespan and reliability. Film caps suit high-frequency tuning and timing circuits.

Mica capacitors

Mica capacitors utilise mica sheets as the dielectric. Silver or metal deposited on mica acts as the electrodes. They have excellent stability and low losses.

Properties:

Shallow leakage current
High accuracy and stability
Low inductance, suitable for RF applications
Operate from -55°C to 125°C
Handle high peak voltages, low DC working voltages
Capacitance ranges from 1 pm to 0.01 μF
Often used in resonant circuits and timing applications

Mica capacitors suit precision high-frequency oscillator and filtering applications. Their low losses and accuracy make them ideal for tuning tank circuits.

INDUCTORS

An inductor, also called a coil, choke, or reactor, is a passive two-terminal electrical component that stores energy in a magnetic field when electric current flows through it. An inductor typically consists of an insulated wire wound into a coil .

Revolution

A recent advance in inductor technology now gaining prominence relates to planar inductor design. This is because planar inductors, which can be fabricated using PCB technology, offer excellent performance, high efficiency, and compact size. They utilize low-profile windings with low leakage and AC winding resistance, minimizing losses .

These inductors can be designed in various shapes such as circular, rectangular, square, hexagonal, or octagonal. The square-shaped spiral inductors, in particular, have become popular for applications like wearables, communication systems, and electronic gadgets.

Planar technology aims to offset the disadvantages of traditional high-frequency inductors. It allows for low-profile magnetics, making it suitable for modern power electronics operating at higher frequencies.

Evolution

Power inductors for automotive LED headlights: Inductor innovation is being applied to higher-power applications as well. TDK’s SPM-VT series meets the high thermal and current demands of the automotive LED headlight environment.

The SPM-VT series is the latest addition to TDK’s lineup of metal-core, wire-wound power inductors. To meet the tough conditions prevailing in harsh automotive environments, these metal-core power conductors have a wide operating temperature range from -55 °C up to +155 °C.

The units offer low DC resistance along with very small size when compared to ferrite wound type inductors. This is made possible due to the DC superimposition characteristics of metallic magnetic materials.

These inductors boast superior DC superposition characteristics in a compact size. TDK claims that the members of the SPM-VT series feature a rated current that is approximately two to three times higher than that of comparable high-temperature products.

Figure 5: Power Inductor (SMD), 1.5 µH, 44.2 A, Shielded, 32.8 A, SPM-VT-D Series

Mainstream inductor types

Apart from these latest technology examples, many types of inductor are widely available. These are summarised in the list below, and described more fully in the content that follows.

Coupled inductors
Air Core Inductor
Laminated Core Inductor
Ferrite Core Inductor
Toroidal core inductors
Bobbin inductors
Axial inductors
Multilayer Chip Inductors
Film Inductor

Coupled inductors

Coupled inductors are a combination of an ideal transformer and an inductor with magnetising Inductance. As with most inductors, the coupled inductor uses the magnetised inductor component to store energy while the transformer transfers it. However, both coils of the inductor increase overall electromagnetic permeability through mutual inductance.

Coupled inductors are typically used in buck-boost DC-DC converters such as:

Single-ended primary-inductor converters
Flyback converters
Ćuk buck-boost converter

Air core inductor

The air core inductor is one of the most common types of inductors. It often uses a ceramic core, but may also be coreless.

Nevertheless, ceramic non-magnetic core air inductors are preferable. This is because the ceramic core gives the inductor its shape and supports it. Additionally, ceramic is an ideal material because of its low thermal coefficient expansion, so can provide high levels of stability when the coil is in use.

Additionally, ceramic is devoid of magnetic properties. Therefore, it has zero permeability and will not store residual energy or interfere with the component’s overall inductance. During the production process, manufacturers dip the inductor in wax or varnish to stabilise it further. This process is essential for coreless air inductors.

Air-core inductors are used in high-frequency applications such as televisions. Other notable applications include interstage coupling, low-frequency applications between 20 Hz and 1 MHz, RF and IF tuning coils, and filter circuits.

Laminated core inductor

The core of these inductors comprise a stack of laminations. The lamination materials used depend on the inductor specifications and purpose.

However, the laminations tend to be steel-based and feature an insulating material between them. The manufacturer must arrange these laminations parallel to the magnetic field to prevent eddy current losses. Other critical components of the laminated core inductor include a coil wrapped around a bobbin.

Applications:

Transformers (Most common use)
Line filters
Noise filters
Filter chokes
Chargers/converters for electric vehicles

Ferrite core inductor

In appearance, ferrite resembles ceramic material. However, unlike ceramic, it’s a ferrous material (ferromagnetic). This means that it magnetises when it is in a magnetic field, and it still retains this magnetism when removed from the field.

Consequently, it has high electromagnetic permeability and a low reluctance path to the magnetic flux. Because the core materials consist of iron oxide, we may also refer to iron core inductors.

There are two types of ferrite cores:

Soft Ferriteonly carries electromagnetism temporarily. Consequently, when the magnetic field is removed, the ferrite loses its magnetism. Thus, it can reverse magnetic polarity.

Hard Ferrite A dense and robust material that can operate in temperatures up to 180°C. However, unlike soft ferrite, it retains its magnetism after the magnetic field is removed. Therefore, we refer to the cores as permanent magnets.

Applications:

Pi Filters
Switching circuits
Various frequency ranges

Toroidal core inductors

Toroids are doughnut-shaped structures, produced in a variety of sizes and materials. The core’s material affects its behaviour over different frequencies and inductance values. Nevertheless, regardless of the material, the toroidal core inductor’s most significant advantage over other inductor types is lower electromagnetic interference (EMI).

Applications:

Power supplies
Electronic circuits
Analogue circuits
Communication systems
Medical devices

Bobbin inductors

A bobbin inductor is designed with the coil wound on a coil form or bobbin. The core structure is then mounted on the bobbin. Bobbin wound Inductors can be designed to operate from 50/60Hz, or line frequency, well into the MHz range .

Applications include:

Switch mode power supplies
Power conversion applications
Filter circuits

Axial inductors

Axial inductors, which resemble resistors, often feature a thin coil around a miniature curved bobbin-like ferrite material. They are marked with bands typically four or five -according to the Electronic Industries Association (EIA) specifications and standards. These rings allow us to discern the value of the inductor or its inductance.

Axial inductors are generally high-frequency inductors. Because of their size and general robustness, they suit applications such as:

Circuit boards
RF Applications
Resonant circuits
Buck-boost converters
Normalising current flow in an electrical circuit
Filters circuits and other designs

Multilayer chip inductors

As the name implies, multilayer chip inductors comprise multiple layers, which usually consist of ferromagnetic material on top of ceramic dielectric materials. Manufacturers print the induction coil on these ferromagnetic sheets using a metallic paste.

Once the layers are placed, the patterns form a singular coil. The manufacturer then moulds and coats the total package. There are connecting terminals on each side of the MLCIs package.

Their compact form factor allows their use in a variety of applications such as:

Wearables
Bluetooth devices
Mobile devices
Computer motherboards
Network adapters
Communications equipment

Film inductor

Film Inductors use a thin film-based material for the coil, and are extremely small and lightweight. They are found in applications such as DC to DC converters in mobile phones and other portable devices. They are also used in resonant circuits.

The future

The future of passive components lies in their continuous evolution, adaptability, and alignment with technological advancements.

The race towards miniaturisation continues; Innovations like Murata Manufacturing’s tiny multilayer ceramic capacitors (MLCCs) measuring just 0.25 x 0.125 mm demonstrate how advanced materials and techniques can shrink passive component sizes while enhancing performance.

Miniaturisation is being helped by integration, as integrated passive devices (IPDs) consolidate various passive components (resistors, capacitors, and inductors) into a single entity. Beyond reducing physical footprint, integration improves performance by minimizing parasitic effects and enhancing signal integrity .

Embedded Passive Components are also poised to be an important innovation in electronic design. They offer cost savings and space efficiency. Additionally, placing bypass capacitors closer to the ideal location is a possibility with embedded passives .

Increasing demand in future electronics, electrification, and digitalisation is also fuelling market growth for high-reliability and custom inductors, resistors, and capacitors for applications in demanding contexts .