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Unit 7
Linear Power Supplies, Switched
Mode Power Supply
Learning Objectives:
Linear Power Supplies:
14.1 Constituents of Linear Power Supply
14.2 Designing Mains Transformer
14.6 Linear IC Voltage Regulators
14.7 Regulated Power Supply Parameters
Switched Mode Power Supplies:
15.1 Switched Mode Power Supplies
15.6 Connecting Power Converters in Series
15.7 Connecting Power Converters in Parallel
14.1 Constituents Linear Power Supply:
•
•
•
•
Stepdown transformer
Rectifier
Filter (C, LC)
Regulator (ICs LM78XX, LM79XX, LM321)
14.2 Designing Mains Transformer:
1. To avoid magnetic saturation – core cross
section area, Ac
Ac = √P / 5.6
where P=power required in Watts
2. t / W ratio =
stack thickness (t)
-------------------------------- = 1.1 to 1.5
Width of central limb (W)
…
3. Turns per volt =
108
----------------------4.44 x f x Ac x B
Where
f=frequency=50 or 60 Hz
B=flux density=50000 lines / square inch
(gauss)
4. Turns ratio (Ns / Np), n
=Vp / Vs
= Ns / Np
5. Primary current =
P
------------------------------------Efficiency x Primary Voltage
…
6. Secondary current =
Primary Current
----------------------n
where n is turns ratio
14.1 Numerical Example:
14.6 Linear IC Regulators:
Features:
• PCB mountable
• Precision
• Best voltage & current regulation
• Wide range of output voltages
• Protection inbuilt: current limit & thermal
shutdown
Three terminal regulators:
Boosting current delivery capability:
14.7 Regulated Power Supply Parameters:
Load regulation:
Change in regulated output voltage as the load
current varies.
VNL – VFL
% load regulation = --------------- X 100
VFL
Line regulation:
is defined in terms of variation of regulated
output voltage for a specified change in line
(input AC) voltage.
Eg.
For input voltage variation, say 170 V to 220V,
output voltage 10V, varies ±1% has lineregulation (1+1)
= ---------- x 100
10
= 0.2 %
Output Impedance:
In Thevenin’s equivalent circuit of RPS, output
impedance comes in series with load
resistance.
It determines load regulation of the power
supply.
Ripple rejection factor:
15.1 Switched Mode Power Supplies:
power supply
Linear
1.
2.
3.
4.
Stepped down,
Rectified,
Filtered,
Regulated
Loss
Costly
Switched
1. The unregulated DC is chopped at a
high frequency (using transistor /
MOSFET / IGBT)
2. The chopped waveform is then rectified
and filtered to get the desired DC voltage.
15.5 Switching Regulator:
1. Buck regulator – step down
2. Boost regulator – step up
3. Buck-boost regulator – both step down / up
15.6 Connecting Power Converters in Series:
• Can be connected in series, provided
specifications allow for the series connections.
• It is possible that the output of one converter
affects the output loop of another.
• Output voltage should not exceed breakdown
voltage of the devices.
15.6 Connecting Power Converters in parallel:
• Again, if specification allow the connection,
then they can be connected in parallel.