Diode (2)

2. Basic parameters of the diode
The basic parameters of the diode mainly include the maximum average rectifier current, the highest reverse operating voltage, reverse current and the highest operating frequency.
(1) Maximum average rectifier current IF (AV)
Maximum average rectifier current refers to the maximum forward average current allowed to pass through the diode when it is working for a long time. It is related to the area, material and heat dissipation conditions of PN junction. In practical application, the operating current should be less than IF (AV); otherwise, the junction temperature may be too high and the PN junction may be burned.
(2) Maximum reverse working voltage VRM
The maximum reverse operating voltage is the maximum reverse voltage allowed when the diode is applied in reverse. In practical applications, when the reverse voltage increases to the breakdown voltage VBR, the diode may be damaged by breakdown, so, usually take (1/2~2/3) VBR.
Diode (2)
(3) Reverse current IR
The reverse current is the reverse current when the diode is not broken down by the reverse. Theoretically, IR=IR (SAT), but in fact, IR is slightly larger considering surface leakage and other factors. The smaller IR is, the better the unidirectional conductivity of the diode is. In addition, IR is closely related to temperature, so attention should be paid when using it.
(4) Maximum working frequency fM
Maximum operating frequency refers to the highest frequency at which the ac signal is allowed to pass when the diode is working properly. In practical applications, do not exceed this value, otherwise the one-way conductivity of the diode will degrade significantly. The size of fM is mainly determined by the capacitance effect of the diode.
2.2.2 Measurement of diodes
Diode pin has positive and negative points, before detection, accurately distinguish pin polarity is the key link of diode detection. In general, the pin polarity of the diode can be identified from the identification information. You can also use a multimeter to judge.
Diodes are made of germanium semiconductor materials and silicon semiconductor materials, in the diode selection, replacement, accurate distinction between diode production materials is also very important. When a digital multimeter measures a diode, it actually measures the forward voltage of the diode, while a pointer multimeter measures the forward and reverse resistance of the diode. Note this difference.
To test the diode with a digital multimeter, first insert the red stylus into the "V ω" jack with polarity of "+", and the black stylus into the "COM" jack with polarity of "-". Adjust the multimeter range knob to set the digital multimeter in diode mode. When the multimeter screen displays in the range of 0.5 ~ 0.7V, it indicates that the diode is on, indicating that the measured diode is silicon diode. At this time, the red pen is connected with the anode of the diode, and the black pen is connected with the cathode of the diode, as shown in Figure 2-21. When "1" is displayed on the multimeter screen, the measured diode is unbiased. If the positive guiding voltage of the measured diode is 0.2 ~ 0.3V, it indicates that the diode is germanium diode, and the pin discrimination is the same as that of silicon tube. If the forward and reverse voltage is infinite or both zero (buzzer sound) or relatively close, it indicates that the diode has been open or short circuit or performance deterioration.
2.2.3 Understanding of rectifier bridge stack
A rectifier bridge reactor is a rectifier device consisting of two or four silicon diodes. Bridge reactor has half bridge, full bridge and three-phase bridge, our conventional equipment is the bridge rectifier circuit composed of four silicon diodes, also known as the full bridge rectifier circuit, commonly known as bridge reactor. The bridge heap text symbol is UR. The rectifier bridge stack can be composed of four silicon diodes or packaged as a whole. The external insulation plastic is used. The high-power rectifier bridge stack is wrapped with zinc metal shell outside the insulation layer to enhance heat dissipation, as shown in Figure 2-22. Figure 2-23 shows the bridge reactor application.
Symbol for full bridge rectifier, full bridge rectifier consists of four diodes with four pins. The connection point of the two diode negatives is the "positive pole" of the full-bridge DC output, and the connection point of the two diode negatives is the "negative pole" of the full-bridge DC output.
2.2.4 Measurement of bridge stack
Bridge reactor test can use digital multimeter diode test block, the test method is similar to diode test, can refer to the diode test method.

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