Diode Specification Sheets Lecture

DIODE SPECIFICATION SHEETS Datasheets Diodes, like other electronic components, have many specifications, parameters and rating which need to be understood when selecting one as a replacement, or for a new electronic design. Data on specific semiconductor devices are normally provided by the manufacturer through a brief description limited to one page or a thorough examination of the characteristics using graphs, tables, and so on. Importance of Datashee Datasheets ts • • • • Understanding the diode specifications, parameters and ratings can be the key to selecting the right electronic component for a particular electronic circuit design. There are different types of diodes being considered and depending on the application, additional data may be needed. The importance of data found in datasheets, that is the different specification parameters, will be self-apparent. Aside from the specifications addressing the electrical performance, the physical packages are also important Diodes come in a variety of packages including wire ended package as well as high power diodes that bolt onto heatsinks. The dimensions of the diode are also included. A brief description of performance is also provided. Inspecting the performance parameters will enable the diode to be assessed for whether it will provide the required performance for its intended function. Diode Specifications Ra Ratings tings and Parameters The list below provides details of the common various diode characteristics, and diode parameters found in the data-sheets and specifications for diodes. 1. Semiconductor material: The semiconductor material used in the PN junction diode is of paramount importance because the material used affects many of the major diode characteristics and properties. Silicon and germanium are two widely used materials: Silicon: Silicon is the most widely used material as if offers high levels of performance for most applications and it offers low manufacturing costs. The technology for silicon is well established and silicon diodes can be made cheaply. The forward turn on voltage is around 0.6V, which is high for some applications, although for Schottky diodes it is less. Germanium : Germanium is less widely used and but offers a low turn on voltage of around 0.2 to 0.3 V. Other materials are generally reserved for more specialist diodes. For example LEDs use compound materials to provide the different colors. 2. Diode typ type: e: Although most diodes have a PN junction as the basis of their construction, different types of diode are formulated to provide different characteristics and sometimes they can operate in different ways. Selecting the right type of diode for any given application is key. 3. Forward vol voltage tage drop, Vf: Any electronics device passing current will develop a resulting voltage across it and this diode characteristic is of great importance, especially for power rectification where power losses will be higher for a high forward voltage drop. Also diodes for RF designs often need a small forward voltage drop as signals may be small but still need to overcome it. The voltage across a PN junction diode arise for two reasons. The first of the nature of the semiconductor PN junction and results from the turn-on voltage mentioned above. This voltage enables the depletion layer to be overcome and for current to flow. The second arises from the normal resistive losses in the device. As a result a figure for the forward voltage drop are a specified current level will be given. This figure is particularly important for rectifier diodes where significant levels of current may be passed. Particularly for power rectification diodes, a graph of the forward voltage drop for various current levels is normally provided within the data sheet. This will have a band of typical figures and using this the range of voltage drop can be determined for the anticipated current levels to be carried. It is possible to then determine the power that will be dissipated into e junction area of the diode. 4. Peak In Inverse verse Volt Voltage, age, PIV: This diode sometimes called the DC Blocking Voltage. characteristics is the maximum voltage a diode can withstand in the reverse direction. This voltage must not be exceeded otherwise the device may fail. 5. Reverse breakdown voltage, V(BR)R: This is a little different to the peak inverse voltage in that this voltage is the point at which the diode will break down. The diode can withstand a reverse voltage up to a certain point, and then it will breakdown. In some diodes and in some circuits it will cause irreparable damage, although for Zener / voltage reference diodes the reverse breakdown scenario is what is used for the voltage reference, although the circuit must be devised to limit the current flowing, otherwise the diode can be destroyed. 6. Maximum forward current: For an electronic circuit design that passes any levels of current it is necessary to ensure that the maximum current levels for the diode are not exceeded. As the current levels rise, so additional heat is dissipated and this needs to be removed. Average forward current - the maximum average amount of current that the diode is able to conduct in forward bias mode. This is fundamentally a thermal limitation: how much heat can the PN junction handle, given that dissipation power is equal to current (I) multiplied by voltage (V) and forward voltage is dependent upon bot current and junction temperature. Peak/Surge forward current – the maximum peak amount of current the diode is able to conduct in forward bias mode. This rating is limited by the diode junction’s thermal capacity, and is usually much higher than the average current rating due to thermal inertia. 7. Junction operating temperature: Like all electronic components, diodes have a maximum operating temperature. In the data sheet there will be a section outlining the maximum junction temperature. As the junction temperature rises, so the reliability will fall over the long term. If the maximum junction temperature is exceeded, the diode is likely to fail, and could even catch fire. It should be remembered that the junction temperature relates to the diode junction itself inside the package and not the package temperature. A good margin should be allowed between the package temperature and the junction temperature. Often curves will be supplied in the data sheet to enable the junction temperature to be determined. It is also possible to calculate the junction temperature from a knowledge of the current, forward voltage drop and the thermal resistance,: specifications that are mentioned in the data sheets and mentioned here as well. 8. Leakage current: If a perfect diode were available, then no current would flow when it was reverse biased. It is found that for a real PN junction diode, a very small amount of current flow in the reverse direction as a result of the minority carriers in the semiconductor. The level of leakage current is dependent upon three main factors. The reverse voltage is obviously significant. It is also temperature dependent, rising appreciably with temperature. It is also found that it is very dependent upon the type of semiconductor material used - silicon is very much better than germanium. The leakage current characteristic or specification for a PN junction diode is specified at a certain reverse voltage and particular temperature. The specification is normally defined in terms of in microamps, µA or picoamps, pA as the levels are normally very low before reverse breakdown occurs. 9. Junction capacitance: All PN junction diodes exhibit a junction capacitance. The depletion region is the dielectric spacing between the two plates which are effectively formed at the edge of the depletion region and the area with majority carriers. The actual value of capacitance being dependent upon the reverse voltage which causes the depletion region to change (increasing reverse voltage increases the size of the depletion region and hence decreases the capacitance). This fact is used in varactor or varicap diodes to good effect, and is widely used in variable frequency oscillator and variable frequency filter RF designs. However for many other applications, especially some RF designs where stray capacitance across the diode could affect the performance, this needs to be minimised. As the capacitance is of importance it is specified. The parameter is normally detailed as a given capacitance (normally in pF as capacitance levels are relatively low) at a given voltage or voltages. Also special low capacitance diodes are available for many RF applications. For many power rectifier applications the capacitance is sufficiently low to not be an issue. As an example the junction capacitance of a 1N4001 and 1N4004 is only 15 pF for a reverse voltage of 4 volts and less as the voltage rises. Higher voltage diodes may be less - a 1N4007 has a junction capacitance of 8 pF for a reverse voltage of 4 volts. Accordingly it is only as the frequencies rise that the effect of the capacitance is noticed. As the capacitance levels are low, frequencies up to around 100 kHz are often not affected by it, and in most cases it can be ignored up to even higher frequencies. 10. Total Power Diss Dissipation ipation (PD): This is the amount of power (in watts) allowable for the diode to dissipate, given the dissipation (P = IV) of diode current multiplied by diode voltage drop, and also the dissipation (P = I2R) of diode current squared multiplied by bulk resistance. This is also fundamentally limited by the diode’s thermal capacity. 11. Thermal Resistance – the temperature difference between junction and outside air or between junction and leads for a given power dissipation. Ideally, this figure would be zero, meaning that the diode package was a perfect thermal conductor and radiator, able to transfer all heat energy from the junction to the outside air (or to the leads) with no difference in temperature across the thickness of the diode package. A high thermal resistance means that the diode will build up excessive temperature at the junction (where its critical) despite best efforts at cooling the outside of the diode, and thus will limit its maximum power dissipation. 12. Reverse Recovery Time Time: This is the amount of time it takes for a diode to “turn off” when the voltage across it alternates from forward-bias to reverse-bias polarity. Ideally, this figure would be zero: the diode halting conduction immediately upon polarity reversal. For a typical rectifier diode, reverse recovery time is in the range of tens of microseconds; for a “fast switching diode”, it may only be a few nanoseconds 13. Package type: Diodes can be mounted in a variety of packages according to their applications, and in some circumstances, especially RF applications, the package is a key element in defining the overall RF diode characteristics. Also for power applications where heat dissipation is important, the package can define many of the overall diode parameters because high power diodes may require packages that can be bolted to heatsinks, whereas small signal diodes may be available in leaded formats or as surface mount devices. Also high power diodes may be available as bridge rectifiers containing four diodes in a bridge suitable to fun wave rectification applications. 14. Diode coding and markings schemes: Most diodes that are used have part numbers that conform to the JEDEC or Pro-Electron schemes. Numbers like 1N4001, 1N916, BZY88 and many more are very familiar to anyone involved in electronics design and manufacture. Graphs A manufacturer may provide graphs, and tables for a thorough examination of the characteristics. We can see in these graphs how a parameter affects another. The graphs are usually line graphs, . Some graphs or curves employ a log i.e. Current vs Voltage, Current vs Temperature, and so on scale. 14 1 2 13 14 4 6 3 8 9 7 11