1 forward voltage drop

pressure drop: The change in the potential (potential) of the diode relative to the same reference point after the current flows through the load is called the voltage drop, or voltage drop for short.

turn-on voltage drop: The corresponding voltage when the diode starts to conduct.

positive characteristics: When a forward voltage is applied to the diode, at the beginning of the forward characteristic, the forward voltage is small enough 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 electric field of the PN junction, the diode conducts in the forward direction, and the current rises rapidly as the voltage increases.

reverse characteristic: When the applied reverse voltage does not exceed a certain range, the current through the diode is the reverse current formed by the drift movement of minority carriers. Since the reverse current is small, the diode is in the off state. After the reverse voltage increases to a certain extent, the diode reverses breakdown.

The relationship between forward voltage drop and on current

When a forward bias voltage is applied across the diode, its internal electric field area is narrowed, 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 "threshold voltage", the germanium tube is about 0.2V, and the silicon tube is about 0.6V), the diode can really be turned on. But is the conduction voltage drop of the diode constant? What is the relationship between it and the forward diffusion current? Through the test circuit shown in Figure 1 below, conduct the conduction current and conduction of the diode of type SM360A at room temperature The relationship between the voltage drop can be tested, and the curve relationship shown in Figure 2 can be obtained: the forward voltage drop is proportional to the current, and the floating voltage difference is 0.2V. Although the voltage difference from the light-load on-current to the rated on-current is only 0.2V, for power diodes, it not only affects the efficiency but also affects the temperature rise of the diode. Therefore, if the price allows, try to choose the on-voltage drop. Small diodes with rated operating current twice as high as the actual current.

Figure 1 Diode conduction voltage drop test circuit

Figure 2 The relationship between the on-voltage drop and on-current

In the process of developing our products, the influence of high and low temperature environment on electronic components is the biggest obstacle to the stable work of products. The influence of ambient temperature on most electronic components is undoubtedly huge, and diodes are no exception. In the high and low temperature environment, we can know from the relationship curve between the measured data table 1 and Figure 3 of SM360A: the conduction voltage drop of the diode Inversely proportional to ambient temperature. When the ambient temperature is -45°C, although the conduction voltage drop is the largest, it does not affect the stability of the diode. However, when the ambient temperature is 75°C, the case temperature has exceeded the 125°C given in the data sheet. It must be derated when used at 75°C. This is also one of the factors why the switching power supply needs to be derated at a certain high temperature point.

Table 1 On-voltage drop and on-current test data

Figure 3 The relationship between the on-state voltage drop and the ambient temperature

2 rated current,Maximum forward current IF

The rated current IF refers to the average current value converted from the operating temperature rise when the diode runs for a long time. At present, the IF value of the maximum power rectifier diode can reach 1000A.

It refers to the maximum forward average current value allowed to pass when the diode works continuously for a long time, and its value is related to the PN junction area and external heat dissipation conditions. Because when the current passes through the tube, the die 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 die will be overheated and damaged. Therefore, under the specified heat dissipation conditions, the diode should not exceed the maximum rectified current value of the diode during use. For example, the rated forward working current of the commonly used IN4001-4007 germanium diode is 1A.

3 Maximum average rectified current Io

Maximum average rectified current IO: In a half-wave rectifier circuit, the maximum value of the average rectified current flowing through the load resistance. A very important value when discounting a design.

4 Maximum inrush current IFSM

Excessive forward current flowing through the operation. Not a normal current, but an instantaneous current, which is quite large.

5 Maximum reverse peak voltage VRM

Even if there is no reverse current, as long as the reverse voltage is continuously increased, sooner or later the diode will be damaged. This reverse voltage that can be added is not an instantaneous voltage, but a forward and reverse voltage that is repeatedly added. Since the AC voltage is applied to the rectifier, its maximum value is an important factor specified. 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.

6 Maximum reverse voltage VR

The above-mentioned maximum reverse peak voltage is the peak voltage that is repeatedly applied, and VR is the value of the continuous DC voltage. For DC current, the maximum DC reverse voltage is important to determine the allowable and upper limit values.

7 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 higher, above 100MHz; the fM of the rectifier diode is lower, generally not higher than several thousand Hz.

8 Reverse recovery time Trr

When the forward working voltage changes from forward voltage to reverse voltage, the ideal situation of diode operation is that the current can be cut off instantaneously. In practice, there is usually a slight delay. The amount that determines the current cut-off delay is the reverse recovery time.

9 Maximum power P

When a current flows through the 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 load diode multiplied by the current flowing through it. This limit parameter is particularly important for Zener diodes and variable resistance diodes.

10 Reverse saturation leakage current IR

Refers to the current flowing through the diode when a reverse voltage is added across the diode, and the current is related to the semiconductor material and temperature. At normal temperature, the IR of silicon tube is nA (10-9A) level, and the IR of germanium tube is mA (10-6A) level.

11 Derating (junction temperature derating)

Derating can improve product reliability and prolong service life. According to the theory that the service life is doubled when the temperature is lowered by 10°C, the minimum derating junction temperature data of tubes with different rated junction temperatures are listed below.

Table 1 Diode Derating

Rating TjM 125℃ 150℃ 175℃ 200℃
TjM available after derating 110℃ 135℃ 160℃ 185℃

12 Safety

In the selection stage, it should be considered whether the device has passed the safety certification, and the main consideration should be the power device. Generally, the safety certification types widely accepted by various countries are UL (North America), CSA (Canada), TUV (Germany), VDE, etc.

13 Design for reliability

Correctly select the circuit design, mechanical design and thermal design of the device and its surrounding to control the working conditions of the device in the whole machine, prevent various inappropriate stresses or operations from causing damage to the device, so as to maximize the inherent characteristics of the device. reliability.

14 Tolerance design

When designing a veneer, the allowable variation range of device parameters (including manufacturing tolerance, temperature drift, and time drift) should be relaxed to ensure that the veneer can work normally when the device parameters change within a certain range.

15 No package selection

It is forbidden 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. Mixed acid (the main component is hydrofluoric acid) needs to be used to wash off the edges. , and then wrap it with silicone and encapsulate it, which is less reliable.

GPP is the abbreviation of GlassivaTIon passivaTIon parts, which is a general term for glass passivation devices. This product is to fire a layer of glass around the P/N junction of the die to be divided on the basis of the existing ordinary silicon rectifier diffuser. Glass and monocrystalline silicon have good bonding characteristics, so that the P/N junction can get the best protection from the intrusion of the external environment, improve the stability of the device, and have excellent reliability.

The heat dissipation of O/J is not as good as that of GPP, and the essential structures of the two are completely different: O/J chips need to be pickled, then welded with copper sheets and packaged with silica gel, and the internal structure is larger than that of GPP; rectifier bridges made of GPP chips The steps of pickling and applying silica gel are omitted, and it is directly welded to the copper connection piece of the rectifier bridge. The internal structure is significantly smaller than that made by the O/J chip. This leads to intuitive and habitual misunderstandings.

Comprehensive evaluation of GPP chips and OJ chips:

1. The glass passivation of the GPP chip is completed in the wafer stage, and the probe testing of VR can be implemented, while the OJ chip can only test the VR after the finished product is made.

2. GPP chips with a VRM of 1000V are usually slotted and glass passivated from the P+ surface, and the mesa has a negative bevel structure (surface electric field strength is higher than the body), while the cutting of OJ chips has no bevel.

3. The glass passivation of the GPP chip is distributed in part of the pn junction area (unlike the GPRC chip that applies glass passivation to the entire section, and the OJ chip that 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. The GPP chip is passivated with special high-temperature molten inorganic glass film, and the stability of Tjm and HTIR is higher than that of OJ products protected by silicone rubber.

6. GPP chips are suitable for miniaturization, thinning, and LLP packaging, while OJ chips are suitable for lead-out packaging.

Differences in production process:

(1) The OJ chip must go through the steps of welding, pickling, passivation, white glue, molding, curing and baking. Its electrical properties (reverse voltage) are closely related to the packaging pickling process, and 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 SMD type.

Reviewing Editor: Tang Zihong

Leave a Reply

Your email address will not be published.