Zener diode
pp 53-60
Zener Diode 3.6 (Zener) and Zener diode applications 3.6.1
When a reverse bias PN diode with an external voltage, the current passing through the IR diode is very small. The more the value of external voltage, the higher (slightly) and reverse currents up to a certain voltage value beyond which the current increases sharply. The reverse trend is called the Zener voltage in honor of the scientist who discovered the phenomenon of the same name.
Indeed, by increasing the reverse voltage increases the electric field and the electrons move through the diode become quite high kinetic energy. The result is that at some value of the voltage electrons break the bonds between atoms of the semiconductor and released a large number of new electrons that contribute to a strong current. The range of voltages and currents where the phenomenon occurs Zener, called region decomposition (breakdown) of the diode.
If in a common pathway, such as those examined so far, applied reverse voltage greater than the breakdown voltage, the diode is destroyed. For this reason, special manufactured diodes operated in the split and said Zener diodes, and they tend Zener (VZ).
Figure 3.6.1 (a) shows the typical current-voltage (IV) Zener diode and a in Figure 3.6.1 (b) shows the symbol of this passage.
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Figure 3.6.1. Diode Zener (a) Typical. (B) Symbol
From the curve it seems that after the threshold voltage (V7K) the minimum voltage of Zener (V7min), the characteristic curve is almost a line segment with slope 1/RZ, where RZ is the resistance of the diode and is valid:
3.6.1
ie Ig is imiathroisma outliers current
the diode.
In contrast, the minimum and maximum Zener voltage is not very different and practical VZ,
The resistance RZ is very small (0 to 500 Ohms). The voltage Zener, V7 is defined as the voltage corresponding to the current Q where:
3.6.2
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KRYSTLLLODIODOI
The equivalent circuit of VZ Zener diode consists of a voltage source, Mr. Ai VWV 1
VZ series resistance as shown RZ - IZ
oeta | in Figure 3.6.2. Shape
The characteristics of the diode Zener, Zener diode equivalent circuit are specified by manufacturers are:
A. The voltage Zener (VZ) the price of which varies from 2.4 up to 200V.
B. The maximum allowable power IZmax <the maximum allowable power Pmax.
C. The dynamic resistance of the diode and RZ
d O temperature coefficient a, which indicates the rate of change of voltage with temperature VZ. O Temperature coefficient is either negative or positive and expressed as a rate per degree Celsius (±% / ° C).
The values of Zener voltage of Zener diode of trade ranging from 2.4 to 200V and roughly follows the E12 and E24 tables of resistances.
Example 3.6.1
For the Zener diode 1N5233, which has a = 3,8 x 10-4 / ° C and VZ = 6 V, to determine the increase in voltage VZ, when the temperature increased by i = 50 ° C.
Solution
Increasing the Zener voltage for temperature increase of 50 ° C
will be:
AV = VZ x a x i = 6,0 x 0,00038 x 50 = 0,114 V.
Example 3.6.2
In the following circuit to calculate the current IZ, when Zener diode has a voltage VZ = 6,8 V, the resistance is R = 950O and the input voltage is V |
= 10 V for the cases:
a) RZ = 0 b) RZ = 50W
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Solution
Applying the law Kirchnoff trends in the circuit will be:
We observe that the price of IZ does not change significantly; be commented;
3.6.2 Using the Zener diode for voltage stabilization
The Zener diode applications are many. But the most important application is to stabilize voltage. Many times it is necessary that a constant voltage, independent of any changes in the prices of electronic circuit elements. Constant voltage may be needed eg or as a reference voltage to other circuits, either to protect a sensitive gauge as a millimeter, overcurrent which may be simple classic katastrofikes.Ena stabilization circuit shown in Figure 3.6.3.
In the above circuit, the voltage across the load resistor RL must remain constant regardless of changes in the voltage V 'or resistance R. The load resistance can be either the input impedance of a feed device or a measuring instrument. we Rz = 0.
a) Rz = 0
, V-Vz, 10 -6,8 V 3,2 V _ L
'Z = ^ | z = ^ 60 - = 9 ^ = 3,36 mA ·
b) Rz = 50W
V-VZ 10-6,8 V 3,2 V
RL VL = VZ
Figure 3.6.3.
Circuit voltage stabilization
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KPYITAMOAIOAOI
Applying the laws of voltages and currents and Ohm's law (Ohm) in the circuit of Figure (3.6.3.) Will apply:
V = IR + VZ,
I = IZ + IL,
3.6.3
Substituting the values of IL and I will be:
V = IZR + ttR + VZ 3.6.4
Equation (3.6.4) gives the change of input voltage versus current and voltage of the Zener diode and plotted a straight line on the diagram of the diode characteristic curve. The straight line is called a load, in analogy to that applied to the PN diode.
In relation (3.6.4) the only constant is the size of the diode voltage Zener VZ. All other sizes can be changed. So there are the following cases:
a load resistance RL or the load current IL remains stable and
changing the input voltage V | and the diode current IZ. b The input voltage V | remains constant and vary the load current IL and pass Iz. In case (a) the load resistor RL is constant, relation (3.6.4) is written:
3.6.5
From equation (3.6.5) can be calculated resistance R. The maximum value of resistance R is when the nominator of equation (3.6.5) maximize and minimize the denominator. Therefore we have:
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Similarly, the calculated resistance value Rmin when the numerator of equation (3.6.5) and minimize the denominator maximized.
3.6.7
Example 3.6.3
The input voltage V 'shape changes from 3.6.3 Vi min = 15 V until V'max = 25 V. If RL = 10KO and VZ = 10V, Rz = 0, 'Zmin = 1mA, to obtain the maximum value of series resistance R.
Solution
The facts are: 15V <V <25V, VZ = 10V, IZmin = 1mA, RZ = 0.
Equation (3.6.6) gives:
Rmax = ^ == ^ ^ G = 7.5 x = 7.5 KO 103O max 'V 10-3 + 10 2 X 10-3
'Zmin + Rl 10 x 103
Example 3.6.4
In the circuit of the previous example, the load current should remain constant IL = 75 mA. Gives the values: R = 200 O, VZ = 7.5 V, IZmin = 2 mA, IZmax = 30 mA. To determine the maximum and minimum input voltage V 'to the load current remains constant.
Solution
Because the load current remains constant and the resistance-loading
is constant, because:
Vmax = 200 x 30ch 10 + (^ + 1 | x 7,5 = 6 + 22,5 = 28,5 V
1100
H relation (3.6.4.) Reads
V = RIZ + (R + 1 VZ where only the Zener current varies.
Therefore the minimum and maximum value of V | is:
V | mi n = 200 x 2 x 10 -3 + (1100 +1 x 7,5 = 0,4 + 22,5 = 22,9 V
SUMMARY 3.4 - 3.6
• The diode Varactor <Varicap is a reverse polarized diode and the capacity varies inversely with the external voltage applied to its ends.
• The diode Varicap corresponds to a variable capacitor as opposed to a fixed capacitor whose capacity remains constant.
• The diode is a Schottky contact metal and semiconductor
advantage of the contact PN is that it takes much less time to go from the state conductivity (correct polarity) in the state cutoff (reverse polarity).
• The Schottky diode is used in low voltage digital circuits because the voltage barrier region stripping is less common than that of a PN diode (0,25 -0,3 V).
• The Zener diode operating in the division, inversely polarized and characterized by a sharp increase in reverse current when the voltage across the reaches a certain price trend called Zener.
• The Zener diode can be used to stabilize voltage, because it provides a constant voltage across it, regardless of the change in input voltage or load current change.
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