Comparison of advantages and disadvantages of the

  • Detail

Comparison of advantages and disadvantages of common resistors

in the past two decades, the electronic industry has developed at an amazing speed. The progress of new technology not only reduces the size of equipment, but also increases the pressure on discrete component manufacturers to develop devices with ideal performance

in these devices, chip resistance has always maintained a high demand, and is the basic component of many circuits. Their space utilization ratio is better than that of split package resistors, which reduces the workload of pre assembly preparation. With the popularization of application, chip resistance plays a more and more important role. The main parameters include ESD protection, thermoelectric electromotive force (EMF), resistance thermal coefficient (TCR), self heating, long-term stability, power coefficient and noise, etc

the application of wire wound resistance in precision circuits will be discussed in the following technical comparison. However, please note that wire wound resistors are not chip type, so they are not used in applications that require precision chip resistors due to weight and size constraints

although upgrading each component or subsystem can improve the overall performance, the overall performance is still determined by the short board in the component chain. Each component in the system has inherent advantages and disadvantages related to the overall performance, especially short-term and long-term stability, frequency response and noise. The discrete resistance industry has made progress in winding resistance, thick film resistance, film resistance and metal foil resistance technology. Considering the unit performance cost, each resistance has many factors that need to be weighed

the advantages and disadvantages of various resistance technologies are shown in Table 1, which shows the influence of thermal stress and mechanical stress on the electrical characteristics of resistance. Table 1: characteristics of different types of resistors

stress (whether mechanical stress or thermal stress) will cause changes in electrical parameters of resistors. When the shape, length, geometric structure, configuration or modular structure changes due to mechanical or other factors due to the influence of 5 experimental steps, the electrical parameters will also change. This change can be expressed by the basic equation: R= ρ L/a, where

r = resistance value, in ohms,

ρ = Material resistivity, in ohmmeter,

l = length of resistance element, in meter,

a = sectional area of resistance element, in square meter

when the current passes through the resistance element, it generates heat, and the thermal reaction will cause mechanical changes of expansion or contraction of each material of the device. Ambient temperature conditions will produce the same results. Therefore, the ideal resistance element should be able to balance itself according to these natural phenomena, maintain physical consistency in the resistance processing process, and do not need to compensate for thermal effect or stress effect in the use process, so as to improve the stability of the system

precision wire wound resistance

wire wound resistance is generally divided into "power wire wound resistance" and "precision wire wound resistance". Great changes will occur in the use of power wire wound resistors, which are not suitable for use under the condition of high precision requirements. Therefore, this resistance is not considered in this discussion

the manufacturing method of wire wound resistance is generally to wind the insulation resistance wire on a spool with a specific diameter. Different wire diameters, lengths and alloy materials can achieve the required resistance and initial characteristics. Precision wire wound resistors have higher ESD stability and lower noise than thin film or thick film resistors. The wire wound resistance also has the characteristics of low TCR and high stability

the initial error of wire wound resistance can be as low as ± 0.005%. TCR (resistance change for every one degree Celsius change in temperature) can reach a typical value of 3 ppm/° C. However, when the resistance value is reduced, the wire winding resistance is generally 15 ppm/° C to 25 ppm/° C. The thermal noise is reduced, and TCR can reach ± 2 ppm/° C within the limited temperature range

during the processing of wire wound resistance, the inner surface of the resistance wire (near the side of the spool) shrinks, while the outer surface stretches. This process produces permanent deformation - relative to elastic deformation or reversible deformation, the resistance wire must be annealed. Permanent mechanical changes (unpredictable) will cause arbitrary changes in the electrical parameters of the resistance wire and resistance. Therefore, there is great uncertainty in the electrical performance parameters of resistance elements

due to the coil structure, the wire wound resistance becomes an inductor, and the capacitance between coils will be generated near the number of turns. In order to improve the response speed in use, special processes can be used to reduce the inductance. However, this will increase the cost, and the effect of reducing the inductance is limited. Due to the inductance and capacitance in the design, the wire wound resistance has poor high-frequency characteristics, especially at frequencies above 50 kHz

it is difficult for two wire wound resistors with the same rated resistance value to ensure accurate consistency within a specific temperature range, and it is more difficult for them to have different resistance values or different sizes (for example, meeting different power requirements). This difficulty will be further aggravated with the increase of resistance value difference. Taking the 1-k Ω resistance as an example relative to the 100-k Ω resistance, this inconsistency is due to the diameter, length, and possibly due to the different alloys used in the resistance wire. Moreover, the resistance core and the number of turns per inch are also different - the influence of mechanical characteristics on electrical characteristics is also different. Because different resistance values have different thermal engine characteristics, their working stability is different, and the designed resistance ratio will change greatly in the life cycle of the equipment. TCR characteristics and ratios are extremely important for high-precision circuits

the traditional wire wound resistance processing method can not eliminate various stresses generated in the winding, packaging, insertion and lead molding processes. In the process of fixing, the axial lead is often tightened and sealed by mechanical force. These two methods will change the resistance, whether powered on or off. From a long-term perspective, as the resistance wire is adjusted to a new shape, the wired components will undergo physical changes

thin film resistance

the thin film resistance is composed of a ceramic substrate with a thickness of 50? To 250? Metal deposition layer composition (using vacuum or sputtering process). The resistance per unit area of film resistance is higher than that of wire wound resistance or bulk metal? Metal foil resistance, and cheaper. When a high resistance value is required and the accuracy requirement is medium, the film resistance is more economical and saves space

they have the best temperature sensitive deposition layer thickness, but the resistance value generated by the best film thickness seriously limits the range of possible resistance values. Therefore, different resistance ranges can be achieved by using various deposition layer thicknesses. The stability of film resistance is affected by temperature rise. The aging process of film resistance stability varies with the film thickness required to achieve different resistance values, so it is variable in the whole resistance range. This chemical/mechanical aging also includes high temperature oxidation of resistance alloys. In addition, changing the optimal film thickness will seriously affect TCR. Because the thinner deposition layer is easier to oxidize, the resistance degradation rate of high resistance films is very high

due to the small amount of metal, the film resistance is very easy to self erode under wet conditions. In the process of immersion packaging, water vapor will bring impurities, and the resulting chemical corrosion will cause open circuit of film resistance within a few hours of low-voltage DC application. Changing the optimal film thickness will seriously affect TCR. Because the thinner deposition layer is easier to oxidize, the resistance degradation rate of high resistance films is very high

thick film resistance

as mentioned above, due to the influence of size, volume and weight, it is impossible to use wafer type for wire wound resistance. Although the accuracy is lower than that of wire wound resistors, thick film resistors are widely used because of their higher resistance density (high resistance value/small size) and lower cost. Like thin-film resistors and metal foil resistors, thick film resistors have fast frequency response, but among the resistance technologies currently used, their noise is the highest. Although the accuracy is lower than that of other technologies, the reason why we discuss thick film resistance technology here is that it is widely used in almost every kind of circuit, including the parts with low accuracy requirements in high-precision circuits

thick film resistance depends on the contact between particles in the glass matrix to form resistance. These contacts constitute a complete resistance, but the thermal strain in operation will interrupt the contact. In most cases, the thick film resistor will not open circuit in parallel, but the resistance value will continue to increase with time and temperature. Therefore, compared with other resistance technologies, the stability of thick film resistance is poor (time, temperature and power)

due to the movement of strings of charges in the structure, the granular structure will also cause high noise in the thick film resistance. Under a given size, the higher the resistance value, the less the metal composition, the higher the noise and the worse the stability. The glass component in the thick film resistance structure forms a glass phase protective layer during the resistance processing, so the humidity resistance of the thick film resistance is higher than that of the thin film resistance

metal foil resistance

it is very important for resistance molding to lay special metal foil with known and controllable characteristics on special ceramic substrate to form thermal engine balance force. Then, the ultra precision process is used to lithography the resistance circuit. This process combines the important characteristics of low TCR, long-term stability, no inductive reactance, no ESD induction, low capacitance, fast thermal stability and low noise into a resistance technology

these functions help to improve the stability and reliability of the system, and there is no need to compromise between accuracy, stability and speed. In order to obtain accurate resistance value, the resistance of large metal foil chip can be trimmed by selectively eliminating the internal "short board". When it is necessary to increase the resistance in a known increment, you can cut the marked area (Figure 2) and gradually increase the resistance in a small amount

the standard temperature coefficient formed by the comprehensive consideration of the alloy characteristics and the thermomechanical balance force between the alloy and the substrate is ± 1 ppm/° C in the range of 0 ° C to + 60 ° C (0.05 ppm/° C for Z foil) (Fig. 3)

when using flat foil, the parallel circuit design can reduce the impedance, and the maximum total impedance of the resistance is 0.08 uh. The maximum capacitance is 0.05 PF. The setting time of 1-k Ω resistance is less than 1 ns below 100 MHz. The rise time depends on the resistance value, but the higher and lower resistance values decrease slightly relative to the middle value. No ringing noise is very important for high-speed switching circuits, such as signal conversion

Under the frequency of

100 MHz, the comparison between the DC resistance of 1-k Ω large metal foil resistance and its AC resistance can be expressed by the following formula:

AC resistance/DC resistance = 1.001

Figure 4: large metal foil resistance structure

metal foil technology comprehensively combines highly ideal resistance characteristics that could not be achieved in the past, including low temperature coefficient (0.05 ppm/° C to + 60 ° C), with an error of ± 0.005% (as low as ± 0.001% when sealing is used), The stability of load life reaches ± 0.005% (50 ppm) at 70 ° C and 2000 hours of rated power on. The consistency between resistors is 0.1 ppm/° C at 0 ° C to + 60 ° C, and the resistance to ESD is as high as 25 kV

performance requirements

of course, not every designer's circuit needs all high-performance parameters. Resistors with relatively poor technical specifications can also be used in a large number of applications. The problems in this regard can be divided into four categories:

(1) existing applications can use all the performance upgrades of large metal foil resistors

(2) existing applications require one or more, but not all "industry best" performance parameters

(3) advanced circuits can only be developed with improved technical specifications using precision resistors

(4) purposefully plan in advance to use precision resistors to meet future upgrading requirements (for example, use resistors instead of active devices to maintain circuit accuracy, so as to save costs, otherwise it will significantly increase costs just to slightly improve performance)

for example, in the second (2) case, a parameter must be adjusted according to the economy of all parameters

Copyright © 2011 JIN SHI