Download Zener Diode

Zener Diode
Zener Diode
2
 The I-V characteristic of a real diode is separated
into three regions:
forward bias – conduct current
reverse bias – block current
breakdown – overheat & damage
Prepared by: Dr. RS
29-Apr-17
Zener Diode
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 A silicon diode that is designed for operation in the
reverse-breakdown region
 Operates the same as a regular diode in forward-
biased connection
Prepared by: Dr. RS
29-Apr-17
Breakdown Voltage
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 The break-down voltage is set by carefully
controlling the doping level during manufacture.
 A zener diode is heavily doped to reduce the
breakdown voltage (very thin depletion region).
Prepared by: Dr. RS
29-Apr-17
Figure 3–2
General diode V-I characteristic.
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Prepared by: Dr. RS
29-Apr-17
Reverse Breakdown
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 There are two breakdown mechanism in a zener
diode:
i. Avalanche breakdown
ii. Zener breakdown
Prepared by: Dr. RS
29-Apr-17
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Avalanche Breakdown
 Occurs in both regular and zener diodes at a
sufficiently high reverse voltage.
Zener Breakdown
 Occurs in a zener diode at low reverse
voltages
 The reverse breakdown voltage remains
almost constant even though the current
changes drastically
Prepared by: Dr. RS
29-Apr-17
Figure 3–3 Reverse characteristic of a zener diode. VZ is usually
specified at the zener test current, IZT and is designated asVZT.
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Prepared by: Dr. RS
29-Apr-17
Figure 3–4 Zener diode equivalent circuit models and the
characteristic curve illustrating ZZ.
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Prepared by: Dr. RS
29-Apr-17
Temperature Coefficient (TC)
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 Specifies the percent change in zener voltage for each
degree centigrade change in temperature
VZ = VZ x TC x T
 A +ve TC means that VZ increases with an increase in
temperature
Prepared by: Dr. RS
29-Apr-17
Power Dissipation
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PD = VZIZ
 Zener diodes are specified to operate at a
maximum power called the maximum dc
power dissipation, PD(max).
Prepared by: Dr. RS
29-Apr-17
Figure 3–7
Partial data sheet for the *1N4728–1N4764 series 1 W zener diodes.
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*
*
Prepared by: Dr. RS
29-Apr-17
Zener – Circuit Analysis
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Assume that the diode is in
breakdown region
2. Treat it as a battery with a
value of VZ
3. Solve for IZ
4. Make sure IZ > IZ(min) to keep
the diode in breakdown
1.
Prepared by: Dr. RS
VS
R
Vo
29-Apr-17
Zener – Resistor Load Circuit
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 As the load resistor gets larger, the load current
decreases.
More current burning through the Zener, putting the
diode deeper into breakdown.
 As the load resistor gets smaller, the load current
increases, drawing more current away from the Zener.
Less current burning through the Zener, bringing it
closer to the knee of its I-V curve.
Prepared by: Dr. RS
29-Apr-17
Alternative Approach
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VS
1.
2.
R
3.
+
RL
Vo
-
Prepared by: Dr. RS
4.
5.
Assume the zener diode is OFF
Replace it with an open circuit
Find the voltage across the
zener diode
If VOC < VZ, the diode is OFF
If VOC > VZ, the diode is in
breakdown
29-Apr-17
Zener Clipper Circuits
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Zener diodes can be used to limit voltage swings
to desired levels
Prepared by: Dr. RS
29-Apr-17
Clipper - Regular Diodes
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Prepared by: Dr. RS
29-Apr-17
Clipper – Zener Diodes
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 Clipper using Zener Diodes can eliminate the need
for the batteries.
 Batteries are not perfect because:
i. Bulky
ii. Run down in time
iii. Can leak nasty chemicals
Prepared by: Dr. RS
29-Apr-17
Zener Limiting
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Prepared by: Dr. RS
29-Apr-17
Voltage Regulator
 Zener diode is a voltage regulator device because it is able to fix the
output voltage at a constant value (DC voltage).
 RS is to limit the zener current, IZ so that it is less than the maximum
current, IZM (to avoid the zener diode from broken).
RS
RS
+
+
VS
+
VZ
VS
-
-
-
A simple regulator circuit
+
VZ
RL
-
A regulator circuit with load resistance
Voltage Regulator
 How to determine whether the zener acts as a regulator or
not??


Use Thevenin Theorem
See example
 If VTH<VZ, regulation does not occur.
 Referring to zener I-V charateristic curve, if the voltage
across the zener diode zener is between 0-VZ, the zener
diode is operating in the reverse bias region, thus it does
not functioned as a regulator.
 VTH must at least the same value as VZ (VTH  VZ) so that the
diode can function as a voltage regulator because it is
operating in reverse breakdown region.
Thevenin Equivalent Circuit