The rectifiers, also called simply diodes, are devices designed to have only one direction of significant conduction, said forward-bias, otherwise when they are working in reverse-bias the conduction is insignificant as long as the voltage value is not exceeded a voltage value called breakdown.
We call rectifier diode any diode used to presents reverse breakdown voltage always greater than the inverse voltage of the power supply. And the function of these diodes is in the final analysis to block any inverse current to the diode conduction.
You consider the following classification of rectifier diodes (bearing in mind that some diodes may also fall into several categories):
- General purpose diodes used for purposes that does not require the optimization of any electrical parameter. Normally they have a few tens of volts of breakdown voltage and direct current maximum limits of some hundreds of mA. An example is the diode BAV17.
- Switching diodes optimized to work with signals which have very fast commutations. They must present small values of recovery time. An example is the model 1N4148. Sometimes the diodes foreseen for high frequency applications (for example VHF tuners) are included in this category and it is very important that they have a small differential resistance. An example of this second type of diode is the device BA243.
- Small signal diodes are foreseen for small currents (at most some hundreds of mA) and not very high breaking voltages (100 ÷ 200 V maximum) and not for specific uses. The 1N4148 and BAV17 diodes already mentioned above are part of this category.
- Rectifier diodes are the rectifiers in the classical sense intended to straighten the mains voltage or in any case alternating voltages often of considerable amplitude: they are therefore foreseen for high currents and voltages. Examples are the devices 1N4001 and 1N4007. Integrated bridge rectifiers are widely used in this category.
- Fast rectifiers-soft recovery are expected diodes to work in power application but with low switching times. They must present small recovery time values. They are particularly suitable for switching power supplies. An example is the BYG20D.
If we apply a voltage to the ideal diode with the potential of the anode higher than the cathode, the device works in forward-bias (a) and has zero resistance, but if the anode potential is less than the cathode, the diode operates in reverse-bias (b) and has infinite resistance.
More precisely, the ideal diode behaves as a null resistance if it is forward-bias (a) and therefore the current flowing on it is worth I = V / R. It behaves instead of infinite resistance if it is reverse-bias (b) and therefore the current is worth I = 0.