DC Voltage Regulators
A voltage regulator is an electronic circuit which maintains the
output voltage (almost) constant in spite of changes within some
specified limits in the load current, input voltage, temperature, etc .
VO = vI – vregulator
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Regulator types
• Parametric regulators (with ZD, without active devices)
• Linear voltage regulators (contain active devices) – the
transistors that adjust the output voltage to the default value
operate in the linear regime (permanent conduction).
• Switching voltage regulators (contain active devices) – the
main transistors that adjust the output voltage to the default
value operate in switching regime, generally at a frequency
≥ 20KHz
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Parametric voltage regulator
Zener Diode
relative regulation factor of ZD
∆vZ
VZ
FZ =
∆iZ
IZ
rz
FZ =
rZ
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Regulation region of ZD
I Z max
Pd max
=
VZ
I Z min < I Z < I Z max
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Parametric voltage regulator
The parametric regulator uses as operating principle
the nonlinear current-voltage characteristic of the ZD.
• Size R
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Parametric voltage regulator –
R value
I O min
Worst case for maxim iZ:
Rmin
vImax − VZ
=
I Z max + I O min
VZ
=
R L max
I O max
VZ
=
RL min
v I − VZ
iZ =
− IO
R
vImax − VZ
iZ max =
− I O min
Rmin
Rmax
vImin − VZ
=
I Z min + I O max
R ∈ (Rmin; Rmax)
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Shortcoming of the parametric voltage
regulator. Solution
Shortcoming:
Narrow range of IO
iZ = i R − I O
Solution:
Voltage follower:
IO increases, RO decreases
Voltage follower
vO = vI
Voltage regulator
VO = VREF
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Voltage follower
with BJT, CC
Voltage regulator
VO = VREF − vBE
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BJT voltage regulator
T – pass transistor
VO = VREF − vBE
iZ = i R −
IO
β
vBE makes the regulation performances worse
Regulation mechanism of VO:
VO ↓ I O ↓ I B ↓ iZ ↑ VZ ↑ VO ↑
NF mechanism
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Op-amp voltage regulators
VO = VREF
A better regulation is provided if R is replaced by a current source
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Op-amp voltage regulators
VO > VREF
VO=?
 R2 
VO = 1 + VREF
 R1 
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Op-amp voltage regulators
VO < VREF
VO=?
Consider that
VREF is given
R2
VO =
VREF
R1 + R2
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Op-amp voltage regulators
Adjustable VO
VO min

R2 
VREF
= 1 +
 P + R1 
VO max
 R2 + P 
VREF
= 1 +
R1 

VO=?
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Increasing the output current
I O max = I O ,OA max
For common use op amp:
I O,OA max ≈ 20mA
? Higher current in the load
Solutions:
• power op amp; e.g. TDA2030, up to 3,5A
• current amplifier between op amp and load
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I O max = βI O ,OA max
T – pass transistor
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Overcurrent and shortcircuit protection
RL → 0
IO → ∞
The current must be limited:
• oversee IO
• when IO exceeds a default value , protection circuit goes on
RP I O < 0.6V ; TP − (off ) ; I P = 0
RP I O ≈ 0.6V ; TP − (a F ) ; I P > 0
I O max =
I O max
When
VRP
RP
+ IP
0,6V
0,6V
=
+ IP ≈
RP
RP
I O = I O max
RL ↓, I O ↑, I O RP ↑, I P ↑, I B ↓, I O ↓
VO ↓
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The output characteristic
I Osc
0.6V
1 0.6V 0.6V
=
+ I O ,OA max −
≈
+ I O ,OA max
RP
RP
β RP
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Selecting the pass transistor
v I ∈ (VImin ; VImax )
I O max
maximum collector current
maximum collector-emitter voltage VCE max = VImax
power dissipated by transistor PdT ≈ I CVCE
maximum power dissipated by transistor
PdT max ≈ max( I O max (VImax − VO ); I OscVImax )
The transistor is selected so that:
I C max > 2 I O max
VCE 0 > VCE max
Pd max 0 ≥ PdT max
without heatsink
Pd max r ≥ PdT max
with heatsink
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Voltage reference
VREF − VZ 9 − 6,3
IZ =
=
= 7,5mA
R3
374
VREF
 R2 
 2,67 
VZ = 1 +
= 1 +
 ⋅ 6,2 = 9V
R1 
5,9 


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Example
VO range ?
VO (I O ) considering the tap to the lower end
Maximum power dissipated by T1 for RL = 2k; RL = 25Ω
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