The diode as rectifier

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An electronic component from the very peculiar behavior is the diode. We have seen that applying a certain voltage to a resistor, the current through it corresponds to the ratio between the applied voltage and the resistance value of the same; this law does not apply to the diode.

PN Diode
                   Figure 1

From the physical-structural point of view, the diode (Figure 1,top) is made by a “p-n” junction, or from a semiconductor containing, adjacent to one another, two regions, doped with a type of impurities ” p ” and one with type impurities” n “.
The P region , being doped with electron defect atoms, it tends to capture electrons as they say, has some holes or gaps.
The N region, being doped with atoms of excess electrons, tends to lose the excess electrons.When the PN junction is reverse biased (Figure 1, middle), or to the P side, a negative voltage is applied to the N side and a positive, both the gaps of the area P that the free electrons in the N area are attracted by the applied electric field , for which the central area is empty; in this area, which is called a “depletion zone”, it creates a potential barrier that prevents the passage of the current; It circulates only a very weak current due to minority charges, known as “drift. This current is the current order of a few uA for germanium diodes, and a few nA for silicon diodes.

When the PN junction is forward biased (Figure 1, bottom), the gaps of the zone P are pushed towards the central area of the junction on the applied positive polarity; similarly, the free electrons in the N area are pushed towards the central zone of the junction from the negative polarity; if the voltage is sufficient to overcome the existing potential barrier, the holes and the electrons combine with each other, giving origin to a current, said diffusion current, which can also become very intense. The voltage required to trigger the flow of this current is 0,2 – 0,3 V in the case of Germanium joints  and 0.5 V in the case of the Silicon junctions.

pn
Figure 2

The diode made with a PN junction as just described, is represented by the symbol that is seen in Figure 2 at the center: the side corresponding to the area P is called “anode”; the area corresponding to the N side is called “cathode.” Under the symbol shows the image of a real diode: the silver band indicates the cathode; in normal use of the diode, the current in the diode flows from anode to cathode

 

Figure 3
Figure 3

In its practical use, the behavior of the diode is represented in the graph of Figure 3.

The voltage applied to the diode is read on the X axis (the horizontal one ), while on the Y axis (vertical) reads the current that flows through it.
With forward bias, that is, when a positive voltage to the anode relative to the cathode is applied, it is noted that no current flows until the VT voltage value, said threshold value; if the voltage applied to the diode is increased beyond this value, it occurs the passage of a current higher, the higher the applied voltage.

If the diode is reverse-biased, and that applies a negative voltage to the anode relative to the cathode, no current flows in practice, except for a very weak current called the “drift”; but if you exceed a certain voltage value, said value of “breakdown”, the resistance of the diode goes down unexpectedly, and has run a place without limits, said “avalanche effect”. As usually a diode is not built to work in the break-down region, you must prevent this from happening, otherwise the irreversible destruction of the diode, due to the sudden increase in power dissipation.

Thanks to the characteristics described so far, the diode is useful in functioning as “rectifier”; for example by inserting a diode in a circuit path from alternating current sine wave, it occurs that the current flows in the circuit only when it has the correct polarity, while it is locked every time the polarity is reversed. In practice, all the negative half-waves of the alternating current are eliminated, for which, downstream of the diode, a voltage is obtained constituted by the positive half-wave .

The rectifier diodes are produced for a wide range of applications; by varying the construction techniques, the percentage of doping of the chip and its size, can be obtained diodes able to bear a maximum current that varies from 1 A to tens and hundreds of amperes, suitable for working voltages from a few dozen to several hundreds of volts.

The main characteristics of a diode are:

  • Maximum reverse voltage: The maximum reverse voltage that the diode can withstand, without resulting in the avalanche effect
  • Rated forward current: the maximum current (average value) that can pass through the diode without destroying it; It depends on the size of the chip, and by its ability to transmit outside the heat produced
  • Maximum forward voltage drop: is the maximum voltage drop across the diode and depends on the current passing through it (in the forward direction)
  • Maximum leakage current : is the leakage current flowing in the diode when it is connected (polarized) in the reverse direction (as long as the applied voltage is not high enough to cause the avalanche effect)
  • Maximum reverse recovery time : is the time it takes for the diode to change from state ON to the OFF state, that is run by the non-conducting; It is in practice the “speed switching”, that is, the switching speed, and depends on the size and the characteristics of the chip.

 

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