Industrial Electronics

Topic 16 – Power Supplies Circuits

A Introduction

1 A power supply is essentially a circuit that converts the AC voltage from the
mains to a stable DC voltage.

In this topic, we will discuss on the operation of the regulated DC Power

supply.

Input Supply

AC Input DC Output
Current
IC
Transformer Rectifier Filter Boost &
Regulator
Limiting

Fig. 16.1 The functional block diagram of a linear power supply

B Principle Operation of regulated DC power Supply

Fig 16.1 shows the functional block diagram of a linear power supply. The
functions of the various block are:

a) The transformer adjusts the input mains AC voltage to an AC

voltage appropriate for the rectifier.

b) The rectifier converts the AC voltage to a pulsating DC voltage.

c) The filter smoothes the pulsating DC voltage by reducing the

fluctuation of the voltage (unregulated DC voltage).

d) The IC voltage regulator maintains a constant output voltage that

is independent of the load conditions, the unregulated DC input
voltage level, and the operating temperature.

e) The current boost and limiting circuit increases the output current
to the required value and provides protection to the power supply
when the output circuit is accidentally shorted.

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Topic 16 – Power Supplies Circuits

C POWER SUPPLY SPECIFICATION

1 We are interested in the following properties and specifications of a power

supply

a) Load Regulation

i) If the power supply is delivering its full rated load current

and then the load is removed, the output voltage will rise.
The load regulation is a measure of this rise.

ii) Load regulation is defined as:

Load regulation = x 100%

b) Line Regulation

i) If the ac mains input voltage varies then the output dc

voltage from the power supply will also vary. The line
regulation is a measure of this variation.

Line regulation = x 100%

D VOLTAGE REGULATION

1 The purpose of voltage regulation is to ensure that the output voltage of a

power supply is constant irrespective of the current drawn from it.

2 The two most important factors that affect the voltage output in a
conventional power supply are:

a) Line Variation

i) When the ac line voltage goes up, the power supply output
voltage goes up. When the line voltage goes down, the
output voltage goes down.

b) Load Variation

i) When the output current fluctuates because of load

resistance variations, the output voltage of the supply will
also change.

ii) This is because the fluctuation output current flows through

the internal resistance Rint, refer to Fig 10.2, which causes
the voltage at the terminals of the supply to vary.

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Topic 16 – Power Supplies Circuits

Fig. 16.2 Power supply with internal resistance shown

3 An unregulated supply has a problem maintaining a constant output voltage

as the load resistance changes.

4 A special circuit can be placed across the output of an unregulated supply

to convert it into a regulated supply. This special circuit is called a voltage
regulator, as shown in Fig 16.3.

Fig. 16.3 A regulated power supply

E DISCRETE TRANSISTOR SERIES VOLTAGE REGULATOR

The series regulator is placed in series with the load, as shown in Fig 16.4.

Fig. 16.4 Series regulator

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Topic 16 – Power Supplies Circuits

1 SERIES VOLTAGE REGULATION

The block diagram for the series voltage regulator is shown in Fig 16.5
.
Pass-
transistor
regulator

Error
Unregulated amplifier Regulated
dc input dc output

Reference Error Sample and

Circuit detector adjust circuit

Fig. 16.5 Series regulator block diagram

2 SERIES REGULATOR CIRCUIT

(a) The circuit for the series voltage regulator is shown in Fig 16.6. The
regulator’s sampling circuit (voltage divider) monitors the output voltage
by feeding a sample voltage back to the error amplifier.

IL

Vin Vout

Reference
Voltage Vzener

Fig. 16.6 Schematic diagram of series feedback regulator

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Topic 16 – Power Supplies Circuits

(b) The reference voltage element (zener diode) acts to maintain a

constant reference voltage that is used by the error amplifier.

(c) The error amplifier compares the output sample voltage with the
reference voltage and then generates an error voltage if there is any
difference between the two.

(d) The error amplifier’s output is then fed to the current-control element
(transistor), which is used to control the load current.

3 CURRENT LIMITING

(a) The primary drawback of series regulator is that the pass transistor
can be destroyed by excessive load current.

(b) A current-limiting circuit is added to the regulator to prevent a shorted

load from destroying the pass transistor as shown in Fig 16.8.

Fig. 16.8 Series regulator with current-limiting circuit

(c) The current-limiting resistor circuit consists of a transistor Q 3 and a

series resistor RS that is connected between its base and emitter
terminals.

(d) In order for Q3 to conduct, the voltage across RS must reach

approximately 0.7 V. This happens when

IL(MAX) =

IL(MAX) =
= 700 mA

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Topic 16 – Power Supplies Circuits

(c) Thus, if load current is less than 700 mA, Q 3 is in cutoff and the circuit
behaves as described earlier.

(d) If the load current increases above 700 mA, Q3 conducts, decreasing
the voltage at the base of Q1.

(e) The decreased VB(Q1) reduces the conduction of the pass transistor,
preventing any further increases in load current.

(f) The maximum allowable load current for the regulator can thus be set
to any value by using the appropriate value of RS.

(g) Current limiting reduces the load voltage when the current becomes
larger than the limiting value.

(h) Power supplies that cross over from constant-voltage to constant-

current operation at some value of load current employ current
limiting.

(i) In Fig 16.9 the supply operates at a constant output of 12 V up to load

currents just over 5A.

Fig. 16.9 Current limiting

(j) The current demand goes beyond 5 A as the load resistance drops
and the output voltage starts to drop.

(k) It continues to drop until it reaches 0 V at short-circuit conditions.

(l) There is little current change from the beginning of limiting to the short-
circuit condition.

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Topic 16 – Power Supplies Circuits

F INTEGRATED CIRCUIT VOLTAGE REGULATORS

1 (a) Integrated circuit voltage regulators are available in a range of output

voltages and usually feature internal fold-back current limiting as well
as thermal shutdown.

(b) These ICs contain the circuitry for reference source, error amplifier,
control device and overload protection all in a single IC chip.

(c) IC voltage regulators are series regulators.

(d) IC voltage regulators provide regulation of either a fixed positive, a

fixed negative, or an adjustable set voltage.

(e) For a particular IC, device specifications list a voltage range over which
the unregulated input voltage can vary to maintain a regulated output
voltage, over a range of load current.

(f) The device specifications also list the load regulation and line
regulation.

(g) An input-output voltage differential must be maintained for the IC to

operate, refer to Fig 16.10.

Output – input
Voltage differential

Load Current ID
IN OUT
+ Voltage
regulator +
Unregulated
input voltage GND
Vin
Regulated Load
output voltage
Vout

- -

Input voltage Load Regulation

range Vout { Line Regulation

Fig. 16.10 Block representation of three-terminal voltage regulator

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Topic 16 – Power Supplies Circuits

2 FIXED INTEGRATED CIRCUIT VOLTAGE REGULATORS

(a) The 78XXX series of positive regulators and the 79XXX series of
negative regulators are the most popular series of three-terminal fixed
voltage regulators.

(b) The three terminals are input, output and ground (or common) as
shown in Fig 16.11.

Fig. 16.11 An integrated circuit voltage regulator

(c) These ICs are available in a variety of current and output voltage
ratings, as indicated by the ‘XXX’ suffix. Current ratings are indicated by
the first part of the suffix (L = 100 mA, M = 500 mA, blank = 1 A, T = 3
A), and the voltage ratings by the last two parts of the suffix (standard
values are 5 V, 9 V, 12 V, 15 V, 18 V and 24 V). Thus a 7805 device
gives a 5 V positive output at a 1 A rating.

(d) Basic fixed voltage regulator circuit using 78 for positive input and
output is shown in Fig 16.12.

Fig. 16.12 Fixed positive-output voltage regulator

(c) The unregulated dc input voltage should be at least 3 V greater than

the desired output voltage.

(d) The ICs give about 60 dB of ripple rejection, so 1 V of input ripple

appears as a mere 1 mV of ripple on the regulated output.

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Topic 16 – Power Supplies Circuits

(e) The input shunt capacitor is used to prevent the input ripple from
driving the regulator into self-oscillations.

(f) The output shunt capacitor is used to improve the ripple reduction of
the regulator.

3 VARIABLE INTEGRATED CIRCUIT VOLTAGE REGULATORS

3.1 (a) The LM317 is a positive regulator with an adjustable output voltage. The
standard configuration is shown in Fig 16.13.

(b) There is an input, an output, and an adjustment terminal.

(c) The external fixed resistor R1 and the external variable resistor R 2
provide the output voltage adjustment.

(d) VOUT can be varied from 1.2 V to 37 V depending on the resistor values.

(e) The LM317 can provide over 1.5 A of output current to the load.

Fig. 16.13 The LM317 adjustable positive voltage regulator

3.2 BASIC OPERATION

The regulator maintained a constant 1.25 V reference voltage (V REF)

between the output terminal and the adjustment terminal as shown in Fig
16.14.

Fig. 16.14 Operation of the LM317 adjustable voltage regulator

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(f) This constant reference voltage produces a constant current (I REF)

through R1, regardless of the value of R2.
IREF = =

(g) There is also a very small constant current at the adjustment terminal of
approximately 50 A called IADJ, which is through R2.

(h) The output voltage is thus

VOUT = VR1 + VR2 = IREFR1 + IREFR2 + IADJR2

= IREF(R1 + R2) + IADJR2

= (R1+ R2) + IADJR2

= VREF(1 + ) + IADJR2

(i) Once the value of R1 is set, the output voltage is adjusted by varying R2.

(j) Note that the VOUT equation for a given adjustable regulator will always
be given on its specification sheet.

G PROTECTION DEVICES AND CIRCUITS

1 Line Filter

A line filter is an LC filter circuit that is inserted into a supply to filter out
unwanted high-frequency interference present in the input line supply as
shown in Fig 16.15

Fig. 16.15 AC line filter

Line filters also can help eliminate the emission of radio frequency
interference by the power supply.

Line filters are placed before the transformer, as shown in the above figure.

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Topic 16 – Power Supplies Circuits

2 Metallic Oxide Varistor (MOV)

a) Line transients are common in the industrial environment. They are

caused by switching of large inductive loads such as motors, and
malfunctions in other parts of the industrial plant.

b) They can cause all sorts of damage to wiring and equipment, and
semiconductors are especially susceptible.

c) Fuses and circuit breakers are too slow acting to prevent transient
damage.

d) A varistor can be used to absorb transient energy safely. They are

voltage-dependent resistors.

e) When the line voltage is normal, a varistor has a very high resistance
and draws very little current from the line.

f) When a transient comes along, the resistance of the varistor drops

sharply.

g) This drop will cause high current in the varistor, and the transient will
be safely absorbed.

h) Four common packages for MOV are shown in Fig 16.16. The axial
devices can absorb 2 joules of transient energy at currents of up to 100
A. The high-energy package is rated up to 6500 J and 50,000 A.

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Topic 16 – Power Supplies Circuits

Fig. 16.16 MOV package styles

i) Example

Suppose a transient reaches 5000 V, lasts 10 S, and causes a

current flow of 100 A in an MOV device, what is the absorbed
transient energy?

Solution

Energy = V X I X t
= 5000 X 100 X 10 X 10-6
=5J

j) Fig 16.17 shows an application of an MOV in a power supply circuit.

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Topic 16 – Power Supplies Circuits

Fig. 16.17 MOV protected power supply

k) The varistor is connected in parallel with the transformer primary.

l) It protects the transformer, the rectifier, filter, regulator and load.

m) Depending on the amplitude and duration of the transient, the fuse

may not blow.

n) The MOVs act in nanoseconds and are therefore 100,000 times

faster than fuses.

3 CROWBAR PROTECTION

(a) Short circuit failure within the series pass element of a regulator can
have catastrophic consequences because the full, unregulated dc
voltage will be transferred directly to the output.

(b) This can be avoided by the incorporation of a ‘crowbar’ over-voltage

protection circuit as shown in Fig 16.18.

Fig. 16.18 Crowbar circuit

(c) This circuit places a virtual short-circuit across the supply whenever the
voltage exceeds 6.1 V.

i) Crowbar voltage, V = VZ + VGT

VZ is the zener voltage and VGT is the thyristor gate trigger voltage
(for the BT152, VGT = 1 V).

(d) Once triggered, the thyristor remains in the conducting state until the
supply is disconnected or the mains fuse ruptures.

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