Hardware design resistance: super comprehensive and detailed
2024-04-18 17:06:04 34
1, the basic principle of resistance
Usually, resistance is defined according to Ohm's law, adding a constant voltage to the resistance, how much current will be generated; Or you can use Joule's law to define how much heat is generated per unit of time when a resistor flows through a current.
Similarly, the actual resistance is non-ideal, there is a certain lead inductance and electrode capacitance, when the application frequency is high, these factors can not be ignored.
Frequency characteristics of a thin film resistor
The high frequency characteristics of the resistance in the figure above are very good, you can see that the electrode capacitance is only 0.03pF, the lead inductance is only 0.002nH, and the resistance of 75Ω can reach 30GHz.
Most of the patch resistors we usually use are thick film resistors, and the performance is far from this, and the lead inductance has several nH, and the electrode capacitance also has several pF, most of which can only be used for several hundred MHz or several GHz.
The image above is from Vishay documentation
Usually resistance values are standard, the figure above gives different precision/tolerance resistance of the standard resistance values, usually multiplied by a multiple of 10 or divided by a multiple of 10, you can get all resistance values.
How to remember the above resistance table? In fact, just pay attention to the following three points:
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Different precision resistors correspond to different precision series. Usually 10% accuracy is E12 series, 2% and 5% are E24 series, 1% is E96 series, and 0.1%, 0.25% and 0.5% are E192 series.
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The number in the series name indicates that the series has several standard resistance values, usually multiples of 6. For example, the E12 series has 12 different resistance values, and the E192 series has 192 different resistance values.
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The resistance values of each series are approximately an geometric series, the common ratio is raised to the power of 10, and the base is 10Ω. For example, the common ratio of the E12 series is 10 ^ 12, and the common ratio of the E96 series is 10 ^ 96.
Interested can count according to the above table, calculate one is not the above law, in addition, according to the IEC regulations, 2% accuracy corresponds to the E48 series has 48 resistance values, interested can calculate which values.
Usually we use the most 5% and 1% chip resistors, generally more than 0603 resistance packages are marked to indicate the resistance value.
For resistance values greater than 10Ω, a 3-digit number is usually used to represent the resistance value, the first two represent the base of the resistance value, and the last digit represents the multiplication of 10 to the power.
For example, the mark 100 represents 10Ω, not 100Ω, and 472 represents 4.7kΩ. Less than 10Ω is usually denoted by R for the decimal point, such as 2R2 for 2.2Ω.
It is usually represented by 2 digits plus A letter, 2 digits represent the number of resistance values in the E96 series, and the letter represents the power of 10, where Y represents -1, X represents 0, A represents 1, B represents 2, C represents 3, and so on.
For example, 47C, counting 47 resistance values from the table, is 30.1, and C stands for multiplying by 10 to the third power, which is 30.1kΩ.
In addition, for the resistance of the axial lead package, the resistance value is marked by a circle of color rings, the specific meaning is shown in the following figure:
From left to right, the first two or three rings represent the number, the next rings represent the multiplier, and the multiplication of the previous numbers is the resistance value. The next ring represents the tolerance of the resistance, and finally the temperature coefficient of the resistance.
2. Process and structure of resistance
There are many kinds of resistance processes, which can be divided into two categories according to whether the resistance value can be changed:
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Fixed resistance
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Variable resistance
2.1 Fixed Resistance
Fixed resistance, as the name implies that the resistance value is fixed, immutable, most of the time, the resistance we use is a fixed value, can be roughly reclassified according to different packages.
The axis lead resistance is usually cylindrical, and the two outer electrodes are axial wires at both ends of the cylinder, which can be divided into a variety of different materials and processes.
The winding resistance is to wind a nickel-chromium alloy wire on an alumina ceramic substrate, and control the resistance size one circle at a time.
The winding resistor can be made as a precision resistor, the tolerance can be 0.005%, and the temperature coefficient is very low, the disadvantage is that the parasitic inductance of the winding resistor is relatively large, and can not be used for high frequency.
The volume of the winding resistor can be made very large, and then an external radiator can be used as a high-power resistor.
The carbon synthesis resistor is mainly made of carbon powder and adhesive together to form a cylindrical resistance body, where the concentration of carbon powder determines the size of the resistance value, adding tinned copper leads at both ends, and finally packaging.
Carbon synthesis resistor process is simple, raw materials are easy to obtain, so the price is the cheapest.
However, the performance of carbon synthesis resistance is not very good, the tolerance is relatively large (that is, it can not do precision resistance), the temperature characteristics are not good, and the noise is usually relatively large.
Carbon synthesis resistance has good voltage resistance, because the interior can be seen as a carbon rod, basically will not be broken down and cause to be burned.
The carbon film resistor mainly forms a layer of carbon mixture film on the ceramic rod, such as directly coating a layer, and the thickness of the carbon film and the carbon concentration in it can control the size of the resistance.
In order to more accurately control the resistance, spiral grooves can be processed on the carbon film, the more spiral the greater the resistance, and finally metal leads are added and resin packaging is formed.
The process of carbon film resistance is more complicated, and it can be a precision resistance, but because of the carbon quality, the temperature characteristics are not good.
Similar to the structure of the carbon film resistor, the metal film resistor mainly uses vacuum deposition technology to form a nickel-chromium alloy coating on the ceramic rod, and then machined spiral grooves on the coating to precisely control the resistance.
The metal film resistor can be said to have good performance and high precision, which can be made into the E192 series, and then the temperature characteristics are good, low noise, and more stable.
Similar to the metal film resistance structure, the metal oxide film is mainly formed in the ceramic rod a layer of tin oxide film, in order to increase the resistance, a layer of antimony oxide film can be added to the tin oxide film, and then a spiral groove is processed on the oxide film to precisely control the resistance. The biggest advantage of metal oxide film resistance is high temperature resistance.
2.1.2 Chip resistance
Metal foil resistance is formed by vacuum smelting nickel-chromium alloy, and then, by rolling into a metal foil, and then the metal foil bonded to the alumina ceramic substrate, and then through the lithography process to control the shape of the metal foil, so as to control the resistance. Metal foil resistors are the best resistors whose performance can be controlled to date.
The thick film resistor adopts the screen printing method, that is, paste a layer of silver palladium electrode on the ceramic substrate, and then print a layer of ruthenium dioxide between the electrodes as the resistor, the resistance film of the thick film resistor is usually relatively thick, about 100 microns, the specific process is shown in the following figure.
Thick film resistor is currently the most used resistor, the price is cheap, the tolerance is 5% and 1%, the vast majority of products are used in 5% and 1% sheet thick film resistor.
The film resistance is formed on the alumina ceramic substrate by vacuum deposition of chromium nickel thin film, usually only 0.1um thick, only one thousandth of the thick film resistance, and then the film is etched into a certain shape by the photolithography process.
2.2 Variable Resistance
Variable resistance is that the resistance value can be changed, there are two kinds: one is the resistance that can be manually adjusted; The other is that the resistance value can change according to other physical conditions.
2.2.1 Adjustable resistance
Remember when we were in middle school, we should all have used sliding rheostat to do experiments, move a sliding rheostat, small bulbs can be bright or dark, sliding rheostat is adjustable resistance, the principle is the same.
Adjustable resistors are usually divided into three types:
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Potentiometer/divider
Potentiometer or pressure divider, which is a three-port device. The potentiometer is divided into two resistors by the middle tap, and the resistance value of the two resistors can be changed by the middle tap, and the voltage can be changed.
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rheostat
In fact, it is a potentiometer, the only difference is that the rheostat only needs to use two ports, purely a resistor that can accurately adjust the resistance value.
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trimmer
In fact, it is also a potentiometer, but it does not need to be adjusted often, such as when the equipment is adjusted at the factory, usually need to use special tools such as screwdrivers to adjust.
2.2.2 Sensitive resistance
Sensitive resistor is a kind of sensitive element, most of this kind of resistor is particularly sensitive to a certain physical condition, the physical condition changes, the resistance value will change, usually can be used as a sensor, such as photoresistor, humidity resistor, magnetic resistor, etc., in the circuit design application more should be thermistor and varistor, commonly used as protection devices.
PTC is a positive temperature coefficient resistance, usually, there are two kinds: one is a ceramic material, called CPTC, suitable for high voltage and high current occasions; The other is a high polymer material, called PPTC, which is suitable for low voltage and small current occasions.
Ceramic PTC, the resistance material is a polycrystalline ceramic, is a mixture of barium carbonate, titanium dioxide and other materials sintered.
PTC temperature coefficient has a strong nonlinear, when the temperature exceeds a certain threshold, the resistance will become large, equivalent to the open circuit, which can play the role of short circuit and overcurrent protection.
At the same time, there is a negative temperature coefficient resistance, that is, NTC will not be detailed.
Varistors are usually metal oxide variable resistors, and their resistance materials are sintered together after mixing zinc oxide particles and ceramic particles.
The characteristic of MOV is that when the voltage exceeds a certain threshold, the resistance drops rapidly and can pass through a large current, so it can be used for surge protection and overvoltage protection.
The zinc oxide ceramics are made into multi-layer varistors using a similar process to MLCC, that is, MLV, the MLV package is smaller, usually sheet, the rated voltage and flow capacity are much smaller than MOV, suitable for low-voltage DC occasions.
3. Application and selection of resistance
Resistance manufacturers are mainly national giant, Panasonic, Rohm, Weishi, and domestic Fenghua high-tech and so on.
3.1 Application of resistance
Basically no circuit board will not use resistance, the most used devices on any circuit board are capacitors and resistors, various pull-down resistors, feedback resistors and so on.
According to Joule's law, the current flowing through the resistance will heat up, and the thermal effect of the resistance has many applications, such as electric blankets, electric fire barrels, and electric kettles.
For some outdoor applications of electronic equipment, especially for some integrated high-performance CPU SOC, the operating temperature requirements are very harsh, most can only meet the commercial applications, winter in the northeast, more than 30 degrees below zero, the temperature is too low, it is likely to be unable to open the machine.
Usually add a high-power resistor to do the pre-heating function, when the temperature comes up, the device starts and then turns off, all of which is turned off, because the power consumption of the device will also heat up, and the temperature can be maintained.
As a hardware engineer, often go to the environmental laboratory to locate the problem, in order to reproduce a high temperature problem, you need to go to the environmental laboratory to build a test environment, the key temperature box is so few, but also to make an appointment, often to queue up too troublesome.
So I made myself a very simple positioning artifact, that is, to cement resistance welding a DC power seat, and then plug in various power adapters, adjust the temperature.
Then put a few minutes on a certain chip, there is no problem, and then change another, the problem is repeated, the problem is focused on a chip, and the positioning of the high temperature problem is completed on your own station.
Zero ohm resistance is also called jumper resistance. In circuit design, it is often used for convenient debugging or compatible design. For example, in pre-research design, in order to test the working current of each group of power supply of the chip during debugging, it is usually necessary to divide the power supply into multiple channels with zero ohm resistance.
When using zero ohm resistance, the most common problem is how to calculate the power consumption, how to determine whether the selected resistance meets the requirements?
At this time, it is necessary to obtain the relevant parameters from the specification of the resistor, as can be seen from the following figure RC0402 zero ohm resistance, its resistance value will not exceed 50mΩ, rated current does not exceed 1A, so you can judge whether the resistance meets the design requirements, usually 0402 zero ohm resistance can meet the current requirements below 1A.
Sometimes a group of tens of MA power supply is needed in the circuit, but its voltage is not used elsewhere in the circuit, at this time it is not appropriate to make a group of DCDC or LDO alone, because the current is too small, at this time you can use a voltage regulator circuit.
Voltage divider such as ADC sampling circuit, DCDC output voltage feedback, level switching, etc.
For high-speed signals, PCB wiring needs to consider the transmission line model to ensure impedance matching and prevent signal reflection from affecting signal integrity.
Impedance matching is to ensure that the load impedance is equal to the characteristic impedance of the transmission line to eliminate reflection, the most commonly used and simplest is the source end series matching, that is, in the signal source end series a resistor, the sum of the resistance and the source internal resistance is equal to the characteristic impedance of the transmission line, so that even if the load end does not match, the signal will be reflected back by the source end signal, will not be reflected again.
In addition, there are a variety of nonlinear sensitive resistors that can be used as sensors, protection circuits, and so on.
3.2 Selection of resistance
Selection, simply put, is to extract key parameters according to the specifications of the device to determine whether it meets the requirements of the application.
3.2.1 Fixed resistance
For the main parameters of common types of resistors, as shown in the following figure, the largest shipments should be thick film resistors and metal film resistors.
3.2.2 Thermistor
The main role of PTC in the circuit is similar to the fuse, that is, overcurrent protection, the difference is that the fuse is one-time, and PTC is recoverable, and many times to change the fuse is unacceptable, affecting the customer experience, PTC is also a safety device, usually required to pass the UL1439 certification.
The figure above shows the impedance temperature characteristics of PTC. When the PTC overflows, the PTC heats up, the temperature rises rapidly, and the impedance of PTC increases rapidly, forming a break. After the break, the current drops, the heat decreases, the temperature drops, and PTC recovers low impedance.
When selecting PTC, first consider the design working current, can not exceed the PTC holding current, at this time PTC can maintain a low impedance state, PTC holding current will decrease with the increase of the operating temperature, therefore, the operating temperature needs to be considered an important factor.
The operating current, that is, the current that the PTC enters the high impedance state and breaks the protection.
That is, the maximum voltage that PTC can withstand, exceeding the rated voltage, PTC may be broken down short circuit, and then cause burning, therefore, the design should consider that the operating voltage of PTC in various cases can not exceed its rated voltage.
When the PTC break protection, it will withstand the entire power supply voltage, PTC selection, the rated voltage is greater than the power supply voltage, usually consider derating to 80%, that is, the power supply voltage of 12V, to choose a PTC with a voltage above 15V.
At the power input port, it is necessary to consider surge protection, at this time to consider the maximum surge current, multiplied by the resistance of PTC, that is, the surge voltage of PTC, can not exceed the rated voltage of PTC.
That is, under the rated voltage, the maximum short-circuit current that PTC can withstand. If the short-circuit current exceeds the rated current, PTC will be damaged.
The presence of PTC DC resistance will cause PTC to have a certain DC voltage drop, and the design should pay attention to the power supply voltage after the voltage drop to meet the requirements.
Compared with fuses, the rated voltage and rated current of PTC are much smaller, and the DC impedance of PTC is usually about two parts of the fuse.
When PTC protection is actually in a high resistance state, there will be a milliampere-level leakage current, and the fuse is a circuit breaker to cut off the current path, and there is basically no leakage current.
3.2.3 Varistor
The characteristics of varistors are similar to voltage regulator diodes and TVS, which are clamp-on devices, mainly used to protect the circuit from transient overvoltage, such as surges.
Ideal voltammetry characteristics of MOV
The selection of protective devices mainly considers two aspects: first, the protective devices cannot be operated or damaged under normal working conditions, and the second is to be able to play the role of protecting the circuit under abnormal circumstances within the design range, that is, the protection ability.
The rated operating voltage can be considered to be the highest continuous operating voltage that MOV can maintain a high impedance state, according to the application, MOV can be divided into AC and DC, the device specifications used in the two occasions are not the same, MOV for DC occasions usually can not be used in AC occasions.
MOV rated operating voltage, ac occasions consider AC rated voltage, that is, Vrms or Vm(ac), the device in the figure above can work normally at 130V RMS AC, more than this voltage, MOV may act or damage, resulting in the circuit can not work.
It is mainly used to protect against transient high voltage, continuous high voltage will cause MOV damage.
MOV is a clamp type device, when encountering transient high voltage, the impedance will decrease, through a large current, the transient high voltage will be suppressed, but will not be reduced to zero, but still maintain a relatively high voltage, usually 2 to 3 times the rated operating voltage.
When selecting MOV, it should be noted that the clamp voltage cannot exceed the highest voltage of the protected device, and when it exceeds, it is necessary to use multiple levels of protection, such as adding a high-power resistance decoupling after the stage, and adding a TVS, using the low clamp voltage of TVS to further reduce the residual voltage.
Lightning strikes or inductive load switching, etc., will produce a large surge current, MOV in addition to clamping high voltage, but also need to release the surge current.
MOV can withstand the surge current, mainly related to the amount of energy MOV withstand in a period of time, too much energy, MOV overheating burned.
The amount of energy is related to the number and waveform of the surge, and in general, the surge capacity of the device is tested according to the 8/20us waveform energy.
MOV in the figure above, a single 3500A 8/20us surge pulse, two consecutive 3000A 8/20us surge pulses, and 20 consecutive 750A 8/20us surge pulses.
In addition, the MOV parasitic capacitance is relatively large, can not be used in higher speed signal lines, MOV response time is slower than TVS, for some fast pulses, like ESD may not work, these are also factors we need to consider.
