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
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), negative
feedback.
• 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, negative feedback.
Parametric voltage regulator
Zener Diode
relative regulation factor of ZD
vZ
VZ
FZ 
iZ
IZ
rz
FZ 
rZ
Regulation region of ZD
Datasheet of a ZD:
Pd max , VZ @ I Z
I Z max
Pd max

VZ max
Can consider
I Z max
To maintain ZD in its
regulation region
I Z min  I Z  I Z max
Pd max

VZ
Excerpt from datasheet
Parametric voltage regulator
The parametric regulator uses as operating principle
the nonlinear current-voltage characteristic of the ZD.
VO  VZ
• Size R
Parametric voltage regulator – R value
Design data
vI  (vImin , vImax ), VO , RL  ( RL min , RL max )
vI  VZ
iZ  iR  I O 
 IO
R
Worst case for maxim iZ:
vImax  VZ
iZ max 
 I O min
Rmin
vImax  VZ
Rmin 
I Z max  I O min
I O min
VZ

R L max
Rmax
I O max
VZ

RL min
vImin  VZ

I Z min  I O max
R  (Rmin; Rmax)
Shortcoming of the parametric
voltage regulator. Solution
Shortcoming:
IO directly affect iZ
iZ  iR  I O
Narrow range of IO to maintain
ZD in its regulation region
Solution:
Voltage follower
with op-amp
Voltage follower (buffer, current amplifier)
between ZD and load
vO  vI
Voltage regulator
with op-amp
VO  VREF
Voltage follower (amplifier)
with BJT
Voltage regulator ?
VO  VREF  vBE
Linear voltage regulators
BJT voltage regulator
T – series pass
transistor
VO  VREF  VBE
IO
iZ  i R 

VBE makes the regulation performances worse
Regulation mechanism of VO:
VO  I O  I B  iZ  VZ  VO 
NF mechanism
Op-amp voltage regulators
VO  VREF
A better regulation is
provided if R is
replaced by a current
source
How does the complete
schematic look like?
Op-amp voltage regulators
VO  VREF
VO=?
 R2 
VO  1  VREF
 R1 
Op-amp voltage regulators
Consider that VREF is given
VO=?
VO  VREF
R2
VO 
VREF
R1  R2
Op-amp voltage regulators
Adjustable VO , VO > VREF
VO=?
VO min

R2 
VREF
 1 
 P  R1 
VO max
 R2  P 
VREF
 1 
R1 

How does the circuit look like for adjustable VO
VO < VREF ?
How does the circuit look like for adjustable VO
VOmin < VREF and VOmax > VREF ?
Increasing the output current
I O max  I O,OA max
Common purpose 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
I O  I O,OA
T – series pass
transistor
Overcurrent and shortcircuit protection
I O   The current must be limited:
RL  0
• oversee IO
• when IO exceeds a default value, protection circuit triggers
RP I O  0.6V ; TP  (off ) ; I P  0
RP I O  0.6V ; TP  (aF ) ; I P  0
I O max 
VRP
RP
I O max
When
I O  I O max
RL , I O , I O RP , I P , I B , I O 
when TP
turns on
0.6V

RP
VO 
The output
characteristic
• voltage regulation region
VO
vO  VO , I O 
RL
• knee point
vO  VO , I O  I O max
0.6V

RP
0.6V
• current
v  VO , vO  I O RL , I O  I O max  I P 
limiting region O
RP
vO  0,
0 .6 V
1 0 .6 V 0 .6 V
0 .6 V

 I O , AO max 

 I O , AO max 
RP
 RP
RP
RP
• short-circuit point
I O  I Osc
Illustration RP  0.5, I O, AO max  20mA, I O max  ?, I Osc  ?
Maximum values of electrical quantities for T
vI  VImin ; VImax 
• maximum collector current:
• maximum collector-emitter voltage
I O max
VCE  VI  VRP  VO
VCEmax appears for short-circuit to the output
VCE max  VImax  VRP  VImax  0.6V  VImax
VCE max  VImax
• maximum power dissipated by the transistor
PdT  I CVCE
PdT max  I O maxVImax
appears for short-circuit to the output
Selecting the series pass transistor
In the transistor data-sheets we can find absolute maximum ratings for
I C max
• collector-emitter voltage VCE 0
Pd tot
• power
• collector current
T should be selected so that:
I C max  2 I O max
VCE 0  VCE max
0,4P d tot
 P dT max
Pay attention for dissipated power. The value in
the data-sheet refers to the maximum power when
T is mounted on an infinite area heatsink. In
practice the maximum power should be consider
to be Pdmax ≈ 0,4Pdtot (acceptable size heatsink).
Voltage reference
IZ 
VREF
VREF  VZ 9  6.3

 7.5mA
R3
374
 R2 
 2.67 


 1 
VZ  1 
  6.2  9V

R1 
5.9 


Example
2.7k
1.5
Current through the Zener diode ?
VO range ?
vO iO  considerin g the tap to the middle position;
Maximum power dissipated by T1 for RL  500; RL  14; short  circuit considerin g :
i) the tap in the top position
ii) the tap in the middle position