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How many of 15 key elements of diode selection do you know?



Forward conduction voltage drop

Voltage Drop: After diode current flows through load, change in potential (potential) about same reference point is called voltage drop, called voltage drop.

Voltage Drop: The voltage at which diode begins to conduct current.

Forward characteristic: When a forward voltage is applied to diode, forward voltage is very small at beginning of forward characteristic, which is not enough to overcome blocking effect of diode. electric field in PN junction, current is almost zero. When forward voltage is large enough to overcome electric field of PN junction, diode is forward-conducting and current rises rapidly as voltage increases.

Reverse characteristic: When applied reverse voltage does not exceed a certain range, current passing through diode is reverse current generated by drifting movement of minority carriers. Since reverse current is very small, diode is in a cutoff state. After increasing reverse voltage to a certain extent, reverse breakdown of the diode occurs.

How many of 15 key elements of diode selection do you know?

Relationship between direct conduction voltage drop and conduction current

When a forward bias voltage is applied to both ends of a diode, its internal electric field area shrinks, and a large forward diffusive current can flow through PN junction. Only when forward voltage reaches a certain value (this value is called “threshold voltage”, germanium tube is about 0.2 V and silicon tube is about 0.6 V), diode can actually conduct current. But is conduction voltage drop of a diode constant? What is relationship between it and direct diffusion current? Through test circuit in Figure 1 below, conduct conduction current and conductance of SM360A diode model at room temperature. voltage drop can get curve dependence shown in Figure 2: forward conduction voltage drop is proportional to conduction current, and floating voltage difference is 0.2V. Although voltage difference from light load conduction current to rated conduction current is only 0.2V, it affects not only on efficiency, but also on diode heating for power diodes, so if price condition allows, try to choose a conduction voltage drop A diode with a low rated operating current, twice actual current.

How many of 15 key elements of diode selection do you know?

Fig. 1. Diode conduction voltage drop test circuit

How many of 15 key elements of diode selection do you know?

Figure 2. Relationship between conduction voltage drop and conduction current

In process of developing our products, exposure to high and low temperatures on electronic components is biggest obstacle to stable operation of products. The effect of ambient temperature on most electronic components is undoubtedly huge, and diodes are no exception. Under high and low temperature conditions, we can learn from relationship curve between measured data of SM360A Table 1 and Figure 3: Diode conduction voltage drop Inversely proportional to ambient temperature . Although conduction voltage drop is maximum at an ambient temperature of -45°C, it does not affect stability of diode, however, at an ambient temperature of 75°C, case temperature exceeds 125°C indicated in Data Sheet At 75°C, it is necessary to derate . This is also one of factors why switching power supplies should be reduced at a certain high temperature point.

Table 1. Conduction voltage drop and conduction current test data

How many of 15 key elements of diode selection do you know?
How many of 15 key elements of diode selection do you know?

Fig. 3. Relationship between conduction voltage drop and ambient temperature


Rated current, Maximum forward current if

The rated current IF refers to average current converted according to rise in operating temperature when diode is in continuous operation. At present, IF value of maximum power rectifier diode can reach 1000A.

refers to maximum direct average current allowed to pass through a diode when it is operated continuously for a long time, and its value is related to area of ​​the PN junction and external conditions of heat dissipation. Because when current passes through tube, tube core will heat up and temperature will rise, and when temperature exceeds allowable limit (about 141 for a silicon tube and about 90 for a germanium tube), tube core will heat up. overheated and damaged. Therefore, under given conditions of heat dissipation, maximum value of rectifying current of diode should not be exceeded during use of diode. For example, a commonly used germanium diode type IN4001-4007 has a rated forward operating current of 1A.


Maximum average rectified current Io

Maximum Average Rectification Current IO: In half-wave rectification, maximum value of average rectification current flowing through load resistance. Very important when transforming a design


IFSM Maximum Inrush Current

Excessive forward current during operation. Not normal current, but instantaneous current, and this value is quite large.


VRM Maximum Reverse Peak Voltage

Even if there is no reverse current, as long as reverse voltage is constantly increased, diode will fail sooner or later. This type of reverse voltage that can be added is not an instantaneous voltage, but a forward and reverse voltage that is repeatedly added. Since rectifier is supplied with AC voltage, its maximum value is an important factor to be determined. The maximum reverse peak voltage VRM refers to maximum reverse voltage that can be added to avoid breakdown. Currently, maximum value of VRM can reach several thousand volts.


Maximum Reverse Voltage VR

The above maximum reverse peak voltage is repeatedly applied peak voltage, and VR is value of continuously applied DC voltage. For DC, maximum DC reverse voltage is important for determining allowable and upper limit values.


Maximum operating frequency fM

Due to capacitance of PN junction, when operating frequency exceeds a certain value, its unidirectional conduction deteriorates. The fM value of point diode is higher, above 100 MHz, fM value of rectifier diode is lower, as a rule, not more than a few thousand Hz.


Reverse cooldown Trr

When forward operating voltage changes from forward to reverse, ideal situation for diode to operate is that current can be turned off instantly. In practice, there is usually a slight delay. The value that determines current cutoff delay is reciprocal of release time.


Max power P

When current flows through a diode, it absorbs heat and raises its temperature. The maximum power P is maximum power value. In particular, voltage across a loaded diode is multiplied by current flowing. This limiting parameter is especially important for zener diodes and diodes with variable resistance.


Reverse saturation leakage current IR

refers to current flowing through a diode when a reverse voltage is applied to diode, which depends on semiconductor material and temperature. At room temperature, IR of a silicon tube is in nA (10–9 A), and IR of a germanium tube is in mA (10–6 A)


Derating (derating due to junction temperature)

Derating can improve product reliability and extend service life. According to theory that service life doubles with a 10°C drop in temperature, the minimum transition temperature data for pipes with different transition temperature ratings is shown below.

How many of 15 key elements of diode selection do you know?


Safety rules

At selection stage, consider whether device has passed safety certification, mainly power devices should be considered. Generally, types of safety certification widely accepted in different countries include UL (North America), CSA (Canada), TUV (Germany), VDE, etc.


Reliability Engineering

Properly select circuit design, mechanical design, and thermal design of device and its environment to control working conditions of device in whole machine, prevent damage to device by various inappropriate loads or operations, and maximize efficiency. device performance The inherent reliability of a device.


Tolerance design

When designing a board, allowable range of device parameters (including manufacturing tolerances, temperature and time drift) must be relaxed to ensure that board operates normally when device parameters change within a certain range.


Package not allowed

It is forbidden to use axially inserted diode blocks, and it is also forbidden to use open-junction diodes.

O/J is an OPEN JOINT plate diffusion process. After plate is dispersed, it is cut into grains. The grain edges are rough and electrical properties are unstable. A mixed acid (mostly composed of hydrofluoric acid) is required. Rinse off edges, then wrap with silicone and encapsulate, which is less secure.

GPP is an acronym for Glassivation Passivation Parts and is a general term for glass passivation devices. Laminated glass, glass and monocrystalline silicon have good adhesion characteristics, so that P/N connection can be best protected from outside environment. , and stability of device can be improved, and reliability is excellent.

O/J's heat dissipation is not as good as GPP's, and basic structure of two materials is completely different: O/J chips need to be etched and then welded with copper sheets to pack in silica gel. , and internal structure is larger than that of GPP; The rectifier bridge eliminates pickling and silica gel steps and is directly welded to copper connector of rectifier bridge. The internal structure is much smaller than O/J chips. This causes an intuitive and habitual misunderstanding.

Comprehensive evaluation of GPP and OJ chips:

1. Glass passivation of GPP chip is completed at substrate stage, after which VR sensor testing can be implemented, while OJ chip can only be tested for VR after finished product has been produced.

2. For GPP chips with VRM 1000V, grooves are usually made from P+ surface and glass is passivated.

3. Glass passivationThe GPP chip is distributed in region of p-n junction part (in contrast to glass passivation of entire section of GPRC chip and silicone rubber protection of entire section of OJ chip).

4. GPP chips leave a layer of cut damage due to mechanical cutting, while cut damage layer of OJ chips can be removed by chemical etching.

5. GPP chips are passivated by a special high temperature molten inorganic glass film, and stability of Tjm and HTIR is higher than OJ products protected by silicone rubber.

6. GPP chips are suitable for miniaturization, thinning and LLP packaging, while OJ chips are suitable for hatcher packaging.

Difference in production process:

(1) The OJ chip must go through welding, pickling, passivation, PVA bonding, shaping, curing and drying. Its electrical properties (reverse voltage) are closely related to packaging and pickling process. Regular packing form pluggable.

(2) GPP already incorporates etching and passivation into chip manufacturing process. Its electrical properties are directly determined by chip, and a common form of encapsulation is SMD.