VOLTAGE REGULATION >A voltage regulator circuit automatically maintains the output voltage of a power supply constant, regardless of - a change in the load - a change in the source
Trang 13.1 ZENER DIODES
K
A
Trang 2VOLTAGE REGULATION
>A voltage regulator circuit automatically
maintains the output voltage of a power
supply constant, regardless of
- a change in the load
- a change in the source voltage
Transformer 4
SOUPCE
Rectifier
(
Trang 3ZENER DIODES
>The simplest of all voltage regulators
is the zener diode voltage regulator
>A zener diode Is a special diode that
is optimized for operation in the
FWD
K BIAS
REGION
VR
REV BIAS
VF
Trang 4ZENER DIODE CIRCUIT
›s The zener diode is typically connected
reverse biased, in parallel with the load
O +
VDC(IN)
- ; Resistor Rs limits current to zener
L)Z
Z
J
CL
-E
Trang 5
ZENER DIODE CHARACTERISTICS
“£In the forward 7
region, the zener
diode acts like a
diode, witha 0.7
REGION volt drop when it
Trang 6ZENER DIODE CHARACTERISTICS
region, a reverse a
leakage current
flows until the
breakdown voltage REV l2
is reached BIAS
REGION
>> At this point, the
reverse current,
current Iz,
increases sharply
Trang 7ZENER DIODE CHARACTERISTICS
-> Voltage after
breakdown is also
called zener
voltage Vz
-> Vz remains nearly
a constant, even
though current |Z
varies
considerably
Trang 8
ZENER DIODE RATINGS
->A zener spec sheet typically provides
- the maximum power rating Pz
- the nominal zener voltage Vz at test
current | ZT
- the maximum DC zener current | ZM
Example:
- 1N752 hasa power rating of 500mW, a
nominal zener voltage of 5.6V at a test
current of 20 mA, a maximum zener
current of 80 mA
Trang 9ZENER DIODE CIRCUIT EXAMPLE
-:In a zener diode voltage regulator circuit,
Q: Calculate IL
RS
| —>
+ 890
20V
VZ
OV
——
100
Trang 10ZENER DIODE EXAMPLE (CONT’D)
Q: Calculate Ir
= [20 - 9J/82O
= 134mA
©
+ $20 _
Trang 11
ZENER DIODE EXAMPLE (CONT’D)
Q: Calculate Iz
A: Iz=I1T-IL
= 134mA-90mA
= 44mA
RS
-
|
VDG(IN) yo /
20V 9V 1002
Trang 12ZENER DIODE EXAMPLE (CONT’D)
Q: If zener Pz = 500mW, is1|z safe 2
- 500 mW / 9V
- 55mA Since circuit |Z
= 44 mA(< 55mA),
zener is safe
Z min
+|z=44mA
lZmax =55mA
R
Trang 13
ZENER DIODE EXAMPLE (CONT’D)
Q: Suppose load changes from 100 © to 200 ©
Find new IL, 1z, lt Is 1z safe 2
RS
| —>
VDG(IN} yo /
20V
VZ
OV
+
200Q
Trang 14ZENER DIODE EXAMPLE (CONT’D)
= [20 - 9]/82 © = 134 mA (no change)
IZ=IT-IL= 134mA- 45mA = 89mA
Since |zm = 55mA, zener is not safe
-
20V 9V 2002
Trang 15ZENER DIODE EXAMPLE (CONT’D)
Q: Suppose load changes from 100 © to 80 Q
Find new IL, 1z, It Is 1z safe ?
A: It= VƯRL= Vz/ R.= 9V/80 O = 112.5 mA
RS
| —>
VDG(IN} yo /
20V
VZ
OV
+
80Q
Trang 16ZENER DIODE EXAMPLE (CONT’D)
A: It= Vrs/ Rs = [Vpciny - VZ]/ Rs
= [20 - 9]/82 © = 134 mA (no change)
IZ=IT-IL= 134mA-112.5mA = 21.5 MA
Since |zm = 55 mA, zener is safe
-
20V 9V 80Q
Trang 17ZENER DIODE EXAMPLE (CONT’D)
Q: Suppose zener regulator circuit has no load
connected, is |Z safe ?
= [20 - 9]/82 © = 134 mA (no change) Iz= Ir= 134 mA this is very Unsafe
RS
| ——>
+
Py
O
Trang 18ZENER DIODE EXAMPLE (CONT’D)
Q: In the previous example, suppose Vociin)
changes from 20V to 30V IsIz safe 2
It = [30-9]/82Q = 256 mA
Iz=IT-IL= 256 mA- 90 mA= 166MmA
Very Unsafe
| —> RS
|Z +
30V ov 1002
Trang 19ZENER DIODE EXAMPLE (CONT’D)
Q: Suppose Vociin) changes from 20V to 18V
lsIz safe 2
lr= [18-9]/82 = 110 mÃÀ Iz=IT-IL= 110 mA- 90 mA= 20 mA
| —> 2 | —>
+ 820 | T
|
18V OV 1002