Introduction to Types of Diodes

Catalog

I Types of diodes

There are many types of diodes: according to materials, there are germanium diodes, silicon diodes, gallium arsenide diodes, etc.; they can be divided into surface contact diodes and point contact diodes according to the production process; according to different purposes, they can be divided into rectifier diodes, detection diodes, Zener diodes, varactor diodes, photodiodes, light-emitting diodes, switching diodes, fast recovery diodes, etc.; according to the type of connection, they can also be divided into semiconductor junction diodes, metal-semiconductor contact diodes, etc.; according to the package form, they can be divided into conventional packaged diodes, special packaged diodes, etc. The following introduces the characteristics of different types of diodes according to different purposes.

1.Rectifier diode 

The role of the rectifier diode is to rectify the AC power into pulsating DC power. It works by using the one-way conductive characteristics of the diode. Because the forward working current of the rectifier diode is large, the surface contact structure is mostly used in the process. The junction capacitance of this structure is relatively large, so the operating frequency of the rectifier diode is generally less than 3kHz.

Rectifier diodes are mainly available in hermetically sealed metal structure packages and plastic packages. Under normal circumstances, the rectifier diode with a rated forward current T of LF above 1 A is packaged in a metal case to facilitate heat dissipation; the rated forward operating current is less than 1 A in an all-plastic package. Besides, due to the continuous improvement of T-technology, there are also many high-power rectifier diodes in plastic packages, which should be distinguished in use.

Figure 1 bridge rectifier circuit

Because the rectifier circuit is usually a bridge rectifier circuit (as shown in Figure 1), some manufacturers package four rectifier diodes together. Such redundant parts are often called rectifier bridges or rectifier full bridges. The shape of a common rectifier diode is shown in Figure 2.

Figure 2 rectifier diode

When selecting a rectifier diode, the parameters such as its maximum rectification current, maximum reverse current, cut-off frequency, and reverse recovery time should be considered. The rectifier diodes used in ordinary series regulated power supply circuits do not have high requirements on the reverse recovery time of the cutoff frequency. As long as the maximum rectifier current and the maximum reverse working current meet the requirements, select a rectifier diode (such as the N series 2CZ series, RLR series, etc.). Rectifier diodes used in rectifier circuits and pulse rectifier circuits of switching regulated power supplies should use rectifier diodes or fast recovery diodes with higher operating frequencies and shorter reverse recovery times.

2.Detection diode

A detection diode is a device that detects low-frequency signals superimposed on a high-frequency carrier wave, which has high detection efficiency and good frequency characteristics.

Figure 3 detection diode

Detector diodes require small forward voltage drop, high detection efficiency, small junction capacitance, and good frequency characteristics. The shape of the detector diode is generally EA glass package structure. The general detection diode uses a germanium material point contact type structure.

When selecting a detection diode, a detection diode with a high operating frequency, a small reverse current, and a sufficiently large forward current should be selected according to the specific requirements of the circuit.

3.Switching diode

A switching diode is a kind of semiconductor diode, which is specially designed and manufactured for "on" and "off" on the circuit. The time required for it to change from on to off or from off to on is shorter than that of ordinary diodes. Commonly there are 2AK, 2DK, and other series, which are mainly used in electronic computers, pulses, and switching circuits. Switching diodes are mainly used in home appliances such as radio cassette recorders, televisions, video players, and electronic equipment, including switching circuits, detection circuits, and high-frequency pulse rectifier circuits.

Figure 4 Switching diode

The semiconductor diode is equivalent to the switch being closed (the circuit is on) when it is on, and equivalent to being open (the circuit is off) when it is off, so the diode can be used as a switch. The commonly used model is 1N4148. Because the semiconductor diode has the characteristic of unidirectional conduction, the PN junction is turned on under positive bias, and the resistance in the on-state is very small, about tens to several hundred ohms; under reverse bias, it is turned off. Its resistance is very large. Generally, silicon diodes are above 10 megaohms, and germanium tubes also have tens of thousands of ohms to hundreds of thousands of ohms. With this feature, the diode will play a role in controlling the current on or off in the circuit and become an ideal electronic switch.

For medium-speed switching circuits and detection circuits, 2AK series ordinary switching diodes can be selected. The high-speed switching circuit can choose RLS series, 1sS series, 1N series, 2CK series high-speed switching diodes. We should choose the specific model of the switching diode according to the main parameters of the application circuit (such as forward current, maximum reverse voltage, reverse recovery time, etc.)

4.Zener diode

Zener diode uses the characteristics that the voltage does not change with the change of current when the PN junction reverse breakdown to achieve the purpose of voltage stabilization. The Zener diode is divided according to the breakdown voltage, and its voltage regulation value is the breakdown voltage value. Zener diodes are mainly used as regulators or voltage reference components. Zener diodes can be connected in series to obtain a higher voltage value.

Figure 5 Zener diode

The selected Zener diode should meet the requirements of the main parameters in the corresponding circuit. The stable voltage value of the Zener diode should be the same as the reference voltage value of the application circuit. The maximum stable current of the Zener diode should be about 50% higher than the maximum load current of the application circuit.

5. Fast Recovery Diode (FR D)

Fast Recovery Diode is a new type of semiconductor diode. This kind of diode has good switching characteristics and a short reverse recovery time. It is usually used as a rectifier diode in high-frequency switching power supplies. The fast recovery diode is characterized by its short recovery time, which makes it suitable for high frequency (such as the line frequency in TV) rectification. The fast recovery diode has an important parameter that determines its performance-reverse recovery time. The definition of the reverse recovery time is that the diode transitions sharply from the forward conducting state to the off state, starting from the output pulse falling to the zero lines.

Superfast recovery diodes (SRDs) are developed based on fast recovery diodes. The main difference between them is that the reverse recovery time is shorter. The reverse recovery time of a common fast recovery diode is several hundred nanoseconds, and the reverse recovery time of an ultra-fast recovery diode (SRD) is generally tens of nanoseconds. The smaller the value, the higher the operating frequency of the fast recovery diode.

Figure 6 Fast Recovery Diode 

When the operating frequency is in the range of tens to hundreds of kHz, the time of normal and reverse voltage change of ordinary rectifier diodes is slower than the recovery time, and ordinary rectifier diodes cannot normally work in unidirectional conduction. But fast recovery rectifier diodes are competent at this time. Therefore, rectifier diodes powered by switching power supplies for color TVs and other household appliances are usually fast recovery diodes and cannot be replaced by ordinary rectifier diodes. Otherwise, electrical appliances may not work properly. The shape of a common fast recovery diode is shown in Figure 6.

6. Schottky diode (sBD)

Schottky diode is an abbreviation of the Schottky Barrier Diode (SBD for short). Schottky diodes are low-power, high-current, ultra-high-speed semiconductor devices produced in recent years. Its reverse recovery time is extremely short (it can be as small as a few nanoseconds), the forward voltage drop is only about 0.4 V, and the rectified current can reach several thousand amps. These excellent characteristics are unmatched by fast recovery diodes.

Figure 7 Schottky diode

Schottky diodes are metal-semiconductor devices made of precious metals (gold, silver, aluminum, platinum, etc.) as positive electrodes and N-type semiconductors as negative electrodes. Schottky diodes are commonly used in high-frequency, high-current, low-voltage rectifier circuits. The appearance of a common Schottky diode is shown in Figure 7.

7. Transient voltage suppression diode

The transient voltage suppression diode is referred to as a TVP tube (transient-voltage-suppressor). It is a semiconductor device developed based on the process of a voltage regulator tube and is mainly used in fast overvoltage protection circuits for voltage. It can be widely used in computers, electronic meters, communication equipment, household appliances, and field-borne / marine and automotive electronic equipment for field operations. It can also be used as a protective element for the overvoltage impact caused by human operation or an electric shock to equipment.

Figure 8 transient voltage suppression diode

Transient voltage suppression diodes can be divided into four categories according to their peak pulse power: 500 w, 1000W, 1500W, 5000w. Each category is divided into several types according to its nominal voltage. When the voltage at both ends of the transient voltage suppression diode is higher than the rated value, it will be turned on instantaneously. Clamped to a predetermined value. The shape of the transient voltage suppression diode is shown in Figure 8.

8. LED

The abbreviation of the light-emitting diode is LED, which is a device made of semiconductor materials such as gallium phosphide and gallium phosphide arsenide, which can directly convert electrical energy into light energy. In addition to the unidirectional conductivity of ordinary diodes, light-emitting diodes can also convert electrical energy into light energy. When a forward voltage is applied to the light-emitting diode, it is also in a conducting state. When a forward current flows through the die, the light-emitting diode emits light and converts electrical energy into light energy.

The color of the light-emitting diode is mainly determined by the material of the tube and the type of impurities. At present, the common light-emitting diodes are mainly blue, green, yellow, red, orange, white, and so on. Among them, the white light-emitting diode is a new type of product, which is mainly used in the fields of mobile phone backlight, LCD backlight, and lighting. The working current of the light-emitting diode is usually 2 to 25 mA. The working voltage (that is, the forward voltage drop) varies with different materials: the working voltage of ordinary green, yellow, red, and orange light-emitting diodes is about 2v; the working voltage of white light-emitting diodes is usually higher than 2.4V; blue light-emitting diodes’ working voltage is usually higher than 3.3V. The working current of the LED must not exceed the rated value too high, otherwise, there is a danger of burnout. Therefore, a resistor R is usually connected in series in the LED circuit as a current limiting resistor.

Figure 9 LED

An infrared light-emitting diode is a special kind of light-emitting diode. Its appearance is similar to light-emitting diodes, but it emits infrared light, which is invisible to the human eye under normal circumstances. Its working voltage is about 1.4v, and the working current is generally less than 20mA. Some companies package two light-emitting diodes of different colors together to make them two-color diodes (also known as color-changing light-emitting diodes). This light-emitting diode usually has three pins, one of which is a common terminal. It can emit three colors of light (one of which is a mixture of two colors), so it is usually used as an indicator for different working states. The shape of a common light-emitting diode is shown in Figure 9.

9. Avalanche Diode

The avalanche diode is a microwave power device developed based on the technology of the voltage regulator tube. It can generate high-frequency oscillation under the action of an applied voltage.

Figure 10 avalanche diode

Avalanche diodes use avalanche breakdown to inject carriers into the crystal. Because carriers take a certain amount of time to cross a semiconductor wafer, their current lags behind the voltage and a delay occurs. A negative resistance effect will occur on the voltage relationship, which will cause high-frequency oscillation. It is often used in microwave communication, radar, tactical missile, remote control, telemetry, instrumentation, and other equipment.

10.DIAC

The DIAC (diode for alternating current) is a diode that conducts electrical current only after its break over-voltage, VBO, has been reached momentarily. DIACs are also called symmetrical trigger diodes. It is a silicon bidirectional voltage triggering switching device. When the voltage applied across the symmetrical trigger diode exceeds its breakdown voltage, the two ends are turned on, and the conduction will continue until the current is interrupted or decreases to the minimum holding current of the device. Shut down again. Symmetrical trigger diodes are commonly used in overvoltage protection circuits, phase shift circuits, thyristor trigger circuits, and timing circuits. 

Figure 11 diode for alternating current

11. Varactor diode

A varactor diode or variable capacitance diode (abbreviated as VCD) is a special semiconductor device that uses reverse bias to change the capacitance of a PN junction. A varactor diode is equivalent to a variable capacitance capacitor. The size of the PN junction capacitance between its two electrodes changes with the magnitude of the reverse voltage applied across the varactor diode. When the reverse voltage applied across the varactor diode increases, the capacity of the varactor diode decreases. Because the varactor diode has this characteristic, it is mainly used in an electric tuning circuit (such as a high-frequency head of a color television) as an automatic trimming capacitor that can be controlled by voltage.

Figure 12 varactor diode

When selecting a varactor diode, it is important to consider whether its operating frequency, maximum reverse working voltage, maximum forward current, and zero bias junction capacitance, and other parameters meet the requirements of the application circuit.    

II Identification and detection of diodes

1. Identification of the diode

Crystal diodes are usually represented by VD plus numbers in the circuit, such as VD5 means the diode with the number 5. In national standard circuits, the symbols of commonly used diodes are shown in Figure 13.

Figure 13 symbols of diode

The identification of a diode is simple: the negative pole of a low-power diode is usually marked with a color ring on the surface; some diodes also use the "P" and "N" symbols to determine the polarity of the diode, "P" represents the positive electrode, and "N" represents the negative electrode; Metal-packaged diodes are usually printed with a diode symbol of the same polarity on the surface; light-emitting diodes usually use positive and negative pins to identify the positive and negative poles, with the long feet being positive and the short feet being negative.

The surface of the rectifier bridge is usually marked with the internal circuit structure or the names of the AC input terminal and the DC output terminal. The AC input terminal is usually represented by "AC" or "~"; the DC output terminal is usually represented by "+" and "~" symbols.

Because of the various shapes of chip diodes, their polarities are also marked in a variety of ways: in a chip diode with leads, the end of the tube with a white color ring is the negative electrode; in a chip diode with leads and no color ring, the longer end of the lead is positive; in the SMD diode without lead, the end with a ribbon or a notch is negative.

2. Diode detection

When using a pointer type multimeter to detect a diode, the smaller value of the primary black test lead is connected to the positive end, and the red test lead is connected to the negative end. Both the forward and reverse resistances are infinite, indicating that the diode has been damaged by the open circuit; if the forward and reverse resistances are all 0, it indicates that the diode has been short-circuited and damaged. Under normal circumstances, the forward resistance of the germanium diode is about 1.6kΩ.

When using a digital multimeter to measure the diode, the red test lead is connected to the positive pole of the diode, and the black test lead is connected to the negative pole of the diode. At this time, the measured resistance is the forward resistance of the diode.  

If the digital multimeter's diode block is used to detect the diode, it is more convenient to put the digital multimeter in the diode block, and then connect the negative pole of the diode to the black multimeter of the digital multimeter, and the positive pole to the red multimeter. Diodes of different materials have different forward voltage drop values: 0.55 to 0.7V for silicon diodes and 0.15 to 0.3V for germanium diodes. If the display shows "0000", it means that the tube is short-circuited; if it shows "0L" or "overload", it means that the diode is internally open or in the reverse state.  

III Main parameters of diodes

Different types of diodes have different characteristics. For beginners, you must know the following main parameters:

1. Rated forward working current

The rated forward working current refers to the maximum forward current value allowed by the diode during continuous long-term T operation. Because when the current passes through the tube, the core will be heated, the temperature will rise, and the temperature will exceed the allowable limit (about 140°C for silicon tube and about 90°C for germanium tub). Therefore, do not exceed the rated forward working current of the diode during use. For example, the commonly used lN400l germanium diode has a rated forward operating current of 1 A.

2. Maximum inrush current

The maximum inrush current is the excess forward current allowed to flow. It is not a normal current, but an instantaneous current. This value is usually about 20 times the rated forward working current.

3．Maximum reverse working voltage

When the reverse voltage applied to both ends of the diode reaches a certain value, the tube will break down and lose its unidirectional conductivity. To ensure safety, the maximum reverse working voltage value is specified. For example, the reverse withstand voltage of the lN400l diode is 50V, and the reverse withstand voltage of lN4007 is 1000v.

4．Reverse current

Reverse current refers to the reverse current flowing through the diode under the specified temperature and maximum reverse voltage of the diode. The smaller the reverse current, the better the unidirectional conductivity of the tube. It is worth noting that the reverse current has a close relationship with temperature. For every 10°C increase in temperature, the reverse current doubles. For example, 2APl type germanium diode at 25°C, the reverse current is 250μA; the temperature rises to 35°C, the reverse current will rise to 500μA; and at 75°C, its reverse current has reached 8mA. Directional conductivity also damages the tube due to overheating. Silicon diodes have better stability at higher temperatures than germanium diodes.

5．Reverse recovery time

When changing from forwarding voltage to reverse voltage, the ideal situation is that the current can be cut off instantaneously. But it will be generally delayed. What decides the amount of current cut-off delay is the reverse recovery time. Although it directly affects the switching speed of the diode, it is not necessary to say that this value is small.

6. Maximum power

The maximum power is the voltage across the diode multiplied by the current flowing. This limit parameter is particularly important for Zener diodes and the like.

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