2 SEMICONDUCTORSOrdinary PN junction diodes can be damaged if they are subjected to their respective breakdown voltages.The reverse current produces more heat than the diodes can safely dissipate.Zener diodes are different in this regard.
3 SEMICONDUCTORSThe zener diode is specifically designed to operate with a reverse bias voltage that is high enough to cause the device to breakdown and conduct a high reverse current.
4 SEMICONDUCTORS The reverse current is low until breakdown occurs. After breakdown, the reverse current increases rapidly.This occurs because the resistance of the zener diode decreases as the reverse voltage increases.
5 ZENER CURRENT IS REPRESENTED BY THE SYMBOL SEMICONDUCTORSOnce the breakdown point is reached the diode is operating in its zener region.The ability of a zener diode to dissipate power decreases as the temperature increases.ZENER CURRENT IS REPRESENTED BY THE SYMBOLIz
6 SEMICONDUCTORSThis is the Zener diode V-I curve showing the breakdown region
7 SEMICONDUCTORSA zener diode is manufactured to have a specific breakdown voltage rating which is the diode’s zener region designated as Vz.Some common Vz ratings are 3.3, 4.7, 5.1, 5.6, 6.2 and 9.1 voltsAlso within the breakdown region is the diode’s current value called the zener test current designated as (Izt)
8 SEMICONDUCTORSZener diodes are available in a variety of voltage ratings which is referred to as the zener volatage (Iv), but so far, we have been unable to make these diodes capable of dropping an exact voltage.Therefore, when you buy a zener diode, it comes with a tolerance value (just like a resistor) of approx %
9 SEMICONDUCTORSThey also specify maximum power that the diode can safely dissipate at a given temperature, the most widely used are rated for 400mW, 500mW and 1W.Power dissipation ratings are based on room temperature and are specified by the manufacture for temp ratings of 25C, 50C or 75C.If the temperature increases the ability of the zener diode to dissipate will decrease.
10 SEMICONDUCTORSThis chart shows and example of a zener diode’s power temperature derating curve.
11 SEMICONDUCTORSThe maximum reverse current that can flow through a zener diode without exceeding it’s power dissipation rating is referred to as the maximum zener current which is designated as Izm.If Izm is not specified it can be determined by the formula:
12 SEMICONDUCTORSZener voltage temperature coefficient is a characteristic that must be considered in certain applications.This means that diodes with a Vz of 5 volts or more have breakdown voltages that increase as temperature increasesMost diodes that have breakdown voltages of 4 volts or less have breakdown voltages that will decrease with an increase of temperature
13 SEMICONDUCTORSZener impedance is another important characteristic which is designated by (Zzt)Zener impedance is determined by the formula:A low zener impedance indicates that the zener voltage changes only slightly with changes in current.
14 SEMICONDUCTORSZener diodes are most widely used in applications where it is continually reversed-biased so that it operates constantly within its zener breakdown region.Under these conditions the zener diode is effectively used to provide voltage regulation and stabilization.Voltage regulation is often required because most solid state circuits require a fixed or constant DC power supply voltage for proper operation.
15 SEMICONDUCTORSFor this reason zener diodes work great because they provide a constant DC output voltage.The input voltage is connected so that the zener diode is reverse biased, the series resistor allows enough current to flow through the diode so it operates within it’s zener region.
16 SEMICONDUCTORSThe DC voltage must be higher than the zener breakdown voltage rating.The voltage across the resistor will be equal to the difference between the diode’s zener voltage (Vz) and the input DC voltage.
17 SEMICONDUCTORSWhen a load (RL) is connected to the output of the regulator circuit, the regulator must supply voltage and current to the load.Here the load resistor (RL) requires a specific load current (IL).
18 SEMICONDUCTORSThe load current (IL) is determined by it’s resistance and output voltage.The current through the zener diode (Iz) combines with (IL) and flows through the series resistor (Rs)
19 SEMICONDUCTORSThe value of (Rs) must be choosen so the (Iz) remains at a sufficient level to keep the diode within it breakdown region and at the same time allow the required value of (IL ) to flow through the load.The zener diode prevents any voltage or current fluctuations from occurring at the load (RL)