Most modern radio circuits use a Schottky diode. Its action is based on a physical effect discovered by the German scientist Walter Schottky, which is why it bears his name. This element has many of the same parameters as conventional diodes, but it also has significant differences.
Content
- Principle of operation and designation
- Positive and negative qualities
- Scope of application and popular models
- Diagnostics of possible malfunctions
Principle of operation and designation
If a conventional semiconductor diode is based on the properties of a p-n junction, then the principle of operation of a Schottky diode is based on the properties of a junction when a metal and a semiconductor are in contact. In physics, such a contact is called the "Schottky barrier". Gallium arsenide (GaAs) is most often used as a semiconductor, and of metals, the following are mainly used:
-
tungsten; - platinum;
- silver;
- gold;
- palladium.
On radio circuits, the Schottky diode designation is similar to the designation of a conventional semiconductor element, but there is a noticeable difference: from the side cathode, where there is a small line perpendicular to the main line, its edges are additionally bent in different directions at right angles or with a smooth bend.
Sometimes it is difficult to graphically designate this element on schematic diagrams, it is drawn like a regular diode, and the type is additionally indicated in the specification.
Positive and negative qualities
The Schottky semiconductor element is widely used in various electronic and radio engineering devices due to its positive properties. These include the following:
- very low voltage drop across the junction, the maximum value of which is only 0.55 V;
- high response speed;
- low barrier (junction) capacitance, which makes it possible to use a Schottky diode in circuits with a high current frequency.
But there are also several negative properties that must be taken into account when using this radio-technical element. Namely:
- instant irreversible failure even with a short-term increase in reverse voltage above the limit value;
- the occurrence of thermal breakdown on reverse current due to heat release;
- diode leaks are common and are difficult to identify.
Scope of application and popular models
The Schottky semiconductor radio element is characterized by the absence of diffuse capacitance due to the absence of minority carriers. Therefore, this element is primarily a microwave diode of a wide range of applications. It is used in the role of the following elements:
- strain gauge;
- radiation receiver;
- light modulator;
- nuclear radiation detector;
- high frequency current rectifier.
A small voltage drop, unfortunately, is observed in most of these elements at an operating voltage in the range of 55-60 V. If the voltage is higher than this value, then the Schottky diode has the same qualities as a conventional silicon-based semiconductor element. The maximum reverse voltage is usually in the order of 250V, but there are special models that can withstand 1200V (for example, VS-10ETS12-M3).
Of the dual models, 60CPQ150 is popular among radio amateurs. This radioelement has a maximum reverse voltage of 150 V, and each individual diode from the assembly is designed to carry a current in direct connection with a force of 30 A. In powerful switching power supplies, you can sometimes find the VS-400CNQ045 model, in which the output current after rectification reaches 400 A.
Schottky diodes of the 1N581x series are popular among radio amateurs. Samples such as 1N5817, 1N5818, 1N5819 have a maximum rated forward current of 1 A, and their reverse voltage is 20-40 V. Voltage drop across the barrier (junction) in the range from 0.45 to 0.55 V. Also in amateur radio practice, there is an element 1N5822 with a direct current of up to 3 A.
On printed circuit boards, miniature diodes of the SK12 - SK16 series are used. Despite their very small size, they can withstand a forward current of up to 1 A, and the "return" voltage ranges from 20 to 60 V. There are also more powerful diodes, for example, SK36. Its direct current reaches 3 A.
Diagnostics of possible malfunctions
There are only three types of possible malfunctions. This is breakdown, breakage and leakage. If the first two types can be diagnosed independently at home using a conventional multimeter, then the third malfunction at home is practically impossible to diagnose.
To reliably determine the failure of the diode, it must be removed from the circuit, otherwise shunting through other circuit elements will distort the readings obtained. During breakdown, the element behaves like a normal conductor. When measuring its resistance in both directions, the meter will be "0". In the event of a break, the part does not transmit an electric current at all in any direction. Its resistance is equal to infinity in every direction.
An indirect sign of a leak in the element is its unstable operation. Sometimes the built-in protection in the power supply of the computer, monitor, etc. can be triggered.
It is impossible to determine the leak with a multimeter, since it occurs during the operation of the element, and measurements must be made when it is disconnected from the circuit.
