Power Supply Design

wistfultitleElectronics - Devices

Nov 24, 2013 (3 years and 6 months ago)

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Power Supply Design


-
an introduction



Linear Regulators


Switching “DC
-
to
-
DC Converter”
Regulators


Design Help


Conclusion

Power Supplies

Overview

Linear Regulators

+
Simple, inexpensive

+ Electrically “quiet” pure DC output

-
Vout < Vin

-
Poor Efficiency

e.g., if Vin = 6V, Vout = 3V, Efficiency

%
?

-
Can be physically large


Switching Regulators


Also known as “DC
-
DC Converters” or
“Switchmode Regulators”

+

Wide range of input voltages

+

Multiple output voltages possible

+

High Efficiency (sometimes >90%!)

+

Compact

-

Complex, more expensive

-

Electrically “noisy” (not pure DC)

Switching Regulators

A few types…


Buck (step
-
down)


Boost (step
-
up)


Flyback (supports multiple outputs;
transformer needed)


Buck
-
boost (step
-
up or

down)


SEPIC, Zeta, Cuk (specialized buck
-
boost)


Forward (requires transformer)


Charge Pump (no inductor)

Switching Regulators

“Buck Converter”

Switching Regulators

“Boost Converter”


A.K.A. “charge pump”
or “flying capacitor”


Inductorless (low cost)


Work well for loads up
to ~100mA


Produce output voltage
equal to

Vin or 2 x Vin


Output noisy and poorly
regulated, but OK for
certain applications

Switching Regulators

Switched Capacitor

Switching Regulators

Design Steps

1.
Define requirements


Range of input voltages


Required output voltage(s)


Required output current

2.
Find a suitable controller IC


Web sites; parametric searches


From a list of suitable IC’s, optimize for efficiency
and ease of procurement

3.
Read datasheet closely
!

4.
Order inductor, capacitor, diode

follow
manufacturer’s recommendations exactly
!

5.
Build prototype

Switching Regulator ICs

Finding Parts


Some manufacturers


www.national.com


www.maxim
-
ic.com


www.linear.com


www.analog.com


www.ti.com


Use parametric search engines to narrow list


Order samples, buy online through mfg., or…


Consider using a catalog/online distributor


www.digi
-
key.com


www.mouser.com


www.future
-
active.com

Switching Regulators

Passives


Capacitors


‘Low
-
ESR’ tantalums: medium ESR, $$


Sanyo’s POSCAP

no explosions!


Aluminum electrolytics: high ESR, $


Cornell Dubilier’s Organic
semiconductor “OSCON”

low ESR,
high cost $$


Solid polymer aluminum: low ESR, $$$


Diodes


Schottky diodes: low voltage drop


SMT are best


Inductors


High current
-
handling, low resistance
required


SMT toroids work very well

Switching
Regulators

Prototyping
Hints


Follow datasheet closely!


Use recommended L, C, Diode


Minimize connection lengths


Use surface
-
mount components


‘Dead bug’ prototyping


Use heavy wire!


Be sure to test output with a
load attached


Most switching regulators need a
load to function at all

In the "dead
-
bug" approach to
prototyping, the ICs mount on
top of a copper ground plane
with the leads up. All
interconnects are short point
-
to
-
point wiring connections.
You bend the ground pins and
solder them directly to the
ground plane. You also solder
low
-
inductance, ceramic, high
-
frequency decoupling
capacitors between each power
pin on the IC and the ground
plane.

Power Supply Design

Helpful Hints


Efficiency not an issue?
Linear regulators may be best.


Battery powered?
Consider switcher for efficiency.


Vout > Vin?
Use a switcher.


With switchers, stick to simple topologies like Boost and
Buck. Advanced topologies not for the faint of heart!


Use a combination of switchers and linear regulators to get
multiple output voltages.


Wall
-
powered (120 volts AC)?
Strongly consider use of ‘wall
wart’ AC adapter.


Electrical noise an issue (e.g., with A/D converters)?
Strongly consider post
-
regulation using a linear regulator.


Need to double (2 x Vin) or invert (
-
Vin)?
Consider a
capacitive ‘charge pump’ (see web sites for details.)


Put batteries in series, not parallel.
Parallel batteries can
rapidly self
-
discharge, wasting battery life.

Power Supplies

Case Studies


2
-
AA batteries to +5V digital


Boost


2
-
AA batteries to +5V for an
A/D


Boost with post
-
regulation


+12VDC ‘wall wart’ to +5V, +3.3V


Buck to +5V, linear regulate to +3.3V


Others?

Power Supplies

Summary


Linear Regulators


a
simple
,
inexpensive

circuit that
produces a
quiet

output voltage
less
than

the input voltage


Switching Regulators


more
complex

circuit that’s
efficient,
compact,

and
versatile

in its
input/output voltages

Appendix A

Some web sites of interest


Intro Switching Supplies


http:// dbserv.maxim
-
ic.com/tarticle/view_article.cfm?article_id=120


http://www.electronics
-
tutorials.com/basics/switched
-
mode
-
power
-
supplies.htm


Switched Capacitor:


http://dbserv.maxim
-
ic.com/tarticle/view_article.cfm?article_id=108


More technical…


http://www.semiconductors.philips.com/acrobat/applicationnotes/
AN120.pdf


http://www.linear
-
tech.com/pdf/an46.pdf


http://www.linear
-
tech.com/pdf/an30.pdf


http://www.linear
-
tech.com/pdf/an25.pdf

Appendix B

Switching Frequency


Typical switching supplies use
switching frequencies between 15
kHz and 300 kHz.


Faster switching frequencies (up to
1GHz+) enable smaller inductors,
simpler filtering


Slower frequencies produce fewer
spurious RF emissions (RFI), are less
sensitive to parasitic
capacitance/inductance in layout