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Principle and selection of diodes
A veteran at the forefront of the semiconductor family, the most obvious feature is its unidirectional conductivity, which means the current should be flowing at one end but not the other (positive to negative).



We classify all kinds of materials as conductors, insulators and semiconductors based on their ability to conduct electricity. Semiconductor is a kind of material with special quality, its conducting ability is between conductor and insulator, so it is called semiconductor. Common semiconductor materials are silicon (Si) and germanium (Ge).



Below we will proceed from the basic knowledge of the secondary tube, the choice of diode, these two aspects to introduce the secondary tube.



Basic knowledge of



01
Classification of diodes



There are many types of diodes, divided into germanium diodes (Ge tubes) and silicon diodes (Si tubes) according to the semiconductor materials used; According to the core structure, it can be divided into point contact type diode, surface contact type diode and plane type diode.



According to the different USES of the diode, it can be divided into detection diode, rectifier diode, voltage regulator diode, switching diode, Schottky diode, led, etc.



02
Type naming method for diodes



(1) According to the model naming method of domestic semiconductor devices: the model naming of diode consists of five parts: main name, material and polarity, category, serial number and specification number (grade of the same class of products).







03
How diodes work



The main principle of diodes is to take advantage of the unidirectional conductivity of the PN junction, and add a lead and package to the PN junction to form a diode.

The crystal diode is a PN junction formed by P-type semiconductor and N-type semiconductor, and a space charge layer is formed on both sides of its interface, and a self-built electric field is built. When there is no applied voltage, the diffusion current caused by the difference in carrier concentration between the two sides of the PN junction and the drift current caused by the self-built electric field are equal and in the state of electrical equilibrium.

When there is a positive voltage bias, the mutual suppression of the external electric field and the self-built electric field causes the diffusion current of carrier to increase and causes the forward current. When there is a reverse voltage bias outside, the external electric field and the self-built electric field are further strengthened, forming a reverse saturation current independent of the reverse bias voltage value within a certain reverse voltage range.

When the applied reverse voltage is high to a certain extent, the electric field intensity in the space charge layer of the PN junction reaches a critical value, generating a multiplication process of carriers, generating a large number of electron hole pairs, and generating a large number of reverse breakdown current, which is called the diode breakdown phenomenon. The reverse breakdown of PN junction can be divided into Zener breakdown and avalanche breakdown.



Diode selection


04
Select by major parameters



(1) Rated forward operating current

Rated forward operating current refers to the maximum forward current allowed to pass through the diode for a long period of continuous operation.



(2) Maximum surge current

The maximum surge current is the excess forward current allowed to flow. It is not normal current, but transient current. Its value is usually about 20 times the rated forward operating current.



(3) The highest reverse operating voltage

When the reverse operating voltage applied to both ends of the diode reaches a certain value, the tube will break down and lose its unidirectional conductivity. In order to ensure the safety of use, the highest reverse working electric value is stipulated. For example, lN4001 diode reverse withstand voltage is 50V, lN4007 reverse withstand voltage is 1000V.



(4) Reverse current

Reverse current refers to the reverse current flowing through the diode under the action of specified temperature and maximum reverse voltage. The lower the reverse current, the better the performance of the one-way guide. The reverse current is closely related to temperature. For every 10¡æ increase in temperature, the reverse current doubles. Silicon diode has better stability than germanium diode at high temperature.



(5) Reverse recovery time

When the forward voltage changes to the reverse voltage, the current generally cannot be cut off instantaneously and has to be delayed for a little time. This time is the reverse recovery time. It directly affects the switching speed of the diode.



(6) Maximum power

The maximum power is the voltage applied to the diode times the current flowing through it. This limit parameter is special for voltage stabilizing diodes, etc.



(7) Frequency characteristics

Due to the junction capacitance, when the frequency reaches a certain degree, the capacitive reactance is small enough to short-circuit the PN junction. As a result, the diode loses its unidirectional conductivity and fails to work. The larger the PN junction area is, the larger the junction capacitance will be, and the less it can work at high frequencies.



05
The choice of different diodes



(1) Detection diode

Detection diode can generally choose point contact germanium diode, choose, should be based on the specific requirements of the circuit to choose a high working frequency, reverse current is small, forward current is large enough detection diode.



(2) Rectifier diode

Rectifier diodes are generally planar silicon diodes used in various power supply rectifier circuits. When selecting rectifier diodes, the parameters such as maximum rectifying current, maximum reverse working current, cut-off frequency and reverse recovery time should be considered.

The rectifying diode used in the common series stabilized voltage power supply circuit does not require high reverse recovery time of cut-off frequency. As long as the rectifying diode with the maximum rectifying current and the maximum reverse working current meet the requirements according to the requirements of the circuit, it is ok.



(3) Voltage regulator diode

The stabilized diode is generally used as a reference voltage source in the stabilized voltage supply or as a protective diode in the overvoltage protection circuit. The selected voltage regulator diode should meet the requirements of the main parameters in the application circuit.

The stable voltage value of the stabilized diode should be the same as the reference voltage value of the application circuit, and the maximum stable current of the stabilized diode should be about 50% higher than the maximum load current of the application circuit.



(4) Switching diode

Switching diodes are mainly used in home appliances and electronic equipment such as radio recorders, TV sets and DVD players, such as switching circuit, detection circuit, high-frequency pulse rectifier circuit, etc.

Medium speed switching circuit and detection circuit, can choose 2AK series of ordinary switching diodes. High-speed switching circuits can choose RLS series, 1SS series, 1N series, 2CK series of high-speed switching diodes, according to the main parameters of the application circuit (such as forward current, highest reverse voltage, reverse recovery time, etc.) to choose the specific type of switching diodes.



(5) Varactor diode

When selecting the variator diode, the working frequency, the highest reverse working voltage, the maximum forward current and the zero bias junction capacitance should be considered, and the variator diode with large variation of junction capacitance, high Q value and small reverse leakage current should be selected.



06
TVS diode selection



(1) Maximum operating voltage: TVS does not work when the circuit is in normal operation, so TVS's cut-off voltage must be greater than the maximum operating voltage of the protected circuit. But not too high, or the clamping voltage will be high. Therefore, when choosing the highest voltage, the working voltage of the protected circuit and the bearing capacity of the circuit should be considered comprehensively.

(2) TVS tube power selection: THE rated transient power of TVS product should be greater than the maximum transient surge power that may occur in the circuit.

(3) Clamping voltage: TVS clamp voltage should be less than the maximum transient safety voltage that the rear stage protected circuit can withstand.

(4) Leakage current: In some low-power circuits or high-precision acquisition circuits, leakage flow may lead to excessive circuit power consumption or excessive signal acquisition accuracy.

(5) Junction capacitance: TVS junction capacitance generally ranges from dozens of PI to dozens of na. For TVS of the same power level, the lower the voltage, the higher the capacitance. In some communication lines, note that the junction capacitance of TVS should not affect the normal operation of the circuit.

(6) Encapsulation: The power of TVS can also be reflected in the form of encapsulation. The smaller the encapsulation volume is, the smaller the power is generally, because the chip area of TVS directly determines the power level of TVS. TVS buyers can select TVS devices that fit the package according to circuit design and test requirements.



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