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Lecture # 10 &11

REGULATED POWER SUPPLIES

Prepared by Engr: Sarfaraz Khan Turk

Lecturer at IBT LUMHS Jamshoro

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REGULATED POWER SUPPLIES Key Points from This Presentation:Regulator.Ideal Power Supply.Unregulated Power Supply.Regulated Power Supply e.g.(Voltage Regulator)Zener Voltage Regulator.Emitter Follower Regulator.Three terminal voltage Regulator.Adjustable Three Terminal Voltage Regulators.

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REGULATOR

A regulator is any circuit that maintain s rated output voltage under all conditions

Either no load(open circuit)supplying no current or full load supplying an out put current.

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IDEAL POWER SUPPLYAn ideal power supply

maintains a constant voltage at its output terminals under all operating conditions. The output voltage of a practical power supply changes with load generally dropping as load current increases as shown in fig. 1. fig. 1

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IDEAL POWER SUPPLY(CONT)The terminal voltage when full load current is

drawn is called full load voltage (VFL). The no load voltage is the terminal voltage when zero current is drawn from the supply, that is, the open circuit terminal voltage.

Power supply performance is measured in terms of percent voltage regulation, which indicates its ability to maintain a constant voltage. It is defined as

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The Thevenin's equivalent circuit of a power supply is shown in fig. 2. The Thevenin voltage is the no-load voltage VNL and the Thevenin resistance is called the output resistance Ro. Let the full load current be IFL. Therefore, the full load resistance RFL is given by

IDEAL POWER SUPPLY(CONT)

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The The venin's equivalent circuit of a power supply is shown in fig. 2.

IDEAL POWER SUPPLY(CONT)

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It is clear that the ideal power supply has zero output resistance.

IDEAL POWER SUPPLY(CONT)

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From the equivalent circuit we have

And the voltage regulation is given by

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Example-1QS A power supply having output resistance 1.5Ω supplies a full load current of 500mA to a 50Ω load. Determine:• percent voltage regulation of the supply• no load output voltage. Answer• Solution:• (a). Full load output voltage VFL = (500mA ) (50Ω) = 25V.• Therefore,

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(b). The no load voltage

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UNREGULATED POWER SUPPLYAn unregulated power supply consists of a

transformer (step down), a rectifier and a filter. These power supplies are not good for some applications where constant voltage is required irrespective of external disturbances. The main disturbances are:

1. As the load current varies, the output voltage also varies because of its poor regulation.

2. The dc output voltage varies directly with ac input supply. The input voltage may vary over a wide range thus dc voltage also changes.

3. The dc output voltage varies with the temperature if semiconductor devices are used.

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UNREGULATED POWER SUPPLY (CONT)An electronic voltage regulator is essentially a

controller used along with unregulated power supply to stabilize the output dc voltage against three major disturbances

1. Load current (IL)

2. Supply voltage (Vi)

3. Temperature (T)Fig. 3, shows the basic block diagram of voltage regulator. where4. Vi = unregulated dc voltage.

5. Vo = regulated dc voltage.

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Fig. 3, The basic block diagram of voltage regulator.

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UNREGULATED POWER SUPPLY (CONT)

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UNREGULATED POWER SUPPLY (CONT)

Since the output dc voltage VLo depends on the input unregulated dc voltage Vi, load current IL and the temperature t, then the change ΔVo in output voltage of a power supply can be expressed as follows

∆VO = VO (Vi, IL, T)

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REGULATED POWER SUPPLIES

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VOLTAGE REGULATORA voltage regulator is a device designed to maintain

the output voltage of power supply nearly constant. It can be regarded as a closed loop system because it monitors the output voltage and generates the control signal to increase or decrease the supply voltage as necessary to compensate for any change in the output voltage. Thus the purpose of voltage regulator is to eliminate any output voltage variation that might occur because of changes in load, changes in supply voltage or changes in temperature

ZENER VOLTAGE REGULATOR Zener diodes break down at some reverse

voltage can buy at specific breakdown voltages as long as some current goes through zener,

it’ll work good for rough regulation

Conditions for working: let’s maintain some minimal current, Iz

through zener (say a few mA) then (Vin Vout)/R1 = Iz + Vout/Rload sets the

requirement on R1

because presumably all else is known if load current increases too much, zener

shuts off (node drops below breakdown) and you just have a voltage divider with the load.

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R1

Z

Vin

Vout = Vz

Rload

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ZENER VOLTAGE REGULATOR(CONT)The regulated power supply

may use zener diode as the voltage controlling device as shown in fig. 4. The output voltage is determined by the reverse breakdown voltage of the zener diode. This is nearly constant for a wide range of currents. The load voltage can be maintained constant by controlling the current through zener.

high slope is what makes thezener a decent voltage regulator

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ZENER VOLTAGE REGULATOR(CONT)

The zener diode regulator has limitations of range. The load current range for which regulation is maintained, is the difference between maximum allowable zener current and minimum current required for the zener to operate in breakdown region.

fig. 4

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ZENER VOLTAGE REGULATOR(CONT)

For example, if zener diode requires a minimum current of 10 mA and is limi ted to a maximum of 1A (to prevent excessive dissipation), the range is 1 - 0.01 = 0.99A. If the load current variation exceeds 0.99A, regulation may be lost.

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IMPROVED ZENER REGULATOR• By adding a transistor to the zener regulator

from before, we no longer have to worry as much about the current being pulled away from the zener to the load– the base current is small– Rload effectively looks times bigger– real current supplied through transistor

• Can often find zeners at 5.6 V, 9.6 V, 12.6 V, 15.6 V, etc. because drop from base to emitter is about 0.6 V– so transistor-buffered Vreg comes out to

5.0, 9.0, etc.• Iz varies less in this arrangement, so the

regulated voltage is steadier

Vreg

Rload

Vz

Vin

Rz

Z

Vin

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EMITTER FOLLOWER REGULATOR

To obtain better voltage regulation in shunt regulator, the zener diode can be connected to the base circuit of a power transistor as shown in fig. 5. This amplifies the zener current range. It is also known as emitter follower regulation

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EMITTER FOLLOWER REGULATOR(CONT)

fig. 5 The Basic Emitter Follower Regulator

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EMITTER FOLLOWER REGULATOR(CONT)

This configuration reduces the current flow in the diode. The power transistor used in this configuration is known as pass transistor. The purpose of CL is to ensure that the variations in one of the regulated power supply loads will not be fed to other loads. That is, the capacitor effectively shorts out high-frequency variations Because of the current amplifying property of the transistor, the current in the zenor dioide is small. Hence there is little voltage drop across the diode resistance, and the zener approximates an ideal constant voltage source.

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EMITTER FOLLOWER REGULATOR(CONT)

Operation of the circuit: The current through resistor R is the sum of zener

current IZ and the transistor base current IB ( = IL / β ). IL = IZ + IB The output voltage across RL resistance is given byVO = VZ – VBE Where VBE » 0.7 VTherefore, VO= constant.

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EMITTER FOLLOWER REGULATOR (CONT)

The emitter current is same as load current. The current IR is assumed to be constant for a given supply voltage. Therefore, if IL increases, it needs more base currents, to increase base current Iz decreases. The difference in this regulator with zener regulator is that in later case the zener current decreases (increase) by same amount by which the load current increases (decreases). Thus the current range is less, while in the shunt regulators, if IL increases by ΔIL then IB should increase by ΔIL / β or IZ should decrease by ΔIL / β. Therefore the current range control is more for the same rating zener.

The simplified circuit of the shunt regulator is shown in fig. 6

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EMITTER FOLLOWER REGULATOR(CONT)

fig. 6 The simplified circuit of the shunt regulator

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EMITTER FOLLOWER REGULATOR(CONT) In a power supply the power regulation is basically,

because of its high internal impedance. In the circuit discussed, the unregulated supply has resistance RS of the order of 100 ohm. The use of emitter follower is to reduce the output resistance and it becomes approximately.

RO = ( Rz + hie ) / (1 + hfe) Where RZ represents the dynamic zener resistance. The

voltage stabilization ratio SV is approximately

SV = ∂ Vo / ∂ VI = Rz / (Rz + R) SV can be improved by increasing R. This increases VCE and

power dissipated in the transistor

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EMITTER FOLLOWER REGULATOR(CONT)

Disadvantages of the circuit are. 1. No provision for varying the output voltage

since it is almost equal to the zener voltage.

2. Change in VBE and Vz due to temperature variations appear at the output since the transistor is connected in series with load, it is called series regulator and transistor is allow series pass transistor.

THREE TERMINAL REGULATORS

DC power regulators are readily available, both fixed voltage and variable voltage types, and for either positive or negative power supplies.

These regulators need decoupling capacitors located close to the devices.

Low, medium and high power versions are available.

Note that a negative power supply can be made with a positive regulator chip, so long as decoupling is carefully considered

POSITIVE fixed regulator NEGATIVE fixed regulator

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THREE TERMINAL REGULATORS(CONT) The most common voltage regulators are the LM78XX (+

voltages) and LM79XX ( voltages)– XX represents the voltage

• 7815 is +15; 7915 is 15; 7805 is +5, etc– typically needs input > 3 volts above output (reg.) voltage

A versatile regulator is the LM317 (+) or LM337 ()– 1.2–37 V output– Vout = 1.25(1+R2/R1) + IadjR2

– Up to 1.5 A– picture at right can go to 25 V– datasheetcatalog.com for details

beware that housing is not always ground

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THE THREE TERMINAL VOLTAGE REGULATOR IC

Can trim down ripply voltage to precise, rock-steady value

Now things get complicated!– We are now in the realm of

integrated circuits (ICs) ICs are whole circuits in small

packages. ICs contain resistors, capacitors,

diodes, transistors, etc.

note zeners

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THREE TERMINAL REGULATORS (CONT)

The output voltage from a three terminal regulator can be increased by the circuit shown above. Supposing you want an 8V supply, but only have 5V regulators to use.

In this case, make the Zener diode 3.3V (nearest preferred value) and run it at a current of around 5mA via R (the regulator chip requires around 0.5mA) so R = 5V/5.5mA = 909 Ohm. Here a 1k resistor will work just as well (the Zener current will then be around 4.5mA). Don’t forget Cin and Cout

POSITIVE variable regulator NEGATIVE variable regulator

ADJUSTABLE THREE TERMINAL REGULATORS

These regulators function by maintaining a fixed level of 1.25V between the OUT and ADJ terminals and by ensuring that the current drawn by the ADJ terminal is very small. Provided the current through the potential divider from the output to the ground rail is large compared to the ADJ terminal, then the regulated output voltage is set by the resistors used for the potential divider. The recommended maximum resistor value between the ADJ terminal and the OUT terminal is 240W for the positive version (220W is OK) and 120W for the negative version.

You want +12V out. R1 = 220W, so current through R1 = 1.25V/220W = 5.68mAV across R2 is 12V – 1.25V = 10.75V, so R2 = 10.75V/5.68mA = 1.892KW which could be 1.8kW + 100W = 1.9kW. This will give an output of 10.792V + 1.25V = 12.042V.

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For student references:

Chapter 17 Regulated Power Supplies (17.1) (17.1.1)(17.1.3)(17.1.4)(17.3.1)(17.3.2) from the book Electronic devices, circuit and systems (Micheal M cirovic)

Chapter 21discrete & integrated voltage regulators (21.1) (21.2)(21.3)(21.4) from the book introductory electronic devices and circuits by author (Robert T .paynter).

Wikipedia and world wide web