Forward conduction voltage drop
Voltage drop : The change in potential (electrical potential) of the diode relative to the same reference point after the current flows through the load is called voltage drop, or voltage drop for short.
Forward voltage drop : The voltage corresponding to when the diode starts to conduct.
Forward characteristics : When a forward voltage is applied to the diode, at the beginning of the forward characteristics, the forward voltage is very small and insufficient to overcome the blocking effect of the electric field in the PN junction, and the forward current is almost zero. When the forward voltage is large enough to overcome the PN junction electric field, the diode conducts forward, and the current rises rapidly as the voltage increases.
Reverse characteristics : When the applied reverse voltage does not exceed a certain range, the current passing through the diode is the reverse current formed by the drift movement of minority carriers. Since the reverse current is very small, the diode is in the cut-off state. When the reverse voltage increases to a certain level, the diode breaks down in the reverse direction.
Relationship between forward voltage drop and on-state current
When a forward bias voltage is applied to both ends of the diode, its internal electric field area becomes narrower, and a larger forward diffusion current can pass through the PN junction. Only when the forward voltage reaches a certain value (this value is called the "threshold voltage", which is about 0.2V for germanium tubes and about 0.6V for silicon tubes), can the diode be truly turned on. But is the on-state voltage drop of the diode constant? What is the relationship between it and the forward diffusion current? Through the test circuit in Figure 1 below, the relationship between the on-state current and the on-state voltage drop of the diode model SM360A is tested at room temperature, and the curve relationship shown in Figure 2 can be obtained: the forward on-state voltage drop is proportional to the on-state current, and its floating voltage difference is 0.2V. Although the voltage difference from the light-load on-state current to the rated on-state current is only 0.2V, for power diodes, it not only affects the efficiency but also the temperature rise of the diode. Therefore, if the price conditions allow, try to choose a diode with a small on-state voltage drop and a rated working current that is twice the actual current.
Figure 1 Diode conduction voltage drop test circuit
Figure 2 Relationship between on-state voltage drop and on-state current
In the process of developing our products, the impact of high and low temperature environments on electronic components is the biggest obstacle to the stable operation of the products. The impact of ambient temperature on most electronic components is undoubtedly huge, and diodes are no exception. Through the relationship curve between the measured data of SM360A in Table 1 and Figure 3 under high and low temperature environments, it can be known that the conduction voltage drop of the diode is inversely proportional to the ambient temperature. Although the conduction voltage drop is the largest when the ambient temperature is -45℃, it does not affect the stability of the diode. However, when the ambient temperature is 75℃, the shell temperature has exceeded the 125℃ given in the data sheet, and the diode must be derated at 75℃. This is also one of the reasons why the switching power supply needs to be derated at a certain high temperature point.
Table 1 Test data of on-state voltage drop and on-state current
Figure 3 Relationship curve between on-state voltage drop and ambient temperature
Rated current, maximum forward current IF
The rated current IF refers to the average current value calculated based on the operating temperature rise when the diode is in long-term operation. Currently, the IF value of the maximum power rectifier diode can reach 1000A.
It refers to the maximum average forward current value allowed to pass through the diode when it is working continuously for a long time. Its value is related to the PN junction area and external heat dissipation conditions. Because when the current passes through the tube, the tube core will heat up and the temperature will rise. When the temperature exceeds the allowable limit (about 141 for silicon tubes and about 90 for germanium tubes), the tube core will overheat and be damaged. Therefore, under the specified heat dissipation conditions, the diode should not exceed the maximum rectification current value of the diode during use. For example, the rated forward working current of the commonly used IN4001-4007 germanium diode is 1A.
Maximum average rectified current Io
Maximum average rectified current IO: The maximum value of the average rectified current flowing through the load resistor in a half-wave rectifier circuit. This is a very important value in conversion design.
Maximum surge current IFSM
Excessive forward current flows during operation. It is not a normal current, but a transient current, and this value is quite large.
Maximum reverse peak voltage VRM
Even if there is no reverse current, as long as the reverse voltage is continuously increased, the diode will be damaged sooner or later. This reverse voltage that can be applied is not an instantaneous voltage, but a forward and reverse voltage that is repeatedly applied. Because the rectifier is applied with an AC voltage, its maximum value is an important factor in the regulation. The maximum reverse peak voltage VRM refers to the maximum reverse voltage that can be applied to avoid breakdown. The current highest VRM value can reach several thousand volts.
Maximum reverse voltage VR
The above maximum reverse peak voltage is the peak voltage applied repeatedly, and VR is the value of the continuously applied DC voltage. For DC current, the maximum DC reverse voltage is important for determining the allowable value and upper limit.
Maximum operating frequency fM
Due to the existence of the junction capacitance of the PN junction, when the operating frequency exceeds a certain value, its unidirectional conductivity will deteriorate. The fM value of the point contact diode is relatively high, above 100MHz; the fM of the rectifier diode is relatively low, generally not higher than several thousand Hz.
Reverse recovery time Trr
When the forward working voltage changes from forward voltage to reverse voltage, the ideal working condition of the diode is that the current can be cut off instantly. In fact, it usually takes a little time to delay. The amount that determines the delay of current cut-off is the reverse recovery time.
When current flows through a diode, it absorbs heat and increases its temperature. The maximum power P is the maximum value of the power. Specifically, it is the voltage across the diode multiplied by the current flowing through it. This limit parameter is particularly important for voltage regulator diodes and variable resistor diodes.
Reverse saturation leakage current IR
Refers to the current flowing through the diode when a reverse voltage is applied to both ends of the diode. This current is related to the semiconductor material and temperature. At room temperature, the IR of a silicon tube is nA (10-9A) level, and the IR of a germanium tube is mA (10-6A) level.
Derating (junction temperature derating)
Derating can improve product reliability and extend service life. Based on the theory that the life span doubles when the temperature is reduced by 10°C, the minimum derating junction temperature data for tubes with different rated junction temperatures are listed below.
Table 1 Diode Derating
|
Rated value TjM
|
125℃
|
150℃
|
175℃
|
200℃
|
|
TjM usable after derating
|
110℃
|
135℃
|
160℃
|
185℃
|
During the selection phase, you should consider whether the device has passed safety certification, especially power devices. Generally, the safety certification types widely accepted by various countries include UL (North America), CSA (Canada), TUV (Germany), VDE, etc.
Correctly select the circuit design, mechanical design and thermal design of the device and its surroundings to control the working conditions of the device in the whole machine, prevent various inappropriate stresses or operations from causing damage to the device, and thus maximize the inherent reliability of the device.
When designing a single board, the range of allowable changes in device parameters (including manufacturing tolerance, temperature drift, and time drift) should be relaxed to ensure that the single board can work normally when the device parameters change within a certain range.
It is prohibited to use axially inserted diode packages and open-junction diodes.
O/J is the wafer diffusion process of OPEN JUNCTION. After the wafer is diffused, it is sliced into grains. The edges of the grains are rough and the electrical properties are unstable. It is necessary to use a mixed acid (the main component is hydrofluoric acid) to wash off the edges, and then wrap them with silicone and encapsulate them into a mold. The reliability is poor.
GPP is the abbreviation of Glassivation passivation parts, which is a general term for glass passivation devices. This product is based on the existing product ordinary silicon rectifier diffuser, and a layer of glass is burned around the P/N junction surface of the tube core to be divided. Glass and single crystal silicon have good bonding characteristics, so that the P/N junction can be best protected from the intrusion of the external environment, thereby improving the stability of the device and making it extremely reliable.
The heat dissipation of O/J is not as good as that of GPP. The essential structures of the two are completely different: O/J chips need to be pickled, welded with copper sheets and packaged with silicone, and the internal structure appears larger than that of GPP; the rectifier bridge made of GPP chips does not require pickling, silicone application and other steps, and is directly welded to the copper connecting sheet of the rectifier bridge. The internal structure is smaller than that made of O/J chips. This leads to intuitive and habitual misunderstandings.
Comprehensive evaluation of GPP chip and OJ chip:
1. GPP chips complete glass passivation at the wafer stage and can be tested for VR probe testing, while OJ chips can only be tested for VR after the finished product is produced.
2. GPP chips with a VRM of 1000V are usually grooved and glass passivated from the P+ surface, and the table surface has a negative bevel structure (the surface electric field intensity is higher than the body), while OJ chips do not have a bevel when cut.
3. The glass passivation of the GPP chip is distributed in the pn junction area (unlike the GPRC chip, which implements glass passivation on the entire section, and the OJ chip applies silicone rubber protection to the entire section.
4. The GPP chip leaves a cutting damage layer due to mechanical cutting, while the cutting damage layer of the OJ chip can be removed by chemical etching.
5. GPP chip is passivated by special high temperature melting inorganic glass film, and its Tjm and HTIR stability are higher than that of OJ products protected by silicone rubber.
6. GPP chips are suitable for miniaturization, thinness, and LLP packaging, while OJ chips are suitable for lead wire packaging.
Differences in production process:
(1) OJ chips must go through steps such as welding, pickling, passivation, white glue coating, molding, curing and baking. Their electrical properties (reverse voltage) are closely related to the packaging pickling process. The conventional packaging form is plug-in type.
(2) GPP already includes pickling and passivation in the chip manufacturing process. Its electrical properties are directly determined by the chip, and the common sealing form is the patch type.
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