Download Electronic_Circuits_Unit-7

Subject Name: Electronic Circuits
Subject Code: 10CS32
Prepared By: Kavyashree.C,Supriya.V.Sullad
Department: CSE
Date:1/10/2014
5/12/2017
Agenda
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Introduction
Constituents of power supply
Designing Mains Transformer
Linear IC Voltage Regulators
Three terminal Regulators
Regulated power supply parameters
Introduction
• Every electronic system requires one or more DC
voltages for its operation.
• The regulated power supply converts the standard 220
volts, 50 or 60 Hz AC available at wall outlets into a
constants DC voltage.
• The DC voltage produce by a power supply is used to
power all the types of electronic circuits, such that
television receiver, stereo system, CD players and
laboratory equipment.
Constituents of power supply
• Transformer: “Downconvert” the AC line voltage to a smaller
peak voltage Vm, usually about 2-3 Volts larger than the
ultimately desired DC output.
• Rectifier circuit changes AC to DC.
• Filter ideally eliminates the fluctuations in the output voltage
of a half –wave rectifier and produces a constant-level dc
voltage.
• Regulator circuit is a feedback circuit that ensures output DC
does not change from its nominal value.
• All power supplies have built in protection circuit.
Designing Mains Transformer
1. Transformer does not saturate at desired load current or
output power.
Ac=√p÷5.6
Ac –core cross section in square inches
2. The ratio of stack thickness (t) to the width of center limb (w)
should be in the range of 1.1 to 1.5.
3. The turns per volt for different windings can be computed as
turns per volt=10^8/(4.44*f*B* Ac )
4. Primary and secondary turns can be computed from primary
and secondary voltages.
5. Primary current= p/(efficiency*primary voltage)
6. Secondary current=primary current/ɳ
ɳ= Ns/Np
Linear IC Voltage Regulators
Functional block diagram into 4 blocks:
1. Temparature compensator zener diode: constant current
source offer zener diode to operate in fixed point.
2. Error amplifier which controls Q1 which acts as a variable
resistor.
3. Series pass resistor: unregulated power supply source is
connected to collector.
4. Current limiter Q2: in short circuit condition.
Frequency compensation controls frequency response of an
amplifier.
High voltage regulator
Low voltage regulator
Three terminal Regulators
• They do not require any external components.
• These are available in both fixed and adjustable
output voltage(both positive and negative).
• The well known IC regulators are:
1) The 78XX series - for positive regulators
(2) The 79XX series - for negative regulators
(3) The LM 317 - for adjustable positive regulators
(4) The LM 337 - for adjustable negative regulators
The 78XX series - for positive regulators
• C1 and C2 are decoupling capacitors.
• C1 is generally used when regulator is located
far from power supply filter.
• The floating regulator could be made into a
variable regulator by replacing R2 with a pot.
However, there are several disadvantages:
– Minimum output voltage is Vreg instead of 0 V.
– IQ is relatively large and varies from chip to chip.
– Power dissipation in R2 can in some cases be quite
large resulting in bulky and expensive equipment.
• There is an input, an output and an adjustable
terminal.
Adjustable voltage Regulator
Vo  Vref
 Vref

 
 I adj  R2
 R1

Regulated power supply parameters
Defines the quality of regulated power supply.
1. Load regulation:
change in regulated output voltage of power supply as load
current varies from zero to maximum rated value.
percentage load regulation= VNL-VFL
*100
VFL
2. Line regulation: Variation of regulated output voltage for
change in input voltage.
line regulation=(0.2/10) *100= 2%
5/12/2017
3. Output Impedance:
• It determines load regulation of power supply.
• Regulator circuit is characterized by low
output impedence.
4.Ripple rejection factor:
• Defined as the ratio of ripple in regulated
output voltage to the ripple present in
unregulated input voltage.
• Calculated in decibels.
• Ripple is nothing but periodic variation in
input voltage.
• V ripple(output)=Vripple(input)/1+loop gain
Switched Mode Power Supplies
Linear v/s Switched Mode Power Supplies
• Linear Power Supplies have good line and load regulation, low
output voltage ripple and negligible radio frequency
interference.
• Switching power supplies have much higher efficiency(8090%) and reduced size and weight for a given power –
delivery capability.
• An improved efficiency and reduced size/weight
• Efficiency in switching supplies do not suffer as the
unregulated input to the regulated output.
• In portable systems operating from battery packs and requiring
higher DC voltages , the Switching supply is the only option.
Switching Regulators
• The buck Converter circuit consists of the switching
transistor, together with the flywheel circuit (Dl, L1
and C1). While the transistor is on, current is flowing
through the load via the inductor L1. The action of
any inductor opposes changes in current flow and
also acts as a store of energy. In this case the
switching transistor output is prevented from
increasing immediately to its peak value as
the inductor stores energy taken from the increasing
output; this stored energy is later released back into
the circuit as a back e.m.f. as current from the
switching transistor is rapidly switched off.
• Transistor Switch ‘on’ Period
• In Fig. 3.1.2 therefore, when the switching
transistor is switched on, it is supplying the
load with current. Initially current flow to the
load is restricted as energy is also being stored
in L1, therefore the current in the load and the
charge on C1 builds up gradually during the
‘on’ period. Notice that throughout the on
period, there will be a large positive voltage
on D1 cathode and so the diode will
be reverse biasedand therefore play no part in
the action.
• Transistor Switch ‘off’ Period
• When the transistor switches off as shown in Fig
3.1.3 the energy stored in the magnetic field around
L1 is released back into the circuit. The voltage across
the inductor (the back e.m.f.) is now in reverse
polarity to the voltage across L1 during the ‘on’
period, and sufficient stored energy is available in the
collapsing magnetic field to keep current flowing for
at least part of the time the transistor switch is open.
The back e.m.f. from L1 now causes current to flow around
the circuit via the load and D1, which is now forward biased.
Once the inductor has returned a large part of its stored energy
to the circuit and the load voltage begins to fall, the charge
stored in C1 becomes the main source of current, keeping
current flowing through the load until the next ‘on’ period
begins.
The overall effect of this is that, instead of a large square wave
appearing across the load, there remains only a ripple
waveform, i.e. a small amplitude, high frequency triangular
wave with a DC level of:
VOUT = VIN x (On time of switching waveform (tON) / periodic
time of switching waveform( T))
• Therefore if the switching waveform has a
mark to space ratio of 1:1, the output
VOUT from the buck Converter circuit will be
VIN x(0.5/1) or half of VIN. However if the mark
to space ratio of the switching waveform is
varied, any output voltage between
approximately 0V and VIN is
• Operating principle
• The key principle that drives the boost converter is
the tendency of an inductor to resist changes in
current by creating and destroying a magnetic field.
In a boost converter, the output voltage is always
higher than the input voltage. A schematic of a boost
power stage is shown in Figure 1.
• (a) When the switch is closed, current flows through
the inductor in clockwise direction and the inductor
stores some energy by generating a magnetic field.
Polarity of the left side of the inductor is positive.
(b) When the switch is opened, current will be reduced as the
impedance is higher. The magnetic field previously created will
be destroyed to maintain the current flow towards the load. Thus
the polarity will be reversed (means left side of inductor will be
negative now). As a result two sources will be in series causing a
higher voltage to charge the capacitor through the diode D.
If the switch is cycled fast enough, the inductor will not
discharge fully in between charging stages, and the load will
always see a voltage greater than that of the input source alone
when the switch is opened. Also while the switch is opened, the
capacitor in parallel with the load is charged to this combined
voltage. When the switch is then closed and the right hand side is
shorted out from the left hand side, the capacitor is therefore able
to provide the voltage and energy to the load. During this time,
the blocking diode prevents the capacitor from discharging
through the switch.
The switch must of course be opened again fast
enough to prevent the capacitor from discharging
too much.
Boost Regulator
Ideal Waveform
• The circuit diagram of a step up operation of
DC-DC converter is shown in Figure . When
the switch S1 is closed for time duration t1, the
inductor current rises and the energy is stored
in the inductor. If the switch S1 is openerd for
time duration t2, the energy stored in the
inductor is transferred to the load via the
didode D1 and the inductor current falls. The
waveform of the inductor current is shown in
figure.
• When the switch S1 is turned on, the output
voltage is given by:
Po=1/2x(L1XIp2 xf)+Vinx(Ip/2)x(tOff/T)
• The voltage across the load can be stepped up
by varying the duty ratio D
• The minimum output voltage is Vs and is
obtained when D = 0
• The converter cannot be switched on
continupusly such that D = 1. For values of D
tending to unity, the output becomes very
sensitive to changes in D
Connecting Power Converters in
Series
• If power converters are connected in series
greater load output can be expected
• While if one power converter fails to operate
,other will fail and the whole system shuts
down
Connecting Power Converters in
Parallel
• When Power converters are connected in
Parallel ,the system works even if one circuit
fails.
• While Parallel Converters suffers from unequal
Load Sharing.