What are the selection criteria for capacitance, resistance, and inductance?

The following content is for personal experience sharing, for reference only.

MP2451 power supply diagram of STC15 actual combat project:

1. Resistance

1. Selection basis

Resistance: resistance value;

Packaging: commonly used packages 0201, 0402, 0603, 0805, 1206, 1812, etc .;

Power consumption: 1 / 16W, 1 / 10W, 1 / 8W, 1 / 4W, 1 / 2W, 1W, 2W, 3W, etc .;

Accuracy: 1%, 5%, etc.

2. Selection method

① Priority is given to the resistance value. For the uncommon resistance value, it can be replaced by series and parallel connection of resistance;

②. Calculate power consumption: P = I²R or U² / R, according to the power consumption, choose a reasonable package, generally the maximum power consumption of 0402 is 1 / 16W, the maximum power consumption of 0603 is 1 / 10W, and the maximum power consumption of 0805 is 1 / 8W, see the resistor selection manual for details.

③, consider accuracy: as the feedback loop, voltage divider resistance, generally choose the accuracy of 1%, others, select the accuracy of 5%;

④, consider the cost: accuracy of 1% is more expensive than accuracy of 5%. Different packages have different prices. Commonly used packages are slightly cheaper;

⑤Consider the PCB size: the smaller the board, the smaller the package as much as possible. For example, mobile phones are generally used for 0201, mini products, 0402 are used more, and general products are mostly 0603;

⑥, consider processing: the larger the size, the lower the requirements for processing technology. 0603 and 0402 can be produced by general processing plants, this is mainly related to the processing plant machines;

⑦, consider maintenance: the smaller the size, the greater the difficulty of welding.

3. Selection examples

According to the MP2451 power supply diagram of the STC15 actual combat project:

①Considering that there is no requirement for the size of the PCB board, manual soldering may be required, and the resistor is usually used in 0603 package;

②, R23 and R24 are feedback resistors, and the accuracy is 1%;

③ The resistance power consumption is relatively small, and no larger package size is selected.

2. Capacitance

1. Selection basis

Capacitance: capacitance value;

Capacitor type: ceramic capacitor, aluminum electrolytic capacitor, tantalum electrolytic capacitor, etc .;

Parasitic parameters: ESR, affect the filtering effect;

Packaging: plug-in packaging, patch packaging;

Price: affect product cost;

Size: affect the structure;

Accuracy: The ceramic capacitor is greatly affected by temperature, and the electrolytic capacitor changes less.

2. Selection method

① Prioritize the capacity value. Capacitors are energy storage devices. The larger the capacitance, the more energy they can provide in an instant. The larger the load instantaneous current is, the larger the capacitance value is selected. If the capacitance value is too small, a large enough current cannot be provided in an instant, and the voltage will be pulled down, producing ripples, affecting other circuits. In addition, due to the existence of parasitic parameters, there is a frequency response, the larger the capacitance is, the better, the best. For example, the internal circuit of the integrated circuit is mainly a switching circuit, and the frequency is relatively high, generally 0.1uF;

② After the capacitance value is determined, select the type of capacitor. Generally, if it is less than 10uF, ceramic capacitor is preferred. If it exceeds 10uF and less than several hundred uF, aluminum electrolytic and tantalum electrolytic capacitors can be selected. If it exceeds several hundred uF, aluminum electrolytic capacitors are generally selected;

③, choose the package again, the package selection needs to consider the withstand voltage, life, structure, processing, cost, etc., such as aluminum electrolytic capacitors, the plug-in is cheaper, and the patch is good for processing;

④ In terms of withstand voltage, ceramic capacitors have better withstand voltage, and tantalum electrolytic capacitors have poorer withstand voltages. The tantalum electrolytic capacitors should have a withstand voltage greater than about twice the circuit voltage;

⑤ In terms of life, aluminum electrolytic capacitors are poor, and the internal electrolyte is greatly affected by temperature. When designing, do not close to the heat source;

⑥ In other aspects, the price of tantalum electrolytic capacitors is higher, but the packaging is small, and the filtering effect is good; aluminum electrolytic capacitors are cheap and the filtering effect is slightly worse. Generally, they are connected in parallel with ceramic capacitors to reduce ESR and improve the filtering effect.

3. Selection examples

According to the MP2451 power supply diagram of the STC15 actual combat project:

①, C24, MP2451 input filter capacitor, the capacitance value is as large as possible, the input voltage is 5-28V, here the 22uF / 35V aluminum electrolytic capacitor is selected to meet the requirements;

② C22 and C23 are connected in parallel to improve the filtering effect.

3. Inductance

1. Selection basis

Inductance value: inductance value;

Inductor type: power inductor, common inductor, common mode inductor, etc .;

Inductance parameters; DCR, saturation current, temperature rise current;

Packaging: plug-in packaging, patch packaging (commonly used);

Size: affect the structure.

2. Selection method

①, preferentially choose the type of inductor: signal filtering, generally choose 0603, 0402 and other commonly used chip packages; power circuit, generally choose power inductor, power inductor shielded EMC effect is better, but the price is slightly more expensive;

②, select the sense value. The filtering used is calculated according to the signal frequency; the power inductance is generally recommended according to the power IC manual, which affects the ripple and the dynamic response of the load;

③, according to DCR, saturation current, temperature rise current to choose the appropriate size, generally speaking, the larger the size, the better the parameters, but the more expensive the price, at the same time, the structure will be limited;

④, the smaller the DCR, the better, according to the power I²R, the greater the DCR, the greater the heat generation, and the lower the power conversion efficiency of the power supply;

⑤ The saturation current must be greater than the maximum current flowing through the inductor. If the inductor is saturated, the inductance value will decrease sharply and lose the energy storage effect, which is equivalent to a wire and damage the circuit;

⑥ The greater the temperature rise current, the better, and the smaller the heat.

3. Selection examples

According to the MP2451 power supply diagram of the STC15 actual combat project:

L1 is the power inductance, the saturation current is 1.4A, which is much larger than the load current.

4. Summary

Resistance, capacitance, and inductance are indispensable passive components in the circuit. They need to be reasonably selected in application, which not only meets the function of the circuit but also meets the requirements of product quality.

There are many knowledge points involved in resistance, capacitance and inductance. This article is only a brief introduction. It only plays a role.

If you want to know more, our website has product specifications for capacitance, resistance, and inductance, you can go to ALLICDATA ELECTRONICS LIMITED to get more information