How the Dual Schottky Limiting Diode Works. Schottky diode assemblies in computer power supplies. Diagnosing Schottky diodes

A semiconductor diode, which uses the barrier effect in principle, is named after the German scientist who described it, Walter Schottky.

Important! The barrier effect is a serious influence of the total space charge on the development of a discharge in a gap with a sharply uneven field.

Additional Information. What is a diode - an electronic element that has an unequal ability to conduct electric current, depending on its direction.

Schottky diode: how it works

The Schottky valve differs from the classical type in that the basis of its operation is a semiconductor-metal pair. This pair is often referred to as the Schottky Barrier. This barrier, in addition to the ability to conduct electricity in one direction, similar to the p-n junction, has several useful features.

Gallium arsenide and silicon are the main suppliers of material for the production of an electronic element in an industrial environment. In more rare cases, precious chemical elements are used: platinum, palladium and the like.

Its graphic conditional expression on electrical circuits does not coincide with classical diodes. Electronic markings are similar. There are also double diodes in the form of an assembly.

Important! A double diode is a pair of diodes combined in a common volume.

Dual Schottky Barrier Diode

For double valves, the outputs of the cathodes or anodes are aligned. It follows that such a product has three ends. Common cathode assemblies, for example, work where switching power supplies are required. Schottky diodes with a common anode are used much less often.

The diodes are in a single package and use the same production technology for their manufacture, therefore, in terms of their set of parameters, they are like twins. They also operate at the same temperature. are in a common space. This property significantly reduces the need to replace them due to loss of performance.

The most important distinguishing properties of the valves under consideration are a slight forward voltage drop (up to 0.4 V) at the moment of transition and high time triggering.

However, the mentioned value of the voltage drop has a narrow range of the applied voltage - no more than 60 V. And this value itself is small, which sets a rather narrow range of applications for these diodes. If the voltage exceeds the specified value, the barrier effect disappears, and the diode begins to work in the mode of a conventional rectifier diode. The reverse voltage for most of them does not go beyond 250 V, however, there are samples with a reverse voltage of 1.2 kV.

When designing electrical circuits, designers often do not graphically highlight the Schottky diode on schematic diagrams, however, in the specification for the order they indicate its use, prescribing it in the type. Therefore, when ordering equipment, you need to pay close attention to this.

Of the inconveniences in working with valves with a Schottky barrier, it is necessary to note their extreme "tenderness" and intolerance to the slightest, even very short in time, excess of the reverse voltage rating. In this case, they simply fail and are no longer restored, which, in comparison with silicon diodes, does not benefit them, because the latter have the property of self-healing, after which they can continue to work as usual without requiring replacement. Also, we must not forget that the reverse current in them critically depends on the degree of transition. When a significant value of reverse current appears, breakdown cannot be avoided.

Increased operating frequency due to low transient capacitance and short recovery period due to serious response speed are positive properties that allow these diodes to be used, for example, by radio amateurs. They are also used at frequencies reaching several hundred kHz, for example, in pulse rectifiers. A large number of of the produced diodes goes away for use in microelectronics. The modern level of development of science and industry makes it possible to use nano technologies in the process of manufacturing Schottky-barrier valves. The valves created in this way are used to bypass transistors. This decision seriously increases the triggering of the latter.

Schottky diodes in power supplies

IN computer units supply very often located Schottky valves. The five-volt voltage provides a serious current of tens of amperes, which is a record for low-voltage power systems. For these power supplies, Schottky valves are used. Basically, dual diodes with a single cathode are used. Not a single high-quality modern power supply unit for computers is complete without such an assembly.

Diagnosis."Burnt out" power supply unit electronic device most often means the need to replace a burnt Schottky assembly. There are only two reasons for the malfunction: increased leakage current and electrical breakdown. When the described conditions occur, the power supply to the computer stops being supplied. Defense mechanisms worked. Let's see how this happens.

There is no voltage at the input of the computer on a permanent basis. The power supply is completely blocked by the protection built into the computer.

There is an "incomprehensible" situation: the cooling fan starts to work, then again the characteristic noise disappears. This means that the voltage at the input of the computer (output of the power supply unit) appears and disappears. Those. protection works out periodic errors, but is in no hurry to block the source completely. Do you have an unpleasant odor from the hot block? Diode block definitely requires replacement. Another way of home diagnostics: under heavy load central processing unit the supply unit has shut down by itself. This is a sign of a leak.

After the repair of the power supply connected with the replacement of the double Schottky diodes, it is necessary to "ring" the transistors. In the reverse procedure, the diodes also need to be checked. This rule is especially true if the cause of the repair was a leak.

Schottky diode test

A household multimeter does a good job of testing any kind of Schottky barrier diode. The test method is very similar to checking an ordinary diode. However, there are secrets. Electronic element with a leak, it is especially difficult to check correctly. First, the diode assembly must be removed from the circuit. This requires a soldering iron. If the diode is broken, then the resistance is close to zero in all possible modes work will tell you about its inoperability. In physical processes, this resembles a closure.

A "leak" is more difficult to diagnose. The most common multimeter for the population is the dt-830, in most cases measurements in the "diode" position will not see the problem. When the regulator is moved to the "ohmmeter" position, the ohmic resistance will go to infinity. Also, the device should not show the presence of ohmic resistance. Otherwise, replacement is required.

Schottky diodes are common in electrical and electronics applications. The area of ​​their use is wide, up to alpha radiation receivers and various spacecraft.

Video

First, with a short-term excess of the maximum reverse voltage, the Schottky diode irreversibly breaks down, unlike silicon diodes, which go into reverse breakdown mode, and provided that the maximum power dissipated on the diode is not exceeded, after a voltage drop, the diode completely restores its properties.

Secondly, Schottky diodes are characterized by increased (relative to conventional silicon diodes) reverse currents, which increase with increasing crystal temperature. For the aforementioned 30Q150, the reverse current at maximum reverse voltage varies from 0.12mA at + 25 ° C to 6.0mA at + 125 ° C. For low-voltage diodes in TO-220 cases, the reverse current can exceed hundreds of milliamperes (MBR4015 - up to 600 mA at + 125 ° C). In case of unsatisfactory heat dissipation conditions, the positive Feedback the heat in the Schottky diode leads to its catastrophic overheating.

The volt-ampere characteristic of the Schottky barrier has a pronounced asymmetric appearance. In the forward bias region, the current grows exponentially with increasing applied voltage. In the region of reverse biases, the current does not depend on voltage. In both cases, with forward and reverse bias, the current in the Schottky barrier is due to the main charge carriers - electrons. For this reason, Schottky barrier diodes are fast-acting devices, since they do not contain recombination and diffusion processes. The asymmetry of the current-voltage characteristic of the Schottky barrier is typical for barrier structures. The voltage dependence of the current in such structures is due to a change in the number of carriers participating in charge transfer processes. Role external stress consists in changing the number of electrons passing from one part of the barrier structure to another.

Schottky diodes in power supplies

IN system blocks power supply, Schottky diodes are used to rectify the current of the + 3.3V and + 5V channels, and, as you know, the magnitude of the output currents of these channels is tens of amperes, which leads to the need to take very seriously the issues of rectifier performance and reduce their energy losses. The solution to these issues can significantly increase the efficiency of power supplies and improve the reliability of the power transistors of the primary part of the power supply.

So, to reduce the dynamic switching losses and eliminate the short circuit mode during switching, in the most high-current channels (+ 3.3V and + 5V), where these losses are most significant, Schottky diodes are used as rectifier elements. The use of Schottky diodes in these channels is due to the following considerations:

· The Schottky diode is a practically inertialess device with a very short recovery time of the reverse resistance, which leads to a decrease in the reverse secondary current and to a decrease in the inrush current through the collectors of the power transistors of the primary part at the moment of switching the diode. This greatly reduces the load on the power transistors and, as a result, increases the reliability of the power supply.

· The forward voltage drop across the Shoki diode is also very small, which, at a current of 15-30 A, provides a significant gain in efficiency.

Since in modern power supplies the + 12V voltage channel also becomes very powerful, the use of Schottky diodes in this channel would also give a significant energy effect, but their use in the + 12V channel is impractical. This is due to the fact that with a reverse voltage of more than 50V (and in the + 12V channel, the reverse voltage can reach 60V), Schottky diodes begin to switch poorly (too long and significant reverse leakage currents occur), which leads to the loss of all the advantages their application. Therefore, high-speed silicon pulse diodes are used in the + 12V channel. Although the industry is now producing Schottky diodes with a large reverse voltage, their use in power supplies is considered impractical due to different reasons, including the economic plan. But there are exceptions to any rules, therefore, in separate power supplies you can find Schottky diode assemblies in + 12V channels.

In modern system power supplies for computers, Schottky diodes are, as a rule, diode assemblies of two diodes (diode half-bridges), which clearly increases the manufacturability and compactness of power supplies, and also improves the conditions for cooling the diodes. The use of individual diodes rather than diode assemblies is now an indicator of a poor quality power supply.

Diode assemblies are mainly available in three types of packages:

· TO-220 (less powerful assemblies with operating currents up to 20 A, sometimes up to 25-30A);

· TO-247 (more powerful assemblies with operating currents of 30 - 40 A);

· TO-3P (powerful assemblies).

The electrical characteristics of diode assemblies most commonly used in modern system power supplies are presented in Table 1.

The interchangeability of diode assemblies is determined based on their characteristics. Naturally, if it is impossible to use a diode assembly with absolutely the same characteristics, it is better to replace it with a device with high values ​​of current and voltage. Otherwise, guarantee stable work power supply will not be possible. There are cases when manufacturers use diode assemblies in their power supplies with a significant power reserve (although more often you have to observe the situation, just the opposite), and during repairs, you can install a device with lower values ​​of current or voltage. However, with such a replacement, it is necessary to carefully analyze the characteristics of the power supply and its load, and all responsibility for the consequences of such a modification, of course, falls on the shoulders of the repairman.

An element like Schottky diode although it was invented quite a long time ago, it appeared in the use of radio amateurs relatively recently and this was due to the fact that the Schottky diode has two very important and useful properties: firstly, very high speed and secondly, a small drop in forward voltage at the junction.
Previously, these two factors were not of particular importance, but in modern equipment operating at higher frequencies than before, Schottky diode simply irreplaceable.

let's consider Schottky diode device(it is also called Schottky barrier diode).

The most interesting thing about a Schottky diode is that it does not have a p-n junction (!). Instead, a metal-semiconductor transition is made (see the picture)

Legend in the figure: 1- semiconductor substrate, 2- epitaxial film; 3 - metal-semiconductor contact; 4 - metal film; 5 - external contact.

When passing electric current through such a transition, the excess of electrons will be distributed over the near-contact area of ​​the metal terminal, creating a kind of barrier (it was called the Schottky barrier) and due to this, rectifying properties are formed. Moreover, the height of the barrier can also be changed, thereby changing the properties of the diode.

Schottky diode designation in the diagram

In the diagrams, the Schottky diode is indicated like this:

How to check a Schottky diode

As mentioned above, a Schottky diode has a low voltage drop across the junction: While the usual silicon diodes have a forward voltage drop of about 0.6-0.7V, germanium about 0.4V, and even less about 0.2V for a Shootka diode. And since the multimeter when checking shows nothing more than a voltage drop at the junction, then the readings will be small: if, when checking ordinary diodes, the multimeter readings are about 300 ... 400 for germanium and 450 ... 650 for silicon diodes, then when checking Schottky diode, the multimeter will show 100 ... 150.

Disadvantages of a Schottky diode

It seems that the Schottky diode is good for everyone: it also works at high-frequency currents and has no reverse capacitance and the voltage drop across it is minimal, but nevertheless, with all its charms, the Schottky diode has drawbacks:

With a short-term excess of the maximum reverse voltage, the Schottky diode irreversibly breaks down (short circuit - short circuit), unlike conventional silicon pn diodes that go into the reversible mode breakdown, and, provided that the maximum power dissipated on the diode is not exceeded after the voltage drop, the diode fully restores its properties.

Schottky diodes are characterized by increased (relative to conventional silicon p-n diodes) reverse currents, which increase with increasing crystal temperature. For the 30CPQ150, the reverse current at maximum reverse voltage ranges from 0.12 mA at +25 ° C to 6.0 mA at +125 ° C. For low-voltage diodes in TO220 packages, the reverse current can exceed hundreds of milliamperes (MBR4015 - up to 600 mA at +125 ° C). Under unsatisfactory heat removal conditions, positive heat feedback in the Schottky diode leads to its catastrophic overheating.

What does a Schottky diode look like? Yes, like the most ordinary diode, and it can only be identified by marking and by circuit designation

During the assembly of power supplies and voltage converters for car amplifiers, there is often a problem with rectifying current from the transformer. Getting powerful pulse diodes is a rather serious problem, so I decided to print an article in which complete list and parameters of powerful Schottky diodes. Some time ago, I personally had a problem with a converter rectifier for an auto amplifier. The converter is quite powerful (500-600 watts), the output voltage frequency is 60 kHz, any common diode that can be found in old junk will immediately burn out like a match. The only available option at that time was the domestic KD213A. The diodes are quite good, they hold up to 10 Amperes, the operating frequency is within 100 kHz, but they also overheated terribly under load.

In fact, almost everyone has powerful diodes. A computer power supply unit is one that powers an entire computer. As a rule, they are made with a power of 200 watts to 1 kW or more, and since the computer is powered by direct current, then there must be a rectifier in the power supply. In modern power supplies for voltage rectification, powerful Schottky diode assemblies are used - it is they who have the minimum voltage drop at the junction and the ability to work in impulse circuits where the operating frequency is much higher than the 50 Hertz mains frequency. Recently, several power supplies were brought for free, from where the diodes were removed for this small review. In computer power supplies, you can find a variety of diode assemblies, there are almost no single diodes here - there are two powerful diodes in one case, often (almost always) with a common cathode. Here is some of them:

D83-004 (ESAD83-004) - Powerful build from Schottky diodes, reverse voltage 40 Volts, permissible current 30A, in pulsed mode up to 250A - perhaps one of the most powerful diodes that can be found in computer power supplies.

STPS3045CW- Double Schottky diode, rectified current 15A, forward voltage 570mV, reverse leakage current 200μA, reverse constant voltage 45V.

The main Schottky diodes that are found in power supplies

Schottky TO-220 SBL2040CT 10A x 2 = 20A 40V Vf = 0.6V @ 10A
Schottky TO-247 S30D40 15A x 2 = 30A 40V Vf = 0.55V @ 15A
Ultrafast TO-220 SF1004G 5A x 2 = 10A 200V Vf = 0.97V at 5A
Ultrafast TO-220 F16C20C 8A x 2 = 16A 200V Vf = 1.3V @ 8A
Ultrafast SR504 5A 40V Vf = 0.57
Schottky TO-247 40CPQ060 20A x 2 = 40A 60V Vf = 0.49V @ 20A
Schottky TO-247 STPS40L45C 20A x 2 = 40A 45V Vf = 0.49V
Ultrafast TO-247 SBL4040PT 20A x 2 = 40A 45V Vf = 0.58V at 20A
Schottky TO-220 63CTQ100 30A x 2 = 60A 100 Vf = 0.69V @ 30A
Schottky TO-220 MBR2545CT 15A x 2 = 30A 45V Vf = 0.65V @ 15A
Schottky TO-247 S60D40 30A x 2 = 60A 40-60V Vf = 0.65V @ 30A
Schottky TO-247 30CPQ150 15A x 2 = 30A 150V Vf = 1V @ 15A
Schottky TO-220 MBRP3045N 15A x 2 = 30A 45V Vf = 0.65V @ 15A
Schottky TO-220 S20C60 10A x 2 = 20A 30-60V Vf = 0.55V @ 10A
Schottky TO-247 SBL3040PT 15A x 2 = 30A 30-40V Vf = 0.55V @ 15A
Schottky TO-247 SBL4040PT 20A x 2 = 40A 30-40V Vf = 0.58V @ 20A
Ultrafast TO-220 U20C20C 10A x 2 = 20A 50-200V Vf = 0.97V at 10A

There are also modern domestic high-current diode assemblies. Here are their markings and internal diagram:

Also produced , which can be used, for example, in the power supply unit of tube amplifiers and other equipment with increased power supply. The list is given below:

High voltage power Schottky diodes up to 1200 V

Although it is more preferable to use Schottky diodes in low-voltage power rectifiers with output voltages of a couple of tens of volts, at high switching frequencies.

The development of electronics requires ever higher standards from radio components. To operate at high frequencies, a Schottky diode is used, which surpasses silicon counterparts in its parameters. Sometimes you can find the name Schottky barrier diode, which basically means the same thing.

• Design
• Miniaturization
• Practical use

Design

The Schottky diode differs from ordinary diodes in its design, which uses a metal-semiconductor, and not a p-n junction. It is clear that the properties are different here, which means that the characteristics should also be different.

Indeed, a semiconductor metal has the following parameters:

• Leakage current is of great importance;
• Low voltage drop across the junction with direct connection;
• Replenishes the charge very quickly, as it has a low charge.

The Schottky diode is made from materials such as gallium arsenide, silicon; much less common, but can also be used - germanium. The choice of material depends on the properties that need to be obtained, however, in any case, the maximum reverse voltage for which these semiconductors can be manufactured is not higher than 1200 volts - these are the highest-voltage rectifiers. In practice, however, they are much more often used at a lower voltage - 3, 5, 10 volts.

On the schematic diagram Schottky diode is designated as follows:

But sometimes you can see this designation:

This means a double element: two diodes in one package with a common anode or cathode, so the element has three leads. Power supplies use such designs with a common cathode; they are convenient to use in rectifier circuits. Often the marking of a conventional diode is drawn on the diagrams, but the description indicates that this is Schottky, so you need to be careful.

Schottky barrier diode assemblies are available in three types:

Type 1 - with a common cathode;

Type 2 - with a common anode;

Type 3 - according to the doubling scheme.

Such a connection helps to increase the reliability of the element: after all, being in the same case, they have the same temperature regime, which is important if powerful rectifiers are needed, for example, 10 amperes.

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But there are also disadvantages. The thing is that a small voltage drop (0.2–0.4 V) in such diodes manifests itself at low voltages, as a rule, 50–60 volts. At a higher value, they behave like conventional diodes. But in terms of current, this circuit shows very good results, because it is often necessary - especially in power circuits, power modules - for the operating current of semiconductors to be at least 10A.

Another major drawback: for these devices, the reverse current cannot be exceeded, even for a moment. They immediately fail, while silicon diodes, if their temperature has not been exceeded, restore their properties.

But there is still more positive. Besides the low voltage drop, the Schottky diode has a low junction capacitance value. As you know: the lower the capacity, the higher the frequency. Such a diode has found application in impulse blocks power supply, rectifiers and other circuits, with frequencies of several hundred kilohertz.

The I - V characteristic of such a diode is asymmetric. When a forward voltage is applied, it can be seen that the current grows exponentially, and with the opposite, the current does not depend on voltage.

All this can be explained if you know that the principle of operation of this semiconductor is based on the movement of the main carriers - electrons. For the same reason, these devices are so fast-acting: they do not have the recombination processes inherent in devices with p-n transitions... All devices with a barrier structure are characterized by an asymmetric I - V characteristic, because it is the number of carriers electric charge due to the dependence of the current on the voltage.

Miniaturization

With the development of microelectronics, special microcircuits, single-chip microprocessors began to be widely used. All this does not exclude the use of hinged elements. However, if radioelements of ordinary sizes are used for this purpose, then this will negate the whole idea of ​​miniaturization as a whole. Therefore, frameless elements were developed - smd components, which are 10 or more times smaller than conventional parts. The I – V characteristics of such components are no different from the I – V characteristics of conventional devices, and their reduced dimensions allow the use of such spare parts in various micro-assemblies.

Smd components are available in several sizes. For manual soldering, smd size 1206 is suitable. They have a size of 3.2 by 1.6 mm, which allows them to be soldered on their own. Other smd elements are more miniature, assembled at the factory with special equipment, and it is impossible to solder them yourself, at home.

Principle of operation smd component also does not differ from its large analogue, and if, for example, we consider the I – V characteristic of a diode, then it will equally be suitable for semiconductors of any size. The current ranges from 1 to 10 amperes. The marking on the case often consists of a digital code, the decoding of which is given in special tables. You can test them for suitability with a tester, like large analogues.

Practical use

Schottky rectifiers are used in switching power supplies, voltage stabilizers, pulse rectifiers. The most demanding current - 10A and more - are 3.3 and 5 volts. It is in such secondary power supply circuits that Schottky devices are most often used. To enhance the current values, they are turned on together according to a scheme with a common anode or cathode. If each of the dual diodes is 10 amperes, then a significant margin of safety will be obtained.

One of the most common failures of switching power modules is the failure of these very diodes. Typically, they are either completely punctured or leaking. In both cases, the faulty diode must be replaced, then the power transistors must be checked with a multimeter and the supply voltage measured.

Testing and interchangeability

You can check Schottky rectifiers in the same way as conventional semiconductors, since they have similar characteristics. It is necessary to ring it in both directions with a multimeter - it should show itself in the same way as a regular diode: anode-cathode, while there should be no leaks. If he shows even insignificant resistance - 2-10 kilo-ohms, this is already a reason for suspicion.

A diode with a common anode or cathode can be tested like two ordinary semiconductors connected together. For example, if the anode is common, then it will be one leg out of three. We put one tester probe on the anode, the other legs are different diodes, another probe is placed on them.

Can I replace it with a different type? In some cases, Schottky diodes are changed to conventional germanium diodes. For example, D305 at a current of 10 amperes gave a drop of only 0.3 volts, and at currents of 2-3 amperes they can generally be installed without radiators. But the main goal of the Schottky installation is not a small drop, but a low capacity, so it will not always be possible to replace.

As you can see, electronics does not stand still, and further options for using high-speed devices will only increase, making it possible to develop new, more complex systems.