Power

1. POWER SUPPLY
Fabrication of Power supply
&
SMPS

2. Block Diagram of Medical Equipments
Power
supply
Input Processing Output
Device Unit Device
Control

3. • The electric power is not normally used in
the form in which it is produced or
distributed.
• Practically all electronic systems require
some form of Power conversion.
• A device that transfers electric energy
from a source to a load using electronic
circuits is referred to as power supply.
• A typical application of a power supply is
to convert utility AC voltage into regulated
DC voltages required for electronic
equipment.

4. Categories of Power Supplies
There are two broad categories of power
supplies:
• Linear regulated power supply
• switched mode power supply (SMPS)
In some cases one may use a
combination of switched mode and linear
power supplies to gain some desired
advantages of both the types.

5. Block diagram of power supply
Parts of a power supply:

6. • The AC voltage is connected to a transformer,
which steps that ac voltage down to the level for
the desired dc output.
• A diode rectifier then provides a full-wave
rectified voltage.
• This is initially filtered by a simple capacitor filter
to produce a dc voltage.
• This resulting dc voltage usually has some ripple
or ac voltage regulation.
• A regulator circuit can use this dc input to
provide a dc voltage that not only has much less
ripple voltage but also remains the same dc
value even if the input dc voltage varies
somewhat or the load connected to the output
dc voltage changes.
• This voltage regulation is usually obtained using
one of the voltage regulator IC units.

8. Transformer
• Transformer convert
Ac electricity from one
voltage to another
with little loss of
power.
• Transformers work
only with AC & this is
one of the reasons
why mains electricity
is AC.

9. TYPES OF TRANSFORMER
• Step-up Transformer
• Step-down Transformer

10. • Step-up transformers increase voltage,
step-down transformers reduce voltage.
• The input coil is called the primary & the
output coil is called the secondary.
• There is no electrical connection between
the two coils, instead they are linked by
the alternating magnetic field created in
the soft iron core of the transformer

11. • The two lines in the middle of the circuit
symbol represent the core.
• Transformers waste very little power , so
the power out is almost equal to
the power in. So, as voltage is stepped
down current is stepped up.

13. RECTIFIER
• In mains supplied electronic systems the AC
input voltage must be converted into a DC
voltage with the right value & degree of
stabilization.
• Rectifier does this work.
• In other words a rectifier circuit is necessary to
convert a signal having zero average value into
one that has a nonzero average.
• Two types of rectifiers :
a. Half wave rectifier.
b. Full wave rectifier.

15. • Figure above uses a center-tapped
transformer with two rectifier diodes.
• Figure below uses a simple transformer &
four rectifier diodes usually known as a
bridge rectifier.

18. SMOOTHING/FILTER
• We need a way to smooth out the
pulsations& get a much cleaner dc power
source for the load circuit.
• This is done by a filter circuit.
• In power supply, a filter must remove or
reduce the ac variations while still making
the desired dc available to the load
circuitry.
• Any given filter involve capacitors,
inductors,&/resistors in some combination.

19. Types of filters
• Capacitor Filter : Used to obtain
essentially a dc voltage with some ripple.
• RC Filter : It is possible to reduce the
amount of ripple across a filter capacitor
by using an additional RC filter .
• LC Filter : This reduces Hum & ripple
(Used where noise should be reduced).

20. CAPACITOR FILTER
• Smoothing is performed by a large value
electrolytic capacitor connected across the DC
supply to act as a reservoir, supplying current to
the output when the varying DC voltage from the
rectifier is falling.
• The diagram shows the unsmoothed varying DC
(dotted line) & the smoothed DC (solid line).
• The capacitor charges quickly near the peak of
the varying DC, & then discharges as it supplies
current to the output.

21. Note: The smoothing significantly increases the average
DC voltage to almost the Peak value(1.4×RMS value).
Ex: A 6V RMS AC is rectified to Dc of about 4.6V RMS,
with smoothing this increases to almost giving
1.4×4.6=6.4V smooth DC.

22. • Smoothing is not perfect due to the capacitor
falling a little as it discharges, giving a small
ripple voltage.
• For many circuits a ripple which is 10% of the
supply voltage is satisfactory & the equation
below gives the required value of the smoothing
capacitor.
C= 5×Io ÷ Vs × f
• A large capacitor will give less ripple.
• Capacitor value must be doubled when
smoothing half-wave DC.

24. RC filter
In order to reduce the ripple still more , we need to extend
the filter a bit.

25. LC filter
RC filter reduces ripple voltage with lot of resistive
losses, we can replace R with L as shown below.

26. REGULATOR
• Regulator - eliminates ripple by setting DC
output to a fixed voltage
• Voltage regulator IC’s are available with fixed
(typically 5,12 &15V) or variable output voltages.
• They are also rated by the maximum current
they can pass.
• Negative voltage regulators are available, mainly
for use in dual supplies.
• Many regulators include some automatic
protection from excessive current (overload
protection) & overheating (thermal protection).

27. Many of the fixed Voltage regulator IC’s have three
leads & look like power transistors, such as shown
here.
They include a hole for attaching a heat sink if
necessary.

28. The regulated DC output is very smooth with no
ripple. It is suitable for all electronic circuits.

29. Zener diode Regulators
• For low current power supplies a simple voltage
regulator can be made with a resistor & a zener
diode connected in reverse as shown in the
diagram.
• Zener diodes are rated by their breakdown voltage
Vz & maximum power Pz.
• The resistor limits the current (like an LED resistor).
• The current through resistor is constant, so when
there is no output current all the current flows
through zener diode & its power rating must be
large enough to withstand this.

31. • Choosing a zener diode and resistor:
• The zener voltage Vz is the output voltage
required
• The input voltage Vs must be a few volts
greater than Vz
(this is to allow for small fluctuations in Vs
due to ripple)
• The maximum current Imax is the output
current required plus 10%
• The zener power Pz is determined by the
maximum current: Pz > Vz × Imax
• The resistor resistance: R = (Vs - Vz) /
Imax
• The resistor power rating: P > (Vs - Vz) ×
Imax

32. • Example: output voltage required is 5V,
output current required is 60mA. Vz = 4.7V
(nearest value available)
• Vs = 8V (it must be a few volts greater than
Vz)
• Imax = 66mA (output current plus 10%)
• Pz > 4.7V × 66mA = 310mW, choose Pz =
400mW
• R = (8V - 4.7V) / 66mA = 0.05k = 50 ,
choose R = 47
• Resistor power rating P > (8V - 4.7V) ×
66mA = 218mW, choose P = 0.5W

33. DUAL SUPPLIES
• Some electronic circuits
require a power supply
with positive & negative
outputs as well zero
volts(0V).
• This is called “dual
supply” because it is like
two ordinary supplies
connected together as
shown in the figure.
• Dual supplies have three
outputs +V, 0V, -V.

34. Negative voltage Regulator

35. Dual Supply Circuit

37. Some Common regulator Circuits
Op-amp based circuit with excellent regulation

39. Advantages of traditional Power
supply
• The linear regulator is the building block of
nearly every power supply used in
electronics.
• The IC linear regulator is so easy to use
that it is virtually foolproof, & so
inexpensive that it is usually one of the
cheapest components in an electronic
assembly.

40. • Power supply discussed till now are also
known as linear regulators and
maintains desired output voltage by
dissipating excess power .
• Thus for proper operation of these
supplies heat sinks are a must.
• The power dissipated by a 15V regulator
is
Power dissipated=(dropout voltage)
(current)
=(18.95-15) (0.5)=1.98W
• The power dissipated by a 5V regulator is
Power dissipated=(8.91-5) (1.0)=3.91W

41. Defects in the Traditional Power Supplies
• Efficiency is very low save 45% to 55% only,

42. • A large amount of energy is wasted.
1. Unstabilized D.C voltage / current should be
greater than the stabilized voltage / current. So,
the energy loss is in the form of heat due to the
power transistor, etc.
2. Energy loss in the form of heat etc. is in the
main transformer.
a. Due to eddy current loss.
b. Copper loss.
• The size & weight of the transformer is also
large for high current.
In transformer, number of turns is
inversely proportional to frequency.
So less frequency means more turns & high
frequency means less turns.

43. • Very large value of capacitor is needed
for reservoir for high current say 10,000
mfd to 50,000 mfd.
Similarly at low frequency, capacitor value
will be increased( so size will increase). At
high frequency, capacitor value will
decrease & hence size will also decrease.
• The low frequency ripple from the main
is always difficult to remove completely,
even large capacitors are used.
• Get shock from the chassis if
accidentally touch it.

44. Answers to many of the problems for
making high current, low
dissipation, light weight, less space
and without low frequency ripple
Is….
Switched Mode power Supply

45. • The SMPS owes its name to dc-to-dc
switching converter for conversion from
unregulated dc input to regulated dc
output voltage.
• Typical frequency range of SMPS is from
50 kHz to several MHz.

46. Requirements of SMPS
• Less bulky Transformers
• Less Energy loss / Eddy currents
• Capacitors of small size

47. Switched-mode power supply
• The input supply drawn from ac mains is first
rectified & filtered using a capacitor.
• The unregulated dc voltage across the
capacitor is then fed to a high frequency
dc- to dc converter.
• Most of the dc-to-dc converters used in
SMPS circuits have an intermediate high
frequency ac conversion stage to facilitate
the use of a high frequency transformer for
voltage scaling & isolation.

48. Block diagram of SMPS
High freq.
dc to dc converter
AC dc to ac ac to dc
converter converter
mains

49. DC to DC conversion in SMPS
• One method is to use a High frequency
transformer for voltage scaling & isolation.
• Another popular method is to use simplified
switching circuits that omits the transformer
action.
In such SMPS, the unregulated input
dc voltage is fed to a high frequency
chopping circuit which switches between
“ON” & “OFF” states.

50. Types of SMPS
SMPS can be classified into four types according
to the input & output wave forms as follows:
• AC in, DC out: rectifier, off-line converter
input stage.
• DC in, DC out: Voltage converter or
current converter, DC to DC converter.
• AC in, AC out: Frequency changer or
cyclo converter.
• DC in, AC out: Inverter

51. AC in DC out (Ordinary SMPS)
This type of power supply previously used in
TV receivers / instruments having used
the main at full voltage to provide a power
for an oscillator / inverter whose output is
in turn is fed to high frequency
transformer, then to rectifier, switcher,
stabilizer and then to regulator. The
correct of the error signal and output
signal is also done by negative feedback
loop from output to the input switch.

52. How an SMPS works

53. View of SMPS

54. Input Rectifier
• The first stage is to convert AC input to
DC output (Rectification).
• The rectifier produces an unregulated DC
voltage which is sent to a large filter
capacitor.
• The current drawn from the mains supply
by this rectifier circuit occurs in short
pulses around the AC voltage peaks.

55. • These pulses have significant high
frequency energy which reduces the power
factor.
• Special control techniques can be
employed by the following SMPS to force
the average input current to follow the
sinusoidal shape of the AC input voltage
thus the designer should try correcting the
power factor.

56. INVERTER
• This converts DC, Whether directly from
the input or from the rectifier, to AC by
running it through a power oscillator,
whose output transformer is very small
with few windings at a frequency of tens or
hundreds of kHz.
• The frequency is usually chosen to be
above 20 kHZ, to make it inaudible to
humans.

57. OUTPUT TRANSFORMER
• If the output is required to be isolated from
input, (as is usually the case in mains
power supplies) the inverted AC is used to
drive the primary winding of a high-
frequency transformer.
• This converts the voltage up or down to
the required output level on its secondary
winding.

58. Output rectifier & filter
• If a DC output is required, the AC output from
the transformer is rectified.
• For output voltages above ten volts or so,
ordinary silicon diodes are commonly used.
• For lower voltages Schottky diodes are
commonly used as rectifier elements (they have
faster times than silicon diodes, thus allowing
low-loss operation at higher frequencies).
• For even lower output voltages, MOSFETs may
be used as synchronous rectifiers.

59. • The rectified output is then smoothed by a
filter consisting of inductors & capacitors.
• For higher switching frequencies,
components with lower capacitance &
inductance are needed.

60. Output
Control Signal
Block Diagram of Switch Mode Power Supply
Main
Voltage
Supply
Oscillator
High
frequency
transformer
Rectifying &
Smoothing Switching Smoothing

61. SMPS having opto-isolator in the
feedback loop.
15 V
Output
Control Signal Feedback Line
Block Diagram of SMPS Using Opto Isolator Transformer
Main
Supply
Inverter
Oscillator
High
Frequency
transformer
Rectifying &
Smoothing
Opto Isolator
Transformer

62. Features & drawbacks
• A lamp from output to Optocoupler.
• When Optocoupler fails, the light will fall on
the photocell, the mains will flow through
feedback circuit & reflect in output. Hence a
shock hazard.
• So, now SMPS using pulse transformer.
Feedback will have mark& space
modulation.

63. Other types of feedback loops in
SMPS
• SMPS having
isolation
transformer
(small size) in
the feedback
loop with pulse
width
modulator.
Output
Block Diagram of S.M.P.S Using Pulse Transformer
Main Supply
Pulse
Amplifier
High
Frequency
Transformer
Rectifying &
Smoothing
Pulse
Transformer
Pulse Width
Modulated
Oscillator.

64. SMPS having a single package (single
IC) is presently used for electronic
circuitry supply.
Rectifier 1
20 KHz
Switched
Mode
Voltage
Regulator
Rectifier 2
Load
T1 (20 KHz)
~ +
~ + ~ -
230 V
AC + + Regulated
Mains DC Output
Input - C1 - Voltage
~ -
Switch Mode D.C. Regulated Power Supply

65. Different Topologies of SMPS
• fly-back
• forward
• push-pull
• C’uk
• Sepic
• Half bridge
• H-bridge circuits.

66. • A particular topology may be more
suitable than others on the basis of one or
more performance criterions like cost,
efficiency, overall weight and size, output
power, output regulation, voltage ripple
etc.
• All the topologies listed above are
capable of providing isolated voltages by
incorporating a high frequency transformer
in the circuit.

67. Popular types of SMPS
Non Isolated SMPS:
• Non isolated power supplies contain an inductor
instead of a transformer. This type includes
Boost converters
Buck converters
Buck-boost converters
• These belong to the simplest class of single
input, single output converters which utilize one
inductor & one active switch (MOSFET).

68. Buck converter
• This reduces the input voltage, in direct
proportion to the ratio of the active switch “on”
time to the total switching period, called the duty
cycle.
For ex: An ideal buck converter with a 10V input
operating at a 50% duty cycle will produce an
average output voltage of 5V.
• A feed back control loop is employed to maintain
(regulate) the output voltage by varying the duty
cycle to compensate for the variations in the
input voltage.

69. Boost & Buck-boost converter
• The output voltage of a boost converter is
always greater than the input voltage.
• The buck-boost output voltage is inverted
but can be greater than, equal to, or less
than the magnitude of its input voltage.
• There are many variations & extensions to
this class of converters but these three
form the basics of almost all isolated &
non isolated DC to Dc converters.

70. Other types of SMPS
• By adding a second inductor the Cuk & SEPIC
converters can be implemented or by adding
additional active switches various bridge
converters can be realised.
• Other types of SMPSs use a capacitor-diode
voltage multiplier instead of inductors &
transformers.
• These are mostly used for generating high
voltages at low currents.
• The low voltage variant is called charge pump.

71. Chopper controller/Regulation
• A feedback circuit monitors the output voltage &
compares it with a reference voltage, which is
set manually or electronically to the desired
output.
• If there is an error in the output voltage, the
feedback circuit compensates by adjusting the
timing with which the MOSFETs are switched on
& off.
• This part of the power supply is called the
switching regulator.

72. • Depending on design/safety requirements, the
controller may or may not contain an isolation
mechanism (such as opto-couplers) to
isolate it from the DC output.
• Switching supplies in Bio-medical instruments
have these opto-couplers to tightly control the
output voltage & isolation of patient .
• Open regulators do not have a feedback circuit.
Instead, they rely on feeding a constant voltage
to the input of the transformer or inductor, &
assume that the output will be correct.

73. Advantage & disadvantage of
SMPS
Advantage:
• Greater efficiency because the switching
transistor dissipates little power in the saturated
state & the off state compared to the
semiconductor state.
• Small size & lighter weight ( elimination of low
frequency transformers)& low heat generation.
Disadvantage:
• Greater complexity
• Generation of high amplitude, high frequency
energy that a low pass filter blocks.

74. • Due to large electronic circuitry and
feedback from output to input it become
difficult to repair as compare to ordinary
Power supplies.
• The Transformer is small in size but it is
tuned to particular high frequency (20 kc/s –
50 kc/s). So, whenever the transformer
gone defective, it must be replaced with the
original one.
• The output load current / voltage must not
be more than 15% up or down the mention
standard value.

75. Application of SMPS in Biomedical
circuits
• Isolation of patients in diagnostic
instruments like ECG etc. (using opto-
couplers in control circuitry)
• SMPS battery chargers for battery based
instruments.

76. • To avoid shocks, in SMPS the input is isolated from
the output. This is achieved at two places in SMPS.
Once at mains & another at feedback using either
transformer or optocoupler.
• +ve feedback for oscillation & -ve feedback for
stabilization.
• To reduce losses, quality of transformer must be
better.
• At high frequency, no iron core will be used. Ferrite
core is used. Basically small particles of ferrite are
bound under pressure, so no shock hazard.
• Now Integrated SMPS are available in LSI & MSI
packages (1394)

77. THANK YOU

78. Comparison of LPS & SMPS
• Unlike a linear power supply, the pass transistor of
a switching mode supply switches very quickly
(typically between 50 kHz and 1 MHz) between
full-on and full-off states, which minimizes wasted
energy. Voltage regulation is provided by varying
the ratio of on to off time. In contrast, a linear
power supply must dissipate the excess voltage to
regulate the output. This higher efficiency is the
chief advantage of a switched-mode power supply.
• Switching regulators are used as replacements for
the linear regulators when higher efficiency,
smaller size or lighter weight are required. They
are, however, more complicated, their switching
currents can cause electrical noise problems if not
carefully suppressed,

79. Linear regulators

80. Switched mode Power supply

81. • SMPS is an electronic power supply unit
(PSU) that incorporates a switching
regulator.
• A linear regulator maintains the desired
output voltage by dissipating excess
power in a “pass” power transistor, the
SMPS rapidly switches a power transistor
between saturation (full on) & cut off
(completely off) with a variable duty cycle
whose average is the desired output
voltage.
• The resulting rectangular waveform is low-
pass filtered with an inductor & capacitor.