stabilised power supplies

Stabilised Power Supplies

Electronic Engineering

© University of Wales Newport 2009 This work is licensed under a Creative Commons Attribution 2.0 License.

The following presentation is a part of the level 5 module -- Electronic Engineering. This resources is a part of the 2009/2010 Engineering (foundation degree, BEng and HN) courses from University of Wales Newport (course codes H101, H691, H620, HH37 and 001H). This resource is a part of the core modules for the full time 1st year undergraduate programme.

The BEng & Foundation Degrees and HNC/D in Engineering are designed to meet the needs of employers by placing the emphasis on the theoretical, practical and vocational aspects of engineering within the workplace and beyond. Engineering is becoming more high profile, and therefore more in demand as a skill set, in today’s high-tech world. This course has been designed to provide you with knowledge, skills and practical experience encountered in everyday engineering environments.

Contents Sample Power Supply Block Diagram Zener Diode Regulator Comparator Reference Feedback Current Limit Series Regulator Operation Switching Regulators Configurations available with Switching Regulators I C Regulators Credits

In addition to the resource below, there are supporting documents which should be used in combination with this resource. Please see:Clayton G, 2000, Operational Amplifiers 4th Ed, Newnes James M, 2004, Higher Electronics, Newnes


Simple Power Supply Block Diagram

Mains Transformer

LowVoltage

AC

Rectification

FluctuatingDC

Smoothing

DCRipple

Filter

DC and lessRipple

MainsDC

Fluctuating DC

Smoothing

DCRipple

Transformer

Low Voltage AC

Filter


This has two main disadvantages:1 D.C. is directly proportional to the mains value.2 D.C. value is affected by changes in the load current.

We therefore need to stabilise the output. This can be done using a Zener Diode.


Zener DiodeThis has the same forward characteristics of a normal diode, but in the reverse direction the diode breaks down at a well-defined level.This breakdown, unlike normal diodes is recoverable.

Zener Diode Characteristic

-8 -6 -4 -2 0 2 4 6 8

Voltage

Cur

rent

It is used in the following way.

Vin Vzd Vout

Rs

IL

If Vin < Vzd the Zener Diode does not conduct, no voltage is dropped across the resistor Rs and Vout = Vin.If V in > Vzd then the zener diode conducts, so as to drop the excess voltage (Vin-Vzd) across the series resistor. Vout = Vzd.


When working normally :

If Vin Vout Vzd Izd IRs VRs Vout .

If IL IRs VRs Vout Vzd Izd IRs VRs Vout .


Zener Diodes

VRBV 400 mW 500 mW 1.3 W VRBV 400 mW 500 mW 1.3 W

2.4 BZX79C2V4 BZX55C2V4 9.1 BZX79C9V1 BZX55C9V1 1N5346B

2.7 BZX79C2V7 BZX55C2V7 10 BZX79C10 BZX55C10 1N5347B


3.3 BZX79C3V3 BZX55C3V3 1N5333B 12 BZX79C12 BZX55C12 1N5349B










8.2 BZX79C8V2 BZX55C8V2 1N5345B


EXAMPLES.

1.

A cars battery voltage is monitored are changes between 10.8v and 14.4v. We need to build a 9v supply to run an electronic device. The maximum current drawn by the device is 100 mA. Design a circuit and suggest component values.

2.

An unstabilised power supply produces 9v 10%. We are using a 1N5338B to produce a fixed output voltage. What current can be certain of driving to the output circuit?

In both cases the zener requires 5 mA flowing through it to maintain its voltage.Stabilised Power Supplies

Practically it is observed that Vout still changes by unacceptable levels if Vin and/or IL fluctuate excessively.By modifying the design we can maintain Vout even if changes are large.

Vin charges excessive.

Vin

Rs1

Vzd2

Rs2

Vzd1

E.g. Vin = 12 volts, Vzd1 = 9 volts, Vzd2 = 5 volts.As Vin changes, Vzd1 will vary by a small amount and Vzd2 by an even smaller amount.

If ILOAD changes excessive.

LOAD

ILOAD

Ib

I b (and hence changes in Izd) will be less than ILOAD

(typically )

Therefore Vout will be much more stable.

100

1

50

1

The disadvantage of the circuits that have been looked at so far is that there is no ability to set the output and the desired level.We can achieve this control by using a series control feedback regulator.

The Block Diagram for such a circuit is shown below:

REGULATOR

COMPARATOR

REFERENCE

FEEDBACK

Vin Vout

This can be realised in the following way.


Regulator

REGULATOR

Q1R

Current flows through R into the base of the transistor turning it on – current flows through Q1. If current is drawn from the base of Q1 then this will reduce the drive to Q1, and it will conduct less.


Comparator

COMPARATOR

Q2

Vref Vfb

Vref and Vfb are compared in the following way:

If Vfb > Vref + 0.6v then Q2 conducts moreIf Vfb < Vref + 0.6v then Q2 conducts less


Reference

REFERENCE

Rs

Zd

Vref

The reference voltage generated will be equal to the zener voltage of the diode.


Feedback

FEEDBACK

R1

R2

Vfb

VoutThe value of the feedback voltage Vfb is given by:

21

2

RR

RVoutVfb


This can now be pieced together to give use a circuit for the Series Control Feedback Regulator.

Q1R

Q2

Vfb

Rs

Zd

Vref

R1

R2

Vin Vout

The output voltage Vout is given by: but

Rearranging gives us:

Variable Output

How can we change the value of Vout?1. Change the value of Vref – change the zener (limited)2. Change the values of the feedback resistors.

If the feedback resistors are replaced by the resistor chain incorporating a variable resistor we can make the voltage variable over a fixed range of values.

21

2

RR

RVoutVfb

vVrefVfb 6.0

2

)21()6.0(

R

RRVrefVout

Ra

Rb

Rv

EXAMPLES

1.

We are using a 5.1v zener, Rv is a 1k variable, Ra is 2.2k and Rb is 4.7k.

Over what range can the output be varied? Are there any conditions which might effect this?

2.

We require supply with a variable output of 10v to 15v suggest components to achieve this.


Current Limit This can be achieved by replacing Q1 with the following circuit:

Q1

Vout

D1 D2

RlimitIload

For D1 and D2 to conduct they require 0.6 + 0.6 = 1.2 volts across them.The voltage which is actually across them is Iload x Rlimit + Vbe of Q1 = Iload x Rlimit + 0.6

So we can see that if Iload x Rlimit 0.6 the diodes conduct. When the diodes conduct the current which is normally flowing into the base of the transistor to turn it on flows through the diodes instead -- this has the effect of limiting the current flow through the transistor.

The limiting point is when Iload x Rlimit = 0.6

Therefore Iload max = 0.6/ Rlimit

or Rlimit = 0.6/Iload max


Series Regulator OperationWe can think of a Series Regulator in the following way.

LOAD

Regulator

We can also control the output voltage by using a parallel regulator in the following way.

LOADRegulator

Resistor


The Parallel Regulator has the following advantages.

1 Built in current limit due to the resistor.

2 Constant current consumption?

3 Load current does not flow through the parallel regulator.


Switching Regulators

With both the series and parallel regulators there is a waste of power in the regulator.Switch-Mode Regulators overcome this by using a switch to control the power.

LOAD

The switch opens and closes and this charges and allows the capacitor to discharge accordingly.


The duty cycle of the switch will determine the output voltage and will be controlled according to the load current.Large Load Resistance (small load current).

On

Off

Desired output

Rapid charge up

Slow discharg

e

Switch state


Small Load Resistance (large load current).

Slow charge up

Rapiddischarg

e


The Block Diagram for this type of regulator is similar to that of a series regulator.

SWITCH

COMPARATOR

REFERENCE

FEEDBACK

Vin VoutP.W.M.

The P.W.M. is a pulse width modulator.


The P.W.M. takes in the signal from the comparator and generates a fixed frequency rectangular waveform whose duty cycle is dependent upon the input.

%100

OFFON

ONtt

tDutyCycle

One design uses a simple triangular wave generator and a comparator.

A comparator compares its two inputs and when the one on the + input is greater than the one on the – input the output is high and when the opposite way the output is low.


To switch

Triangular wave generator

Signal from comparator

-

+

Signal to the switch

Triangular wave

Signal to P.W.M.

The operation is shown below


In addition to the advantages of lower wasted power in the regulator, the switching regulator can also be configured to provide outputs, which the series and parallel regulator could not produce.


Configurations available withSWITCHING REGULATORS

Step-down Regulator L

C

D

S

VoutVin

S is controlled by the regulation circuitry.When the switch S is closed it conducts - current flows through L to charge C and to supply the output. The inductor has a magnetic field and the potential across the inductor is positive + on the left, and negative - on the right. The diode will not conduct.

+

-

+ -+

-

+

-

L

C

D

S

VoutVin

When the switch is opened, the magnetic field in the inductor collapses generating a reversed voltage. This maintains a charging current to the capacitor via the diode.

This ensures that the ripple due to the switching on and off is minimised as the output is always being supplied.

- +

+

-

+

-


Step Up Regulator.

L CD

SVoutVin

With the switch S closed it conducts and a magnetic field is built up within the coil. Polarity is positive + on the left, negative - on the right. The diode does not conduct as its anode is at ground potential.

+ -+

-


L CD

SVoutVin

With S open the field in the inductor collapses - the polarity reverses and adds to the input. This now charges the capacitor C via the diode D. As the inductor voltage and the input voltage are summed, Vout > Vin.

- ++

-

+

-


Inverting

L

CD

S

Vout

Vin

With S closed, current flows through L building up a magnetic field, positive + at the top. The diode does not conduct as the cathode is positive.

+

-

+

-


L

CD

S

Vout

Vin

+

-

-

+

-

+

-

+

With S open the magnetic field collapses and the voltage across it reverses and C charges negatively to give a negative value for Vout.Note. With the second two methods of connecting to the output, only limited current can be drawn from the output.

This is useful where we have a single supply and we need reference voltage of other levels or polarities around a circuit:

5vsupply

Step up

Invert

9v

5v

-5v

GND


I. C. Regulators

There are available a range of integrated circuit regulators, examples of these are:-

Positive OutputLM 340 series and the 78XX series

e.g. LM 340-12 and 7812 + 12 volt regulators

LM 340 -5 and 7805 + 5 volt regulators

Negative OutputLM 320 series and the 79XX series

e.g. LM 320-9 and 7909 - -9 volt regulatorLM 320-18 and 7918 - -18 volt

regulatorStabilised Power Supplies

Input

1

Output

2

3

LM 34078XX

Positive

3

Output

2

1

InputLM 32079XX

Negative

Maximum current 5A


Maximum current 1A/1.5A7805T – 1A 7805K – 1.5A

Maximum current 100mA


It is possible to use a fixed regulator to produce a different (larger) output. This is done in the following way.

7805Vin Vout

R1

R2

I

As this is a 5 volt regulator. The voltage across R1 = 5 V

11

5

RIR

As long as I is small compared to IR1 12 IRIR

1

22

1222

55

R

RR

RRIRVR

but )1(55

51

2

1

221

R

R

R

RVRVRVout

Varying OutputThere is available an I.C. specifically designed for varying output.

LM317Vin Vout

R1

R2 )1(25.11

2 R

RVout


This resource was created by the University of Wales Newport and released as an open educational resource through the Open Engineering Resources project of the HE Academy Engineering Subject Centre. The Open Engineering Resources project was funded by HEFCE and part of the JISC/HE Academy UKOER programme.

© 2009 University of Wales Newport

This work is licensed under a Creative Commons Attribution 2.0 License.

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stabilised power supplies

Education

vin changes

ifvin vzd

vin rs

unstabilised power supply

zener diode conducts

regulator current flows

load current

excess voltage vinvzdacross