1. Ch. Regulators and DC-DC Converters
1. Goals
Understand the usage and operating method of regulators and DC-DC converters
and implement a circuit that produces a desired voltage
2. Objectives
1. Understand the characteristics and operating method of the regulators
2. Understand the characteristics and operating method of the DC-DC converters
3. Understand the characteristics and operating method of sensors
3. Theory
3.1. Regulator
A voltage regulator is a device that maintains an output of a certain voltage
level regardless of the change in input voltage and / or environmental variables.
Since using the power supply as the supply voltage directly onto the circuit can
introduce various noise and distortions, most circuits receive the voltage from
regulators.
There are mainly two types of voltage regulators : a linear type and a
switching
type.
A
linear
type
have
simple
circuit
structures,
utilizes
cheap
components and has an excellent output voltage characteristics, but it tends to have
bad efficiency that introduces much heat, requiring, in some cases, heat insulating
boards. Some of the most utilized linear type ICs are the 78XX series, where XX
refers to the output voltage. For example, 7805 is a voltage regulator IC with the
output voltage level of 5Vs.
Switching type utilizes the pulse width modulation (PWM) and controls the
on/off duty of the pulse to control the average voltage level. While this type
introduces few heating, it is more expensive and has a more complicated circuit
structure. Some of the most utilized switching type ICs are 2575-XX series, where
XX again regers to the output voltage.
In general, linear type regulators require at least 2V+supply voltage. In
other words, to produce the output voltage level of 5V, the input voltage must be
at least 7V. On the other hand, LDOs require the input voltage to be only 0.2~0.5V
higher than the output voltage. LDO is a kind of linear type regulators and is an
acronnym for Low Drop Output.
Another kind of linear type regulators is the regulated constant voltage
source. Regulated constant voltage source like figure 1.1 detects the output voltage,
compares it with the difference between an appropriate ratio of the measurement
value of Vm and the reference voltage(Vref), and changes the output current to
change the measurement value to equal the reference voltage, which in turn keeps
the output voltage constant.
IE
IB
Comparator
VS
R1
Vm
Vref
Vo
RL
R2
Figure 1-1 Structure of the regulated constant
voltage source
From figure 1-1, when
Vref = Vm , the base current of the BJT IBQ and the
desired output voltage VOQ, if the output voltage becomes lower than VOQ, Vm also
decreases which in turn produces a negative difference between Vref and Vm. If the
current of BJT IB is increased to be larger than IBQ, the BJT emitter current IE
increases and thus the current through the load resistors. This effectively increases
the output voltage. On the other hand, if the output voltage is larger than VOQ
which makes Vm larger than Vref, IE can be decreased to become lower than IEQ,
which can decrease the output current and the output voltage. Such architecture
where the output of a device is detected and is used to control the output of the
upcoming signal is called feedback and is used in many microelectronic circuits.
Figure 1-2 shows an example of figure 1-1 implemented in circuits.
Q1
R1
RB
Vm
Q2
VS
Vo
VBE
Vz
RL
R2
Figure 1-2 imlementation example of the regulated
constant voltage source
From figure 1-2, since the collector current of Q2 only flows if the base-to-emitter
voltage is larger than 0.7V, the reference voltage Vref is given as follows,
(Eq. 1-1)
Note that RB is used to provide Q1 its operating point current IEQ. The
measurement voltage Vm, considering the base current of Q2 to be small enough to
be negligible, can be calculated as follows by the voltage divider of R1 and R2.
(Eq. 1-2)
Since the Vref and Vm is equal, from Eq. 1-1 and 1-2, the output voltage can be
calculated to be
(Eq. 1-3)
Therefore, the output voltage can be adjusted by the ratio of R1 and R2. This also
means that by using a variable resistor in place of R1 or R2 can create a regulated
variable voltage source.
3.2 DC to DC converter
DC-DC converter creates a DC voltage of a different level from a DC
supply voltage of a certain level. There are two types of DC-DC converters: buck
DC-DC converter (or step-down DC-DC converter) which creates a voltage level
lower than that of the supply voltage, and boost DC-DC converter which creates a
voltage level higher than that of the supply voltage.
3.2.1. Buck DC-DC converter
Buck DC-DC converters using inductors comprise of a switch, a diode, an
inductor, and a capacitor, as shown in figure 1-3. For the sake of analysis, diode
is considered to be an ideal diode with zero threshold voltage and the capacitor to
be large enough to make the change of Vo according to time negligible.
L
iS
Vs
iL
iD
Vo
C
RL
Figure 1-3 Architecture of buck DC-DC converter
The switch on figure 1-3 infinitely repeats the cycle of turning on and off
with the period of T. While the switch is on, the diode is reversely biased, and
current is injected into the capacitor through the switch and the inductor as shown
on figure 1-4(a).
L
iS
Vs
L
iL
C
(a) on
Vo
iL
iD
C
(b) off
Figure 1-4 Buck DC-DC converter
iC
The inductor current while the switch is on is given as follows,
(Eq. 1-4)
where iL(0) is the initial current through the inductor before the switch is turned
on. If the switch was on for the duration of D, the inductor current at time D is
calculated as;
(Eq. 1-5)
The current through inductor when, at time D, the switch is turned off as
shown on figure 1-4(b) is calculated as:
(Eq. 1-6)
where iL(D) is the from value Eq. 1-5, the initial current through the inductor just
before the turning off of the switch. When, at time T, the switch is turned on
again, the current through the inductor is calculated as:
(Eq. 1-7)
When the switch is turned on and off infinitely, the inductor current at time
T is equivalent to iL(0) of the eq. 1-4. Therefore, the following equation can be
derived from eq. 1-7.
(Eq. 1-8)
From eq. 1-8, the output voltage Vo is calculated as:
(Eq. 1-9)
Since D<T, the output voltage is lower than the input voltage. The ratio of D to T,
or the ratio of the duration with the switch on to that with the switch off is called
the duty. The duty of 50% means that the duration with the switch on is the same
as the duration with the switch off.
3.2.2. Boost DC-DC converter
To obtain a voltage level higher than that of the supply voltage, boost
DC-DC converter such as figure 1-5 is used. Same as the buck DC-DC converters,
the switch infinitely repeats the turning on and off cycle.
iD
L
iL
Vs
iS
iC C
Vo
RL
Figure 1-5 Architecture of the boost DC-DC converter
From the schematic of figure 1-5, the equivalent circuit with the switch on
and off can be drawn as figure 1-6. If the switch is off, the diode is reversely
biased and does not have a current flow through itself. Charge in the capacitor,
therefore, is discharged to the load, providing the current.
L
Vs
iL
L
Vs
(a) on
iL
C
Vo
(b) off
Figure 1-6 Operating principle of the boost DC-DC converter
The inductor current while the switch is on is calculated as:
(Eq. 1-10)
where iL(0) refers to the initial current through the inductor right before the switch
is turned on. The inductor current at time D where the switch is turned off is
calculated as:
(Eq. 1-11)
The inductor current with the switch off can be calculated as:
(Eq. 1-12)
where iL(D) denote the initial current through the inductor just before the switch is
turned off at D. When the switch is turned on again at time T, the inductor current
is calculated as:
(Eq. 1-13)
Assuming that the turning on and off of the switch is repeated infinitely,
the iL(0) of equation 1-10, and the following equation can be derived:
(Eq. 1-14)
Therefore, the output voltage can be derived from equation 1-14 as:
(Eq. 1-15)
Since D<T, it is evident that the output voltage is larger than the input
voltage.
4. Prelab Report
Prelab reports should include how the experiment would be performed according to
the design specifications. The following information must be included, and it is
recommended that any extra experiment is included in the report as well.
4.1. Regulator
1. Refer to the datasheet of LM7805 and draw the circuit schematic and the
component values that satisfy the specification of 6.1.1 and 6.1.2.
2. Research the characteristics of regulators.
4.2. DC to DC converter
1. Refer to the datasheet of VASD1-DIP and draw the circuit schematic and the
component values that satisfy the specification of 6.2.1.
2. Research the characteristics of DC-DC converters.
4.3. Sensors
1. Refer to the datasheet of a range-finding sensor GP2YOA21YK0F and research
the operating principle of the sensor.
5. Equipments and components
Power Supply, Digital Multimeter, Oscilloscope
LM7805, LM7905, uA741, VASD1-S5-D5-DIP, Sensor (GP2Y0A21YK0F)
Resistor, Capacitor
6. Design specifications
6.1 Regulator
6.1.1 fixed output voltage
Variable
Input
Regulator
(LM7805)
5V
LM7805
Input Voltage
Variable
Output Voltage
5V
Using Regulators, design a circuit that produces a constant voltage despite changes
in the input voltage that is higher than a certain voltage.
6.1.2 variable output voltage
Variable
Input
Regulator
(LM7805)
Variable
Output
LM7805, uA741
Input Voltage
Variable
Output Voltage
8V, 12V, 15V
Using regulators and OP-AMPs, design three circuits with fixed output voltages 8V,
12V, and 15V.
6.1.3 fixed output voltage
Variable
Input
Regulator
(LM7905)
-5V
LM7905
Input Voltage
Variable
Output Voltage
-5V
Using Regulators, design a circuit that produces a constant voltage despite changes
in the input voltage that is higher than a certain voltage.
6.2 DC to DC converter
Variable
Input
Regulator
(LM7805)
5V
DC-DC
Converter
(VASD1-S5-D5DIP)
+5V
-5V
VASD1-S5-D5-DIP
Input Voltage
5V
Output Voltage
5V
Utilizing the output from regulators as the input to the DC-DC converter, design a
circuit that produces 5V as its output.
6.3 Sensor
Variable
Input
Regulator
(LM7805)
5V
Sensor
Voltage
Let the supply voltage of the sensor be from the regulator and observe the output
voltage according to the change in the distance. Let the input of the oscilloscope
be AC and observe the wave pattern while changing the distance between the
sensor and the object.
7. Final report
Final reports should be in a format of regular reports and should include
the experiment's goal, design procedure, measurement results and the discussion
thereof. Information below should also be included as well as a picture of the
implemented board. Inclusion of any extra experimentation is recommended as well.
7.1 Regulator
1. Compare and contrast the characteristics of 6.1.1, 6.1.2, and 6.1.3.
2. Grasp the characteristics of regulators that were found during this experiment.
7.2. DC-DC converter
1. Grasp the characteristics of DC-DC converters that were found during this
experiment.
7.3 Sensor
1. Compare and contrast the measured output voltage according to the distance
with the waveform on the datasheet. If any discrepancy exists, explain.
2. Check if there is a difference in the results when the input of the oscilloscope
is AC and DC. If any discrepancy exists, explain.