Power Supply Circuits

1

9/7/2005 221308 Engineering Electronics I, Asst. Prof. Dr. MONTREE

SIRIPRUCHYANUN

1

221308 Engineering Electronics I

Power Supply CircuitsPower Supply Circuits

Asst. Prof. Dr. Montree SiripruchyanunMatheepot Pattanasak

Dept. of Teacher Training in Electrical EngineeringFaculty of Technical Education

King Mongkut’s Institiute of Technology North Bangkok


SIRIPRUCHYANUN

2

Power Supply Block Diagram

2


SIRIPRUCHYANUN

3

Capacitor Filters

Ov

Time

(e)

rVMV

LV

t't TPT

'/ '/t t RCo M Mv t V e V eτ− −= =


SIRIPRUCHYANUN

4

'/T RCr MV V e−=

/(1 )T RCr M L MV V V V e−= − = −

/ '1T RC TeRC

− ≅ −

'r M

TV VRC

⎛ ⎞≅ ⎜ ⎟⎝ ⎠

pr M

TV V

RC⎛ ⎞

≅ ⎜ ⎟⎝ ⎠

3


SIRIPRUCHYANUN

5

12 p

fT

=

2M

rVVfRC

=


SIRIPRUCHYANUN

6

More Equations . . .

Rearranging the previous equations: VP = Vdc + 1.736 VrThe ripple voltage as a percentage of the dc voltage is:

100% xVVripple

dc

r=

The diode(s) must be rated to withstand the surge current:

W

Psurge R

VI =where RW is the transformer winding’sresistance given by:

FL

FLNLW I

VVR −=

4


SIRIPRUCHYANUN

7

Comparison of Different Types of Rectifiers

Half-wave rectifier needs only a single diode but ripple is twice those of the other types.Full-wave rectifier requires a centre-tapped transformer and its output voltage is about half those of the other types.Bridge-type rectifier is best overall even though it requires four diodes because the diode bridge is often available in a single package. However, if a single diode in the bridge is defective, the whole package has to be replaced.


SIRIPRUCHYANUN

8

Line Regulation

i

o

VVVmVregulationLine

ΔΔ

=)/(

oi

o

Vx

VVregulationlineΔΔ

=

is a measure of the effectiveness of a voltage regulatorto maintain the output dc voltage constant despitechanges in the supply voltage.

5


SIRIPRUCHYANUN

9

Load Regulation

is a measure of the ability of a regulator to maintain aconstant dc output despite changes in the load current.

L

o

IVAmVregulationLoad

ΔΔ

=)/(

oL

o

Vx

IVregulationload 100%

ΔΔ

=


SIRIPRUCHYANUN

10

Other Specifications

A common definition for voltage regulation is:

100(%) xV

VVregulationVoltageFL

FLNL −=

The ability to reduce the output ripple voltage is:

)(

)(log20)(inr

outr

VV

dBrejectionRipple −=

Source resistance of regulator is: ΩΩΔΔ

= morIVR

L

os

6


SIRIPRUCHYANUN

11

Zener Diode Voltage Regulator

Circuit

I-V Characteristic

IZM


SIRIPRUCHYANUN

12

Notes on Zener Diode Regulator

VZ depends on I and temperature.Zener diodes with rated voltage < 6 V have negative temperature coefficient; those rated > 6 V have positive temperature coefficient.In order to maintain a constant Vo, IZT varies in response to a change of either IL or Vi. For example, when RL increases, IL decreases, then IZT has to increase to keep the current through Rsconstant. Since the voltage drop across Rs is constant, Vo stays constant.

7


SIRIPRUCHYANUN

13

Formulae for Zener Regulator Circuit

Rs establishes the zener bias current, IZT:

LZT

Zi

Rs

Zis II

VVI

VVR+−

=−

=

For fixed Vi, but variable RL:

ZMRs

Z

L

ZL

Zi

Zs

Rs

ZL

IIV

IVR

VVVR

IVR

−==

−==

(min)

.max

.min


SIRIPRUCHYANUN

14

Formulae (cont’d)

For fixed RL, but variable Vi:

LZMR

ZsRi

ZL

sLi

IIIwhere

VRIV

VR

RRV

+=

+=

+=

(max)

(max).max

.min

The output ripple voltage of the zener regulator is:

)()( ////

inrsZL

ZLoutr V

RRRRRV+

= where RZ = ac resistanceof zener diode.

8


SIRIPRUCHYANUN

15

Transistor Series Voltage Regulator

The simple zener regulatorcan be markedly improvedby adding a transistor.Since VBE = VZ - VL anytendency for VL to decrease or increase will be negatedby an increase or decrease in IE. The dc currents for thecircuit are:

RVVI

RVV

RVI Zi

RL

BEZ

L

LL

−=

−== ;

IL = hFEIB; IZT = IR - IB


SIRIPRUCHYANUN

16

Transistor Shunt Voltage Regulator

Since VBE = VL - VZ,any tendency for VLto increase or decreasewill result in a corresponding increase or decrease in IRs. This willoppose any changes in VL because VL = Vi - IRsRs.

S

BEZiRs

L

BEZ

L

LL R

)VV(VI;R

VVRVI +−

=+

==

IE = IRs - IL = hFEIZT

9


SIRIPRUCHYANUN

17

Op-Amp Voltage Regulators

Zo VRRV ⎟⎟

⎠

⎞⎜⎜⎝

⎛+≅

3

21

Series Shunt


SIRIPRUCHYANUN

18

Notes on Op-Amp Voltage Regulator

More flexibility possible in design of voltage output than IC voltage regulator packages.The essential circuit elements are: a zenerreference, a pass or shunt transistor, a sensing circuit, and an error/amplifier circuit.Equation indicates that Vo depends on R2, R3, and VZ.The shunt configuration is less efficient but R2offers short-circuit current limiting.

10


SIRIPRUCHYANUN

19

Constant Current Limiting

can be used for short-circuit or overload protection ofthe series voltage regulator.

4(max)

7.0R

IL =

Output currentis limited to:


SIRIPRUCHYANUN

20

Fold-back Current Limiting

is a better method of short-circuit protection.

oLooBBE VRIVRR

RVVV −++

=−= )( 465

622

11


SIRIPRUCHYANUN

21

Design Equations for Fold-back Current Limiting

Maximum load current without fold-back limiting:

64

655(max)

)(7.0RR

RRVRI oL

++=

Output voltage under current limiting condition:

L

Lo RRRR

RRRV564

65 )(7.0'−+

=

The short circuit current (i.e. when Vo = 0) is:

64

65 )(7.0RR

RRIshort+

=


SIRIPRUCHYANUN

22

Characteristics of Fold-back Limiting

Notice that Ishort < IL(max)and that Vo is regulated (i.e. constant) only after RL > a certain critical value.For designing purpose, R5+ R6 = 1 kΩ and if Ishortand IL(max) are specified then

(max)4 7.0)7.0(

07

Loshort

o

IVIVR−+

=

Vo

IL

12


SIRIPRUCHYANUN

23

Transistor Current Regulators

are designed to maintain a fixed current through aload for variations in either Vi or RL.

For the BJT circuit, VEB = VZ - VRE.Any tendency for IL to change willcause an opposing change in VEB,thus nullifying the perturbation.

For the JFET circuit, IL = ID = IDSS aslong as VL < VSS - VP.


SIRIPRUCHYANUN

24

IC Voltage Regulators

There are basically two kinds of IC voltage regulators:

Multipin type, e.g. LM723C3-pin type, e.g. 78/79XX

Multipin regulators are less popular but they provide the greatest flexibility and produce the highest quality voltage regulation3-pin types make regulator circuit design simple

13


SIRIPRUCHYANUN

25

Multipin IC Voltage Regulator

LM 723C Schematic

The LM723 has an equivalent circuit that contains most of the parts of the op-amp voltage regulator discussed earlier.It has an internal voltage reference, error amplifier, pass transistor, and current limiter all in one IC package.


SIRIPRUCHYANUN

26

Notes on LM723 Voltage Regulator

Can be either 14-pin DIP or 10-pin TO-100 canMay be used for either +ve or -ve, variable or fixed regulated voltage outputUsing the internal reference (7.15 V), it can operate as a high-voltage regulator with output from 7.15 V to about 37 V, or as a low-voltage regulator from 2 V to 7.15 VMax. output current with heat sink is 150 mADropout voltage is 3 V (i.e. VCC > Vo(max) + 3)

14


SIRIPRUCHYANUN

27

LM723 in High-Voltage Configuration

External pass transistor andcurrent sensing added.

Design equations:

2

21 )(R

RRVV ref

o

+=

21

213 RR

RRR+

=max

7.0I

Rsens =

Choose R1 + R2 = 10 kΩ,and Cc = 100 pF.To make Vo variable,replace R1 with a pot.


SIRIPRUCHYANUN

28

LM723 in Low-Voltage Configuration

With external pass transistorand foldback current limiting

sens5

54o4(max)L RR

)RR(7.0VRI ++=

sens5

54short RR

)RR(7.0I +=

(max)Loshort

osens I7.0)7.0V(I

V7.0R−+

=

L4sens5

54Lo RRRR

)RR(R7.0'V−+

=

Under foldback condition:

21

ref2o RR

VRV+

=

15


SIRIPRUCHYANUN

29

Three-Terminal Fixed Voltage RegulatorsLess flexible, but simple to useCome in standard TO-3 (20 W) or TO-220 (15 W) transistor packages78/79XX series regulators are commonly available with 5, 6, 8, 12, 15, 18, or 24 V outputMax. output current with heat sink is 1 ABuilt-in thermal shutdown protection3-V dropout voltage; max. input of 37 VRegulators with lower dropout, higher in/output, and better regulation are available.


SIRIPRUCHYANUN

30

Basic Circuits With 78/79XX Regulators

Both the 78XX and 79XX regulators can be used to provide +ve or -ve output voltagesC1 and C2 are generally optional. C1 is used to cancel any inductance present, and C2 improves the transient response. If used, they should preferably be either 1 μF tantalum type or 0.1 μF mica type capacitors.

16


SIRIPRUCHYANUN

31

Dual-Polarity Output with 78/79XX Regulators


SIRIPRUCHYANUN

32

78XX Regulator with Pass Transistor

Q1 starts to conduct when VR2 = 0.7 V.R2 is typically chosen so that max. IR2 is 0.1 A.Power dissipation of Q1 is P = (Vi - Vo)IL.Q2 is for current limiting protection. It conducts when VR1 = 0.7 V.Q2 must be able to pass max. 1 A; but note that max. VCE2 is only 1.4 V.

max1

7.0I

R =2

27.0

RIR =

17


SIRIPRUCHYANUN

33

78XX Floating Regulator

It is used to obtain an output > the Vreg value up to a max.of 37 V.R1 is chosen so thatR1 0.1 Vreg/IQ, where IQ is the quiescent currentof the regulator.

21

RIR

VVV Q

regrego ⎟⎟

⎠

⎞⎜⎜⎝

⎛++=

1

12

)(RIV

VVRR

Qreg

rego

+

−=or


SIRIPRUCHYANUN

34

3-Terminal Variable Regulator

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.

A variety of 3-terminal variable regulators are available, e.g. LM317 (for +ve output) or LM 337 (for -ve output).

18


SIRIPRUCHYANUN

35

Basic LM317 Variable Regulator Circuits

Circuit with capacitorsto improve performance

Circuit with protectivediodes

(a) (b)


SIRIPRUCHYANUN

36

Notes on Basic LM317 Circuits

The function of C1 and C2 is similar to those used in the 78/79XX fixed regulators.C3 is used to improve ripple rejection.Protective diodes in circuit (b) are required for high-current/high-voltage applications.

21

RIR

VVV adj

refrefo ⎟⎟

⎠

⎞⎜⎜⎝

⎛++=

where Vref = 1.25 V, and Iadj isthe current flowing into the adj.terminal (typically 50 μA).

1

12

)(RIV

VVRR

adjref

refo

+

−=

R1 = Vref /IL(min), where IL(min)is typically 10 mA.

19


SIRIPRUCHYANUN

37

Other LM317 Regulator Circuits

Circuit with pass transistorand current limiting

Circuit to give 0V min.output voltage


SIRIPRUCHYANUN

38

Block Diagram of Switch-Mode Regulator

It converts an unregulated dc input to a regulated dcoutput. Switching regulators are often referred to asdc to dc converters.

20


SIRIPRUCHYANUN

39

Comparing Switch-Mode to Linear Regulators

Advantages:70-90% efficiency (about double that of linear ones)can make output voltage > input voltage, if desiredcan invert the input voltageconsiderable weight and size reductions, especially at high output power

Disadvantages:More complex circuitry Potential EMI problems unless good shielding, low-loss ferrite cores and chokes are used


SIRIPRUCHYANUN

40

General Notes on Switch-Mode Regulator

The duty cycle of the series transistor (power switch) determinesthe average dc output of the regulator. A circuit to control theduty cycle is the pulse-width modulator shown below:

21


SIRIPRUCHYANUN

41

General Notes cont’d . . .

The error amplifier compares a sample of the regulator Vo to an internal Vref. The difference or error voltage is amplified and applied to a modulator where it is compared to a triangle waveform. The result is an output pulse whose width is proportional to the error voltage.Darlington transistors and TMOS FETs with fT of at least 4 MHz are often used. TMOS FETs are more efficient.A fast-recovery rectifier, or a Schottky barrier diode (sometimes referred to as a catch diode) is used to direct current into the inductor.For proper switch-mode operation, current must always be present in the inductor.


SIRIPRUCHYANUN

42

Step-Down or Buck Converter

When the transistor is turned ON, VL is initially high but falls exponentially while IL increases to charge C.When the transistor turns OFF, VL reverses in polarity to maintain the direction of current flow. IL decreases but its path is now through the forward-biased diode, D.Duty cycle is adjusted according to the level of Vo.

22


SIRIPRUCHYANUN

43

V & I Waveforms for Buck Regulator

PWMoutput

VL

IL

Vo

Normal Low Vo High Vo


SIRIPRUCHYANUN

44

Equations for Buck Regulator

Tt

ttt

VV on

offon

on

i

o =+

=

Selecting IL = 0.4Io where Iois the max. dc output current:

oscio

oio

fVIVVVL )(5.2 −

=

oscrms

o

oscpp

o

fVIor

fVIC

ΔΔ=

01768.005.0

where V is the ripple voltage

23


SIRIPRUCHYANUN

45

Notes on Operation of Buck Regulator

When IL = 0.4Io was selected, the average minimum current, Imin, that must be maintained in L for proper regulator operation is 0.2Io.If IL is chosen to be 4% instead of 40% of Io, the 2.5 factor in the equation for L becomes 25 and Imin becomes 0.02Io.L and C are both proportional to 1/fosc; hence, the higher fosc is the smaller L and C become. But for predictable operation and less audible noise, fosc is usually between 50kHz to 100 kHz.


SIRIPRUCHYANUN

46

Step-Up, Flyback, or Boost Regulator

Assuming steady-state conditions, when the transistor is turned ON, L reacts against Vin. D is reverse-biased and C supplies the load current.When the transistor is OFF, VL reverses polarity causing current to flow through D and charges C. Note that Voutis > Vin because VL adds on to Vin.

24


SIRIPRUCHYANUN

47

Equations for Boost Regulator

Tt

VVV on

o

io =−

oscoo

ioi

fVIVVVL 2

2 )(5.2 −=

Assuming IL = 0.4Io:

rmsoosc

oio

ppoosc

oio

VVfIVVor

VVfIVVC

Δ−

Δ−

=)(3536.0)(


SIRIPRUCHYANUN

48

Voltage-Inverting or Buck-Boost Regulator

Vo can be either step-up or step-down and its polarity is opposite to input.During ON period, Vin is across L, and D is reverse-biased.During OFF period, VL reverses polarity causing current to flow through C and D.

25


SIRIPRUCHYANUN

49

Equations for Buck-Boost Regulator

Tt

VVV on

oi

o =+

For IL = 0.4Io:

oscioo

oi

fVVIVVL

)(5.2+

=

oscoirms

oo

oscoipp

oo

fVVVVIor

fVVVVIC

)(3536.0

)( +Δ+Δ=


SIRIPRUCHYANUN

50

Basic Push-Pull Power Converter

Operates as a class D power amplifier. Output rectifier converts the square-wave to dc. Each transistor must withstand 2xVin plusvoltage spikes.

26


SIRIPRUCHYANUN

51

Basic Half-Bridge Power Converter

Each transistor “sees” approx. Vin. Full flux reversal in the transformer and capacitors across DS prevent voltage spikes.


SIRIPRUCHYANUN

52

Basic Full-Bridge Power Converter

Either Q1 & Q3 or Q2 & Q4 are turned ON simultaneously.Ideal for high power applications.

27


SIRIPRUCHYANUN

53

Single-Package Switch-Mode Regulator

The LH1605 is a 5A step-down switching regulator.Vo is adjustable from 3 to 30 V by using a pot. for R1.In the circuit above, Q1 turns ON when voltage across Rsens is 0.7 V. Q2 then turns ON shorting Vref to ground and driving Vo to zero. .


SIRIPRUCHYANUN

54

Equations for LH1605 Switching Regulator

200080000125.05.2

1

1

−=+=

o

o

VRorRV

oscT f

C40000

1=

oscio

oio

fVIVVVL )(5.2 −

=With IL = 0.4Io:

oscpp

o

oscrms

o

fVIor

fVIC

ΔΔ=

05.001768.0

max

7.0I

Rsens =

Typically, CF = CC = 10 μF; RB = 10 kΩ

Documents

Power Supply Circuits