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The most common device employed in voltage regulation schemes is the Zener diode Zener diodes function on the basis of the reverse portion of the i-v characteristic of the diode discussed in Section 82 Figure 810 in Section 82 illustrates the general characteristic of a diode, with forward offset voltage V and reverse Zener voltage VZ Note how steep the i-v characteristic is at the Zener breakdown voltage, indicating that in the Zener breakdown region the diode can hold a very nearly constant voltage for a large range of currents This property makes it possible to use the Zener diode as a voltage regulator The operation of the Zener diode may be analyzed by considering three modes of operation: 1 For vD V , the device acts as a conventional forward-biased diode (Figure 850) 2 For VZ < vD < V , the diode is reverse-biased but Zener breakdown has not taken place yet Thus, it acts as an open circuit 3 For vD VZ , Zener breakdown occurs and the device holds a nearly constant voltage, VZ (Figure 851) The combined effect of forward and reverse bias may be lumped into a single model with the aid of ideal diodes, as shown in Figure 852 To illustrate the operation of a Zener diode as a voltage regulator, consider the circuit of Figure 853(a), where the unregulated DC source, VS , is regulated to the value of the Zener voltage VZ Note how the diode must be connected upside
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Figure 850 Zener diode model for forward bias
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Figure 851 Zener diode model for reverse bias
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8
Semiconductors and Diodes
"Forward" branch
"Reverse" branch
rD + V _
rZ _ VZ + The Zener diode may be modeled as consisting of two parallel branches, representing forward and reverse operation
Complete model for Zener diode
Circuit symbol
Figure 852 Complete model for Zener diode
Series current Unregulated limiting resistor source RS iS
Voltage regulator iZ +
Load iL RL vS
RS iS iZ + VZ _ iL RL
VZ _
Figure 853 (a) A Zener diode voltage regulator; (b) Simpli ed circuit for Zener regulator
down to obtain a positive regulated voltage Note also that if vS is greater than VZ , it follows that the Zener diode is in its reverse-breakdown mode Thus, one need not worry whether the diode is conducting or not in simple voltage regulator problems, provided that the unregulated supply voltage is guaranteed to stay above VZ (a problem arises, however, if the unregulated supply can drop below the Zener voltage) Assuming that the resistance rZ is negligible with respect to RS and RL , we replace the Zener diode with the simpli ed circuit model of Figure 853(b), consisting of a battery of strength VZ (the effects of the nonzero Zener resistance are explored in the examples and homework problems) Three simple observations are suf cient to explain the operation of this voltage regulator: 1 The load voltage must equal VZ , as long as the Zener diode is in the reverse-breakdown mode Then, VZ iL = RL
(820)
2 The load current (which should be constant if the load voltage is to be regulated to sustain VZ ) is the difference between the unregulated supply current, iS , and the diode current, iZ : iL = iS iZ (821) This second point explains intuitively how a Zener diode operates: Any
Part II
Electronics
current in excess of that required to keep the load at the constant voltage VZ is dumped to ground through the diode Thus, the Zener diode acts as a sink to the undesired source current 3 The source current is given by vS VZ iS = (822) RS In the ideal case, the operation of a Zener voltage regulator can be explained very simply on the basis of this model The examples and exercises will illustrate the effects of the practical limitations that arise in the design of a practical voltage regulator; the general principles will be discussed in the following paragraphs The Zener diode is usually rated in terms of its maximum allowable power dissipation The power dissipated by the diode, PZ , may be computed from PZ = iZ VZ (823)
Thus, one needs to worry about the possibility that iZ will become too large This may occur either if the supply current is very large (perhaps because of an unexpected upward uctuation of the unregulated supply), or if the load is suddenly removed and all of the supply current sinks through the diode The latter case, of an open-circuit load, is an important design consideration Another signi cant limitation occurs when the load resistance is small, thus requiring large amounts of current from the unregulated supply In this case, the Zener diode is hardly taxed at all in terms of power dissipation, but the unregulated supply may not be able to provide the current required to sustain the load voltage In this case, regulation fails to take place Thus, in practice, the range of load resistances for which load voltage regulation may be attained is constrained to a nite interval: RL min RL RL max (824)
where RL max is typically limited by the Zener diode power dissipation and RL min by the maximum supply current The following examples illustrate these concepts
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