* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Fixed Resistors Resistors Developments Developments meeting
Survey
Document related concepts
3D television wikipedia , lookup
Lumped element model wikipedia , lookup
Valve audio amplifier technical specification wikipedia , lookup
Two-port network wikipedia , lookup
Valve RF amplifier wikipedia , lookup
Transistor–transistor logic wikipedia , lookup
Resistive opto-isolator wikipedia , lookup
Printed circuit board wikipedia , lookup
Electrical ballast wikipedia , lookup
Invention of the integrated circuit wikipedia , lookup
Network analysis (electrical circuits) wikipedia , lookup
Transcript
application note Philips Passive Components g for Hig ho h ity ll bil Smaporta for we Po for "Standard" resistors of s rounmic ub d sys ing tem s P for recis Ra ion dar Power rs u pp ly Fixed Resistors Developments meeting market needs ic nsing ohem e nt s r Loow r u rc f Low com munnoise icati f o r ons Fixed resistors − developments meeting market needs Though all performing the same basic function, today's fixed resistors come in a wide range of technologies and executions to meet the equally wide ranging requirements of modern industry. A clear understanding of their characteristics, strengths and limitations is therefore a useful aid to designers wishing to achieve optimum performance and reliability from their circuits. Another important element stimulating resistor development has been cost, especially in the consumer industries. However, the drive for ever lower costs in the consumer sectors has always been in conflict with the drive for higher performance in the industrial sectors, and as each new technology was introduced to further reduce costs, the industrial sectors would look closely at it to determine if it would yield some special features useful for specific applications. Inevitably, of course, new technologies also introduced drawbacks of their own resulting, in many instances, in continued use of the older technologies. From 'standard' to 'application specific' resistors When the first resistors were introduced last century, principally to meet the needs of the then rapidly growing radio telegraphy industry, reliability and ease of manufacture were the main requirements. Those early products were constructed of carbon compositions compressed into tubular containers and fitted with end-caps. Another early resistor was the wirewound resistor in which a resistive wire was spiralled around a tubular body. owe for P Both the carbon-composite and wire-wound resistors were manufactured individually and it wasn't until the introduction of bulk processing that real breakthroughs came in manufacturability, reliability and cost reductions. The first resistor to be manufactured in a bulk process was the carbon-film resistor. Though the process brought significant cost advantages, it also led to higher temperature coefficients than those exhibited by wire-wound products. For some applications, of course, low temperature coefficient is important and for these applications wire-wound resistors continues to be used. This was the first application-specific requirement of a resistor. Power r su pply So from a standard product performing a standard function, today we see a broad range of fixed-resistor technologies and executions, all tailored to specific application areas (Fig.1). P for recisi Rad on ar ic ensing ohrm ent s Low r cur fo ow n comLm unicoaise fo tions r Hig all rtability Sm for po for h ohm of suground ic b sy ing stem s "Standard" resistors Fig.1 The standard resistor developed into a broad range of application-specific types From 'leaded' to 'surface-mount' resistors By the late 1970s, the reliability of resistors was so high that the major cause of failure was the mounting. This was chiefly because the flexible leads of the components caused stresses on the solder joints which subsequently led to cracks in the solder. Later, new resistor materials were developed such as metal film to further improve reliability and manufacturability. Later still, the development of radar and multichannel communication stimulated additional requirements such as high precision, and with the arrival of portable equipment, size too became important. The introduction of surface-mount technology in the 1980s by companies such as Philips Components greatly reduced this problem. By eliminating the flexible leads, surface-mount technology allowed for 2 Philips Passive Components much stronger solder joints and greatly enhanced reliability. Surface-mount technology brought other advantages too such as easier, cheaper mounting. It also offered greater possibilities for miniaturization since not only could the surface-mount devices be more densely packed onto the PC board, but the components themselves, without the need for external leads, could be smaller. available in surface-mount, the initial penetration of surface-mount technology was accomplished relatively easily. A problem arises, however, when industrial equipment manufacturers wish to move over to full surface mount since many of the functions in their equipment require application-specific resistors and these are currently available only in leaded versions. The future will, however, see this gap being filled as companies such as Philips Components address this problem. The result of all this is that today almost 70% of all resistors are in surface-mount execution, and this figure is still rising. Resistor technologies and their principal applications The technology gap − application-specific surfacemount resistors The first large-scale users of surface-mount resistors were manufacturers of consumer electronics equipment. These use mostly standard resistors, and low price being an important driver in this area, they naturally went for the most cost-effective solution. During the development of resistors, various manufacturing methods have been adopted, each offering specific advantages and limitations. Moreover, each specific part of a resistor such as its resistive element and terminations will influence its performance in a given application. Here we give an overview of the advantages and limitations of the most commonly used technologies. So to protect their market position, manufacturers of leaded resistors targeted their products at the application-specific industrial and professional sectors. This helped to advance leaded-resistor technology but also meant that standard leaded resistors were almost entirely supplanted by surface-mount versions. Let's consider each in turn. General industrial driven trend Application specific Consumer driven trend Leaded Every resistor comprises the following elements: − resistive element − resistor body − contacts − protective means (encapsulation etc.). Resistive element The resistive element can be a solid material such as a metal strip, metal wire or compressed carbon composite. It can also be in the form of a metal film, metal-glaze or carbon film on a carrier (usually a ceramic body). All these materials offer advantages and disadvantages. General purpose SMD Fig.2 The trend toward application specific surface-mount resistors is being driven by general industrial market segments that have consumer characteristics, i.e. telecom, computers, automotive Resistors with solid elements can generally absorb high pulse loadings. Film resistors, on the other hand, are less expensive to manufacture and metal resistors generally have more predictable and controllable temperature coefficients. These days we see high-ohmic carbon-composite resistors increasingly being replaced by metal-glaze resistors as the performance of metal glaze approaches that of carbon composite. Today, however, improved capabilities together with increasing pressures on price in the industrial sectors has compelled manufactures of industrial equipment to look seriously at surface-mount components. Since industrial equipment uses mostly standard resistors which, thanks to the consumer market, were readily 3 Philips Passive Components Tolerances are strongly influenced by the manufacturing method though material properties are also important. Metal-film generally offers the narrowest tolerances though metal-strip and wire-wound resistors can also offer narrow tolerances. With these latter two technologies, however, narrow tolerances are more difficult to achieve. Moreover, with wirewound resistors the material is again a critical factor. Resistor contacts For resistors with rectangular ceramic bodies or metal-strip resistive elements, contact with the resistive element is made via terminations fixed directly to the body. For all other resistor types, leaded contacts are used. A major disadvantage of the termination method is that heat generated in the resistive element is conducted directly to the solder joint, limiting the maximum power that can be applied to the resistor. Leaded contacts, on the other hand, must be attached to the resistor body via caps which can be relatively expensive. The (current) noise is also strongly influenced by the material of the resistive element: the more homogeneous the material, the lower the noise. Trimming, in particular, degrades noise behaviour as it leads to cracks in the material that can be a source of noise. Lacquering/encapsulation The resistive element is generally lacquered to protect it from the environment. Lacquer is, however, not used when a leaded resistor is adapted for surface-mount and with resistors for high-power applications. In the first example, standard leaded resistors are removed from the production cycle before lacquering. They are then covered in a moulded plastic case after which the leads are flattened and bent. In the second example, the resistor is fitted into a ceramic case to improve heat transfer to the environment. Where high power dissipation and small size are important, the resistors are fitted into metal cases attached to heatsinks. The table compares performance criteria of the main resistor technologies. Fixed- resistor performance criteria compared resistive element resistance pulse-load TC capability noise price metal strip very low +++ 5 − 15 ++ − wire-wound low/medium +++ 100 + 0 carbon composite medium +++ 5000 − 0 metal-film medium + 50 − 200 + 0 ++ > 300 − − 200 0 +/− carbon-film medium metal-glaze medium/high +/0 Expanding possibilities Each new development within each of the basic resistor functions described above resulted in a series of new possibilities. For example, a solution to the problem of measuring low-ohmic values during production also led to an important breakthrough enabling the production of surface-mount (1206) low-ohmic resistors. Later the same technological breakthrough also led to the development of lowohmic surface-mount (1218) power resistors. So breakthroughs in one area can lead to important developments in several related areas. Resistor body The body of the resistor may be the resistive element itself, or a separate body supporting the element, for example, a rod around which the element is spiralled or a rectangular block on which the element is deposited. The body is usually of ceramic but other materials such as glass fibre, glass or silicon are sometimes used. Rods are very popular with leaded resistors and with the exception of carbon composite resistors, all leaded resistors currently use rods. A major disadvantage is poor high-frequency behaviour due to inductive effects caused by the spiral shape of the resistive element. In this respect, Philips has been actively working on metal-film technology for surface-mount resistors. The most important result of this work so far has been its successful MPC01 high-precision surfacemount (1206) resistor series. As this technology matures, other products will also benefit, such as the MPC01's little brother due to be introduced later this year and offering the same capabilities as the The size of the body strongly influences the resistor's power handling capacity since, as may be expected, the larger the body the more heat it can dissipate and hence the lower the body temperature. So resistors intended for high power applications need larger bodies than standard or low-power resistors. 4 Philips Passive Components MCP01 but in size 0805. Besides its precision properties, other positive features of this new technology are being explored such as its pulse handling capabilities leading to highly pulse-resistant surface-mount resistors and its well-defined fusing characteristics which could open the way to the development of low-ohmic fusible resistors. Power Power resistors are usually larger than standard resistors and are often distinguished from them by their coating, which can withstand higher temperatures to accommodate greater power dissipation. General-purpose power resistors come in leaded metal-film technology allowing for dissipation up to 3 W, and in surface-mount thick-film technology with dissipations up to 1 W. Other technologies are used for higher power dissipation or to satisfy other power-related requirements − very low ohmic values for current sensing, for instance, or good pulse-load behaviour for transient-sensitive circuitry. For surface-mount resistors, the major factor limiting power handling is not the resistor itself or its coating but the temperature of the solder joint. Principal application areas For each application, a proper choice of resistive element, body, contact and lacquering/encapsulation needs to be made. There are, however, some general guidelines that can be followed. The principal applications can be roughly divided into 6 categories: standard or general-purpose, power, precision, grounding, fusible and trimmable. Standard resistors Carbon composite Standard resistors are available in both leaded and surface-mount executions though 80% of the market is currently in surface mount. Leaded resistors employ carbon- or metal-film technology while surface-mount resistors employ metal-glaze technology. The resistors normally exhibit tolerances from 2% to 5%, dissipation up to 0.5 W and TCs of between 100 to 200. The TC can be even higher for carbon-film resistors and their noise behaviour is inferior to metal-film. So for leaded resistors, metalfilm technology is often preferred. Pulse resistant Leaded Standard Thick film Wirewound Thick film Application specific Power Thick film Metal film Leaded General purpose SMD Fig.4 Power resistors require different technologies for specific power-related applications With its 1218-sized PRC201 power series, Philips has made an important contribution to the field of surface-mount power resistors and is now planning to introduce metal-film surface-mount products offering high pulse-load capability. Application specific Carbon film Metal film Current sensing Precision Precision resistors are normally in leaded metal-film technology. Surface-mount versions are currently available with TCs as low as 25 and tolerances down to 0.1%, but new developments are leading to even narrower tolerances and better TC values. Philips is currently expanding its range of surface-mount highprecision resistors with the introduction of its 0805 sized MPC11. Metal-film is not ideal for low-ohmic precision resistors for current sensing, however, and metal-strips (for very low-ohmic values) or wirewound leaded resistors (for power applications) are often used instead. General purpose SMD Fig.3 Standard resistors are widely available in surface mount execution Philips is highly active in this area and currently offers a broad range of standard resistors in both leaded and surface-mount execution. 5 Philips Passive Components Wirewound Current sensing Metal strip Fusible resistors Coated with a special non-flammable lacquer, fusible resistors are produced in metal-film technology since this allows for the introduction of well-defined fusing characteristics. Not only do they provide the protection of a true fuse, they also function as a standard resistor under normal operation. Formerly available only as leaded products, surface-mount (thick-film) versions have recently become available with the development of new resistive pastes and improved laser-trimming techniques. Philips offers the most complete range of fusible resistors in the industry and is currently developing low-ohmic surface-mount types. Application specific Precision Metal film General purpose Leaded SMD Fig.5 Precision is still the domain of leaded resistors but surfacemount technology is developing fast in this direction Grounding Though standard high-ohmic (> 100 kΩ) resistors can provide circuit coupling with low leakage current, application-specific types are normally needed. Since it's commonly high-voltage environments that require grounding, special highvoltage types are necessary, usually leaded metal-glass resistors. In this respect, Philips' metal-glass VR series offers good stability and excellent high-voltage behaviour. Metal film Leaded Carbon film Metal film Leaded Standard Thick film SMD Fig.7 Fusible resistors have recently becoome available in surfacemount execution Trimmable resistors Available only in surface-mount, trimmable resistors perform the basic function of a potentiometer with a limited adjustment range but with the important advantages of being small and relatively inexpensive. Though laser trimming is necessary, for large-scale production this is not a disadvantage since the purchase of a laser system by the circuit manufacturer will then prove cost-effective. Incidentally, used untrimmed, trimmable resistors can offer advantages such as significantly improved pulse-load capability and noise. Application specific Circuit Grounding Application specific General purpose Surface-mount types are not available for these applications since the very close terminal distances would result in sparking across the terminals. Metal glaze Thick film Fusible General purpose SMD Fig.6 High-voltage resistors are not available in surface-mount execution since the contact distance needs to be greater than the open air spark voltage (1 kV/mm) 6 Philips Passive Components Thick film Trimmable Application specific Cost Trimmable Potentiometer General purpose Leaded SMD Volume Fig.8 Trimmable resistors are an attractive alternative to potentiometers Fig.8 Adjustment costs per unit. Initial costs of laser-trimming equipment is high but is paid back when trimmable resistors are used in large volume 7 Philips Passive Components