Table of Contents - Jugandijugandi.com/ebooks/Power Supplies/pdf_version/04 Design Examples.pdf•...

An Introduction to Power Supplies i

Table of Contents

4.0 Design Examples

4.1 Generic SMPS ICs

4.2 LM78S40 Universal Switching Regulator

4.3 MAX641 Step-Up Switching Regulator

Assignment Questions

For Further Research

_____ Notes _____

An Introduction to Power Supplies 4 - 1

4.0 Design Examples

Objectives

• Identify various types of switch mode converters.

• Determine the approximate current waveforms in SMPS circuits.

•

4.1 Generic SMPS ICs Of these components, only the switch and diode can be integrated; the inductor and capacitor are external components. In high power applications, even the switch and diode are discrete. It may therefore seem pointless to integrate a switching regulator. However, this is not the case. The real trick is to control the switch.

Switching converters can achieve a power conversion efficiency of 70 – 90%. However, they also create electro-magnetic interference (EMI), which can have adverse effects on other nearby electronics. Low-loss ferrite materials, high permeability magnetic shielding, and smaller semi-conductors can reduce EMI.

Bipolar switching transistors, with a gain-bandwidth product in excess of 4 MHz are often used as the principle switching element. To minimize adverse effects from inductive kickback, Schottky or fast-recovery diodes are also used.

Buck Switching Regulator

Switch Mode Power Supplies _____ Notes _____

4 - 2 An Introduction to Power Supplies

Boost Switching Regulator

Most switching regulators consist of:

Switching transistor

Diode clamp

LC filter

Control logic

It is possible to design switching power supplies to operate directly from the hydro input. This eliminates the need for a transformer, rectification, and pre-filtering.

A switching regulator IC contains the four basic components found in a linear regulator, but adds an oscillator and some control logic in order to control the transistor switch (control element).

Generic Switching Regulator IC

There are several different control methods that can be used to control the conduction in the series control element:

PWM – pulse width modulation: the frequency is held constant and the ON time is varied. This is the most common technique used.

PFM – pulse frequency modulation: the on or the off time is held constant and the frequency is varied.



PBM – pulse burst modulation: the oscillator frequency and duty cycle is held constant and oscillator cycles are gated on or off. This technique is used in simple converters below 10 watts.

As the switching frequency increases, the size of the magnetic components decreases and the switching losses increase. If the switching frequency is in the audio range, it is possible for the coil windings to vibrate, thus creating an annoying singing tone. This is a common phenomenon in TV sets, where the flyback transformer operates at 15.75 KHz.

The sampling circuit generally consists of a simple voltage divider. Under normal operating conditions, it produces an output equal to the built-in reference voltage. An op amp is used to compare the sampled and reference voltages. This creates a difference signal that used to control the series pass device (switch). A similar technique is used in linear regulators, the principle difference is that the series device is operated at the extremes of its load line instead of the linear region. This subtle difference is what gives switching converters their high power conversion efficiency.

4.2 LM78S40 Universal Switching Regulator

LM78S40 Datasheet by National Semiconductor

uA78S40 Datasheet by OnSemi

AN-711 LM78S40 Applications by National Semiconductor

Many of the functional blocks in this circuit are disconnected. This gives the designer a great deal of design flexibility. If required, the transistor switch can be used directly in low power applications, or it can be used to drive a high power series pass element.



Each SMPS IC has its own design peculiarities. In some cases, the restriction placed on the designer may preclude the use of any SMPS IC and a completely discrete circuit design must be considered. This situation however, is beyond the scope of this presentation.

Transistor Driver and Switch

The 78S40 uses a Darlington pair in the switching arrangement. The collectors of both transistors are brought out to external pins. This allows them to be connected together as is the standard configuration, or an external resistor can be placed in the driver collector to control the switch saturation current.

Oscillator

The 78S40 chip is designed to operate within a switching frequency range of 100 Hz to 100 KHz. Increasing the switching frequency increases the electromagnetic radiation and PCB layout problems but decreases the size of the inductor. Most designs based on this IC have an operating frequency of 20 - 30 KHz.

The oscillator, the charge/discharge ratio is preset to approximately 6:1. The overall switching duty cycle can be varied from approximately 17% to 50% by means of two feedback loops.

The switching frequency and duty cycle are controlled by current and voltage feedback. This will at times make triggering an oscilloscope to the switching waveforms somewhat problematic. As the load increases, the switching frequency tends to increases.

Reference Voltage

The 78S40 has an internal 1.245 volt temperature compensated, band-gap voltage reference which is available at an external pin. This reference voltage should be bypassed by a 0.1µfd capacitor to ground to help insure stability.

Current Feedback

The current feedback circuit modifies the switch ON time. A current sensing resistor RSC generates a voltage proportional to the switching current. When this potential exceeds approximately 0.3 volts, the oscillator (and hence switch) is turned OFF. This control mechanism takes priority over voltage feedback.

Voltage Feedback

The voltage feedback loop, consisting of the voltage divider and comparator controls the switch ON time. If the output voltage is too low, the ON time is increased.



Current and Voltage Limitations

The internal Darlington transistor switch can handle a maximum peak current of 1.5 amps during the ont period, and a maximum of 40 volts during the offt

period.

An external transistor switch is needed if the design requires either more current or a higher input voltage.

Voltage Sensing Resistors

The voltage divider at the output represents a minimum load. The voltage at the junction of the two resistors must equal the 1.245 reference voltage when one resistor is attached to ground and the other to the output.

Efficiency

The efficiency of a well-designed power converter can be in excess of 90%.

The output voltage divide constitutes a minimum load on the switching converter and therefore reduces the efficiency. The current drawn by the divider can be as low as 100 µa, but is more typically in the region of 1 ma. This may not be significant with high load currents, but it becomes more dominant as load current decreases.

The saturation voltage of the Darlington transistor can be as high as 1.3 volts. This decreases efficiency as load current and ont increase.

The internal Darlington transistor has a switching speed of 300 – 500 nSec. During this time, the transistor is neither ON nor OFF, and therefore dissipates power.

Any current sensing resistor in series will also dissipate power.

In the following formulas:

inV = Nominal input voltage

outV = Desired output voltage (determined by the ratio R2/R1)

satV = ON voltage drop across the switching element

DV = Forward voltage drop across the flyback diode.

outI = Desired output current.

sI = Voltage divider sampling current (~ 1 ma)

rippleV = Desired peak-to-peak ripple voltage.

SCR = Short circuit current sensing resistor.



NOTE:

When breadboarding these circuits:

It may be necessary to reduce RSC to 0 Ω.

Always keep the circuit leads as short as possible.

Always use a large decoupling capacitor at the circuit input.

Spread Sheet Design

Buck (Step down) Circuit

Simplified Circuit

Buck Design Formulas

Formula Comment

+=

1

21RR

VV refout The internal reference voltage is Vref = 1.245 volts.

(max)2 outpk II = pkI is the peak inductor current.

(max)outI is the maximum output

load current.

pkSC I

R33.0=

The value for the current sensing resistor is a given, not derived, formula.

outsatin

Dout

off

on

VVVVV

tt

−−+

= ton and toff > 10 µs ton + toff < 50 µs

offpk

Dout tI

VVL

+=

Dout

pkoff VV

LIt

+=



offT tC 51045 −×= Timing capacitor

( )ripple

offonpkO V

ttIC

8

+=

Minimum output filter capacitor

Dout

out

in

Dsatin

VVV

VVVV

+

+−

Converter efficiency.

+−+

=Dsatin

Doutpkloadavein VVV

VVII

2))(max( Input current.

Buck Design Example (AN711)

Vin = 25 volts Vout = 10 volts

Iout (max) = 500 ma Vripple < 1%

Step 1 - Calculate the peak current

(max)2 outpk II = = 1 amp

Step 2 - Calculate the current sense resistance

pkSC I

R33.0= = 0.33 Ω

Step 3 - Calculate the ton/toff ratio

8.0101.125

25.110 ≈−−

+=−−

+=

outsatin

Dout

off

on

VVVVV

tt

Step 4 - Calculate the values for ton and toff

Since offon tt 8.0= and sec50µ≤+ offon tt

Let sec22µ=offt then sec18µ=ont

Step 5 - Calculate the timing capacitor CT

fdtC offT µ01.0102210451045 655 ≈×××=×= −−−



Step 6 – Determine the inductor size

HtI

VVL off

pk

Dout µ25010221

25.110 6 ≈×

+=

+= −

Step 7 – Determine the minimum storage capacitor size

( ) ( )fd

V

ttIC

ripple

offonpkO µ50

1.08102210181

8

66

≈×

×+×=+

=−−

Step 8 – Determine the feedback resistor sizes

Ω=×

== − KI

VR

s

ref 25.1101245.1

32 (Select 1.3 K)

Ω=×−=

−= − K

I

VVR

s

refout 76.8101

245.11031 (use a 10 K potentiometer)

Final Circuit

Note: the above schematic is missing the reference and input bypass capacitors.



Basic Waveforms

Boost (Step Up) Circuit

Simplified Circuit



Boost Design Formulas

Formula Comment

+=

1

2125.1RR

Vout The internal reference voltage is 1.245 volts.

−−+

=satin

satDoutoutpk VV

VVVII (max)2

pkI is the peak inductor current.

(max)outI is the maximum output load

current.

pkSC I

R33.0=


satin

inDout

off

on

VVVVV

tt

−−+


offpk

inDout tI

VVVL

−+=

inDout

pkoff VVV

LIt

−+=


( )ripplepk

offoutpkO VI

tIIC

2

2−=


satDout

out

in

satin

VVVV

VVV

−+

−


2))(max(pk

loadavein

II =

Input current.

B

refin

I

VVR

−=3

Driver collector resistor

βpk

B

II = , assume 20≈β

Boost Design Example (AN711)

Vin = 5 volts Vout = 15 volts



ampVV

VVVII

satin

satDoutoutpk 1

45.0545.025.115

15.022 (max) ≈

−−+×=

−−+

=




pkSC I

R33.0= = 0.33 Ω


5.245.05

525.115 ≈−

−+=−

−+=

satin

inDout

off

on

VVVVV

tt





nfdtC offT 5.4101010451045 655 ≈×××=×= −−−


HtI

VVVL off

pk

inDout µ12510101

525.115 6 ≈××

−+=

−+= −


( ) ( )fd

VI

tIIC

ripplepk

offoutpkO µ24

15.012101015.01

2

622

≈××

××−=−

=−


Let maI s 1=

Ω=×

== − KI

VR

s

ref 25.1101245.1

32 (Select 1.3 K)

Ω=×−=

−= − K

I

VVR

s

refout 8.13101




Step 9 – Determine the base drive resistor size (if desired)

B

refin

I

VVR

−=3 and

βpk

B

II = , assume 20≈β

Ω≈−= 75201245.15

3R

Final Circuit




Basic Waveforms

Inverter Circuit In some designs (such as the one below) the switch current is extremely large. As a result, an external switching transistor may be required.

Simplified Circuit



Inverter Design Formulas

Formula Comment

+=

1

2125.1RR

Vout The internal reference voltage is 1.245 volts

−−++

=satin

satDoutinoutpk VV

VVVVII (max)2

pkI is the peak inductor

current. (max)outI is the

maximum output load current.

pkSC I

R33.0=


satin

Dout

off

on

VV

VV

tt

−+


offpk

Dout tI

VVL

+=

Dout

pkoff VV

LIt

+=


( )ripplepk

offoutpkO VI

tIIC

2

2−=


Dout

out

VV

V

+


+−++

=Dsatoutin

Doutpkloadavein VVVV

VVII

2))(max( Input current.

Inverter Design Example (AN711)

Vin = 12 volts Vout = -15 volts



−−++

=satin

satDoutinoutpk VV

VVVVII (max)2

6.2212

225.115125.02 ≈

−−+−+

×=pkI




pkSC I

R33.0= = 0.13 Ω


625.1212

25.115=

−+−

=−+

=satin

Dout

off

on

VV

VV

tt





nfdtC offT 5.4101010451045 655 ≈×××=×= −−−


HtI

VVL off

pk

Dout µ7010106.2

25.115 6 ≈××

+−=

+= −


( ) ( )fd

VI

tIIC

ripplepk

offoutpkO µ60

15.06.2210105.06.2

2

622

≈××

××−=−

=−


Ω=×

== − KI

VR

s

ref 25.1101245.1

32 (Select 1.3 K)

Ω≈×+=

+= − K

I

VVR

s

refout 5.17101




Step 9 – Determine the switching transistor bias resistors

Select Ω≤≤Ω 300100 3R

βpk

BETsatin

IVVVV

R−−−

≈4

where vVT 3.0= (current sense threshold voltage)

4FEh=β (of the external transistor)

Ω≈−−−≈ 7201906.2

7.03.03.1124R

Final Circuit




Basic Waveforms



4.3 MAX641 Step-Up Switching Regulator

MAX641 Datasheet

MAX641 Block Diagram

This particular IC requires the designer to only select the value of the inductor.

Boost Design Example

maIma

SectSec

KHzf

vVv

vV

vV

vV

load

on

osc

sw

D

out

in

45015

128

%2050

75.025.0

4.0

15

%105

≤≤≤≤∴±=

≤≤==

±=

µµ

To determine the value of the inductor, it is necessary to make two calculations, one for a maximum size and the other for minimum.

Case 1 Maximum L

loadswin

inDoutpk I

VVVVV

I ××−

−+= 4

mamaI pk 17415475.05.4

5.44.015 =××−

−+=

onpk

swin tI

VVL ×

−=



HSecma

L µµ 1728174

75.05.4 =×−=

Case 2 Minimum L

From the datasheet Ipk(max) = 450ma.

HSecma

L µµ 14012450

25.05.5 =×−=

A value of 160 µH would be a reasonable compromise.



Assignment Questions

On-Line Test

Quick Quiz

1.

Composition Questions To answer these questions, it will be necessary to do some research.

1.

Analytical Questions

1. Create a spreadsheet to implement the design of a Buck Regulator based on the LM78S40.

2. Create a spreadsheet to implement the design of an Inverter based on the LM78S40.

3. Create a spreadsheet to implement the design of a Boost Regulator based on the LM78S40.

4.



For Further Research

Table of Contents - Jugandijugandi.com/ebooks/Power Supplies/pdf_version/04 Design Examples.pdf•...

Documents

Transcript of Table of Contents - Jugandijugandi.com/ebooks/Power Supplies/pdf_version/04 Design Examples.pdf•...