What is Zener diodes

Chapter 3.11 Zener diodes

Lessons In Electric Circuits Volume III – Semiconductors Book
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Lessons In Electric Circuits Volume III – Semiconductors Book

  • 3.11. ZENER DIODES1353.11Zener diodesIf we connect a diode and resistor in series with a DC voltage source so that the diode isforward-biased, the voltage drop across the diode will remain fairly constant over a wide rangeof power supply voltages as in Figure 144,3.56 (a).According to the “diode equation” (page 110,101), the current through a forward-biased PNjunction is proportional to eraised to the power of the forward voltage drop. Because this is anexponential function, current rises quite rapidly for modest increases in voltage drop. Anotherway of considering this is to say that voltage dropped across a forward-biased diode changeslittle for large variations in diode current. In the circuit shown in Figure 144,3.56 (a), diode currentis limited by the voltage of the power supply, the series resistor, and the diode’s voltage drop,which as we know doesn’t vary much from 0.7 volts. If the power supply voltage were to beincreased, the resistor’s voltage drop would increase almost the same amount, and the diode’svoltage drop just a little. Conversely, a decrease in power supply voltage would result in analmost equal decrease in resistor voltage drop, with just a little decrease in diode voltage drop.In a word, we could summarize this behavior by saying that the diode is regulatingthe voltagedrop at approximately 0.7 volts.Voltage regulation is a useful diode property to exploit. Suppose we were building somekind of circuit which could not tolerate variations in power supply voltage, but needed to bepowered by a chemical battery, whose voltage changes over its lifetime. We could form a circuitas shown and connect the circuit requiring steady voltage across the diode, where it wouldreceive an unchanging 0.7 volts.This would certainly work, but most practical circuits of any kind require a power supplyvoltage in excess of 0.7 volts to properly function. One way we could increase our voltageregulation point would be to connect multiple diodes in series, so that their individual forwardvoltage drops of 0.7 volts each would add to create a larger total. For instance, if we had tendiodes in series, the regulated voltage would be ten times 0.7, or 7 volts in Figure 144,3.56 (b).≈ 7.0 V≈ 0.7 V≈7.0 V(a)(b)Figure 3.56:Forward biased Si reference: (a) single diode, 0.7V, (b) 10-diodes in series 7.0V.So long as the battery voltage never sagged below 7 volts, there would always be about 7volts dropped across the ten-diode “stack.”If larger regulated voltages are required, we could either use more diodes in series (an in-elegant option, in my opinion), or try a fundamentally different approach. We know that diodeforward voltage is a fairly constant figure under a wide range of conditions, but so is reversebreakdown voltage, and breakdown voltage is typically much, much greater than forward volt-age. If we reversed the polarity of the diode in our single-diode regulator circuit and increasedthe power supply voltage to the point where the diode “broke down” (could no longer withstand