Milwaukee School of Engineering
© MSOE 2015
Dr.-Ing. Joerg Mossbrucker
Electronic Devices
Linear Regulated Power Supply
1 Rectifier Circuits
One important application of diodes is the design of rectifier circuits used in dc power
supplies. One topology is the so-called linear regulated power supply shown in Figure 1.1.
vO
AC
Power
Voltage Transformer Rectifier
Source
Filter
Regulator
Load
FIGURE 1.1: TOPOLOGY OF A LINEAR REGULATED DC POWER SUPPLY
The AC voltage source is in general a 120Vrms, 60Hz ac signal. The power transformer is
designed to provide a particular secondary voltage (and also guarantees isolation from the
mains voltage).
1.1
Half-Wave Rectifier
The rectifier in a linear power supply converts an alternating current into a current flowing only in one direction. A single pn-junction diode can be used as a rectifier. Since the
secondary voltage of the power transformer is much larger than VD(on), the piecewise linear model of the pn-junction diode can be used. A so-called half-wave rectifier and its
voltage transfer characteristic is shown in Figure 1.2.
vO
vS
vO
0
VD(on)
FIGURE 1.2: HALF-WAVE RECTIFIER AND VOLTAGE TRANSFER CHARACTERISTIC
vS
2
For vS<VD(on), the diode is reverse biased, leading to zero current and vO=0. When
v S >V D(on) , the diode becomes forward biased and a diode current is flowing and
vO=vS-VD(on).
Assuming that vS is a sinusoidal signal, the output voltage is unidirectional and non-zero
for the positive half of the input voltage. This is shown in Figure 1.3.
vS
vO
VD(on)
t
t
FIGURE 1.3: INPUT AND OUTPUT SIGNAL OF A HALF-WAVE RECTIFIER CIRCUIT
One drawback of the half-wave rectifier circuit is that the output voltage is zero for more
than 50% of each cycle.
1.2
Bridge Rectifier
A circuit which gives a much higher utilization of the power transformer is the bridge rectifier. Its circuit and voltage transfer characteristic are shown in Figure 1.4.
vO
vS
vO
-2VD(on) 0
2VD(on)
vS
FIGURE 1.4: BRIDGE RECTIFIER AND VOLTAGE TRANSFER CHARACTERISTIC
The output voltage of a bridge rectifier is shown in Figure 1.5.
2VD(on)
vS
vO
t
FIGURE 1.5: INPUT AND OUTPUT SIGNAL OF A BRIDGE RECTIFIER CIRCUIT
t
3
Because of the much higher utilization of the power transformer, the bridge rectifier circuit is almost always used in linear power supplies.
1.3
Filter Capacitor
The output voltage of the bridge rectifier is zero for a brief moment every cycle. In order
to provide a continuous dc output voltage, an energy storing element is needed, that provides the necessary current. A so-called filter capacitor can be used for this purpose. The
capacitor is connected in parallel to the output voltage. The circuit and the output voltage of the filter capacitor are shown in Figure 1.6.
C
vS
vO
R
vO
Vr
VM
t
vS
FIGURE 1.6: CIRCUIT AND OUTPUT VOLTAGE OF A BRIDGE RECTIFIER WITH FILTER CAPACITOR
The ripple voltage Vr is defined as the difference between the maximum and the minimum of the output voltage, and can be approximated to be:
VM
V r = -------------2fRC
(1.1)
with f=120Hz (for a 60Hz ac input voltage) and VM being the maximum output voltage.
2 Zener Diode Circuits
The applied reverse-biased voltage on a pn-junction diode cannot increase without limit.
At some point, breakdown occurs and the current in the reverse-bias direction increases
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rapidly. The voltage at this point is called the breakdown voltage. The complete I-V characteristic is shown in Figure 2.1.
iD
-VZ
vD
breakdown
region
slope=1/rZ
FIGURE 2.1: COMPLETE I-V CHARACTERISTIC OF A PN JUNCTION DIODE INCLUDING BREAKDOWN
So-called Zener diodes are pn-junction diodes that provide a specified breakdown voltage
VZ. The slope of the I-V characteristic in the breakdown region is quite large, so the
incremental resistance rZ is small, typically in the range of a few ohms to tens of ohms.
2.1
Zener Regulator
Keeping the Zener diode in the breakdown region keeps the voltage across the diode
nearly constant. This effect can be used to regulate a voltage, as shown in Figure 2.2.
IS
RS
IZ
VS +-
ZD
IL
RL
VO
FIGURE 2.2: A ZENER DIODE VOLTAGE REGULATOR CIRCUIT
VS may be the output voltage of a filter capacitor which is not constant over time. For
proper operation of the circuit, the Zener Diode must operate in the breakdown region.
Assuming that the Zener resistance is zero, the output voltage VO=VZ. This leads to:
V S – VO
I Z = --------------------- – I L
RS
(2.1)
Equation 2.1 assumes operation in the breakdown region (IZ>0). In other words:
RL
V S -------------------- > V Z
RL + RS
(2.2)
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If equation 2.2 is not satisfied, the Zener diode operates in the reverse bias region and
IZ=0. The circuit shown in Figure 2.2 then operates as a voltage divider.
The power dissipated by the Zener diode is limited by the specification of the diode. It is
bounded by the following conditions:
The current in the diode is at minimum when the load current is at maximum and
the source voltage is at minimum.
The current in the diode is at maximum when the load current is at minimum and
the source voltage is at maximum.
We can solve for the series resistance Rs for both cases:
V S min – V Z
R S = ---------------------------------I Z min + I L max
(2.3)
V S max – V Z
R S = ---------------------------------I Z max + I L min
(2.4)
Reasonably it can be assumed that the input voltage range, the load current range, and
the Zener voltage are known. A typical requirement is that the minimum Zener current to
be one-tenth of the maximum Zener current. This leads to:
I L max ( V S max – V Z ) – I L min ( V S min – V Z )
I Z max = -----------------------------------------------------------------------------------------------------V S min – 0.9V Z – 0.1V S max
(2.5)
Using the maximum current thus obtained from Equation 2.5, the maximum power rating of the diode can be determined. Combining Equation 2.5 with either Equation 2.3 or
Equation 2.4 leads to the series resistance RS.
2.2
Zener Resistance and Percent Regulation
In actual Zener diodes, the Zener resistance is not zero. The result is that the output
voltage will fluctuate slightly with a fluctuation in the input voltage and will fluctuate
with changes in the load resistance. Two important regulation characteristics are defined
for a voltage regulator. The Source Regulation is the measure of the change in output
voltage with a change in source voltage and is defined as:
∆V
Source Regulation = ----------L- ⋅ 100%
∆V S
(2.6)
The Load Regulation is the measure of the change in output voltage with a change in
load current and is defined as:
V L no load – V L full load
Load Regulation = --------------------------------------------------- ⋅ 100%
V L full load
(2.7)
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3 Laboratory Assignment
Purpose: Bridge Rectifier, Filter Capacitor, Voltage Regulator, Source and Load Regulation
This laboratory assignment consists of two parts.
The first circuit is a bridge rectifier followed by a filter capacitor as shown in Figure 1.6.
Input ac voltage is specified as 20VP/60Hz.
The maximum load current IL max=10mA.
Calculate the required filter capacitor so that the ripple voltage Vr<2V.
Simulate the circuit and show the load voltage at the maximum load current.
Build the circuit and verify the simulation result.
The second circuit is a Zener regulator, as shown in Figure 2.2.
Nominal load voltage VO=VZ=7.5V.
Minimum load current IL min=0.
Maximum load current IL max=10mA.
Assume a Zener resistance rZ=5Ω.
Assume an input voltage VS=20V
Minimum Zener current to be one-tenth of the maximum Zener current.
Calculate the power rating of the Zener diode.
Calculate the required series resistance RS.
Simulate the circuit and determine the source and load regulation.
Build the circuit and verify your simulation results.