THE POWER DIODE
Diode is a Switch
Among all the static switching devices used in power electronics (PE), the power diode is perhaps the simplest. Its circuit symbol, is a two terminal device, and with terminal A known as the anode and terminal K known as the cathode. Meanwhile, Note, that If the terminal A experiences a higher potential compared to terminal K, the device is said to be forward biased and a forward current (IF) will then flow through the device in the direction shown. However, these causes a small voltage drop across the device (<1 V), which under ideal conditions is usually ignored. By contrast, when a diode is reverse biased, it does not conduct and the diode then experiences a small current flowing in the reverse direction called the leakage current.
Both forward voltage drop and leakage current are ignored in an ideal diode. In PE applications a diode is usually considered to be an ideal static switch. The characteristics of a practical diode depart from the ideals of zero forward and infinite reverse impedance In the forward direction, a potential barrier associated with the distribution of charges in the vicinity of the junction, together with other effects, leads to a voltage drop.
In the case of silicon this is in the range of 1 V for currents in the normal range. In the reverse direction, within the normal voltage operating range, a very small current flows that is largely independent of the voltage. For practical purposes the static characteristics are often represented in diagram ,the forward characteristic is expressed as a threshold voltage VO with a linear incremental or slope resistance r. The reverse characteristic remains the same over the range of possible leakage currents irrespective of voltage within the normal working range.
PROPERTIES OF PN JUNCTION
From the forward and reverse-biased condition characteristics, one notices that when the diode is forward biased, current rises rapidly as the voltage is increased. Current in the reversed biased region is significantly small until the breakdown voltage of the diode is reached. Once the applied voltage is over this limit, the current will increase rapidly to a very high value limited only by an external resistance.
DC DIODES PARAMETERS
The most important are the following:
1) Forward voltage VF is the voltage drop of a diode across A and K at a defined current level when it is forward biased.
2)Breakdown voltage VB is the voltage drop across the diode at a defined current level when it is beyond reversed biased level. This is known as avalanche.
3) Reverse current IR is the current at a particular voltage, and which is below the breakdown voltage
COMMON TYPES OF DIODE
Depending on their applications, diodes can be segregated into
The following major divisions:
Small Signal Diode. These are the semiconductor devices used most often in a wide variety of applications. In general purpose applications, they are used as a switch in rectifiers, limiters, capacitors, and in wave shaping.
The common diode
parameters a designer needs to know include forward voltage, reverse breakdown voltage, reverse leakage current, and recovery time.
Silicon Rectifier Diode.
These are the diodes that have high forward-current carrying capability, typically up to several hundred amperes. They usually have a forward resistance of only a fraction of an ohm while their reverse resistance is in the megaohm range. Their primary application is in power conversion, such as for power supplies, UPS, etc. In case of current exceeding the rated value, their case temperature will rise. For stud mounted diodes, their thermal resistance is between 0.1 to 1 C=W Zener Diode. Its primary applications are in the voltage reference or regulation. However, its ability to maintain a certain voltage depends on its temperature coefficient and impedance.
The voltage reference or regulation application of Zener diodes are based on their avalanche properties. In the reverse-biased mode, at a certain voltage the resistance of these devices may suddenly drop. This occurs at the Zener voltage VX, a parameter the designer knows beforehand. Under normal operating conditions, the transistor will transmit power to the load (output) circuit. The output power level will depend on the transistor base current. A very high base current will impose a large voltage across the Zener and it may attain Zener voltage VX, at which point it will crush and limit the power supply to the load.
Photodiode.
When a semiconductor junction is exposed to light, photons generate hole-electron pairs. When these charges diffuse across the junction, they produce photo current. Hence this device acts as a source of current that increases with the intensity of light. Light-Emitting Diode (LED). Power diodes used in PE circuits are high-power versions of the commonly used devices employed in analog and digital circuits. They are manufactured in many varieties and ranges. The current rating can be from a few amperes to several hundreds while the voltage rating varies from tens of volts to several thousand volts.
Typical Diode Ratings
Voltage Ratings, For power diodes, a data sheet will give two voltage ratings. One is the repetitive peak inverse voltage (VRRM) and the other is the nonrepetitive peak inverse voltage.
The nonrepetitive voltage (VRM ) is the diode’s capability to block a reverse voltage that may occur occasionally due to an overvoltage surge. On the other hand, repetitive voltage is applied on the diode in a sustained way.
