CHAPTER 18: Power Supplies ppt

• Regulator circuits keep DC voltage constant.. • Some power supplies convert one DC voltage into another DC voltage... • Regulation is a measure of how well a power supply can hold it

CHAPTER 18

Power

Supplies

Describe and Analyze:

• Power Supply Systems

• Electronic equipment requires DC power

But electricity is distributed as AC

• Power supplies convert AC to a steady DC

• They must work with minimum AC voltage

as well as maximum AC voltage

• Regulator circuits keep DC voltage constant

• Some power supplies convert one DC

voltage into another DC voltage.

Block Diagram

<insert figure 18-2 here>

• Regulation is a measure of how well a power supply

can hold its DC output steady as its operating point changes

• Two things make up the operating point:

– The AC input voltage

– The current drawn by the load on the DC output

• Line regulation measures the effect of the AC input

• Load regulation measures the effect of the DC load

• A value of 0% means perfect regulation

Load Regulation

• A perfect power supply would have a constant DC output voltage as the DC load current varied from 0

to the maximum level

• The output of real power supplies changes slightly with the load current

V NL = DC output voltage with no load current

V FL = DC output voltage with maximum load current

Load Regulation = ([V NL – VFL ] / V FL)  100%

Line Regulation

• A perfect power supply would have a constant DC

output voltage as the AC input voltage varied

between specified minimum and maximum levels

• The output of real power supplies changes slightly

with the AC input voltage

• Line Regulation can be calculated as a percentage of

rated DC output (%R) or as a percentage per volt

(%R/ VAC) of AC change:

%R = [ Vout / Vout(rated)]  100%

%R / VAC = %R /  VAC

Linear vs Switching

Low efficiency limits linear to low-power applications

Linear vs Switching

• Switchers are more efficient, but also more

complicated

• Switching control circuitry available in an IC

• Switchers require high-speed transistors

• Switching speeds from 50 kHz to 500 kHz or higher are common Can generate electrical noise (EMI)

• Switcher efficiency due to transistor being either ON

or OFF

• Linears are simple, and can be inexpensive

Linear Supplies

A typical linear supply design

Linear Supplies

• Linears require a large, heavy, 60 Hz transformer

• Require large filter capacitors

• Dissipate heat in the series pass transistor Requires a heat sink, and maybe a fan

• Easier to have an adjustable DC output voltage than it is with switchers

• Often used for “bench” supplies for powering circuits

under test

• Linears often have better regulation and less ripple and noise than switchers

Linear Supplies

Typical linear regulator circuit

3-Terminal Regulators

A typical circuit, good for about an Amp or less

• 79XX are negative voltage regulators (7905 = –5

Volts, 7912 = –12 Volts, etc.)

• Typically housed in a TO-220 case, but available in a TO-92 case for currents under 100 mA

• LM317 is an adjustable 3-terminal regulator

Switching Regulators

<insert figure 18-22 here>

Switching Regulators

Typical switching waveforms

Switching Regulators

• The previous slide showed the basic components of a

switching regulator:

– A Switch: typically an E-MOSFET

– An Inductor: often a few turns of wire on a ferrite core – A Switching Diode: must be fast; it carries the inductor discharge current when the switch opens

– A Filter: typically a Tantalum electrolytic; a few F

– The Load: unlike linears, switchers don’t like to be run without a load Typically, switchers achieve higher

efficiency with higher load current

Switching Regulators

• There are many types of switchers Here are a few common ones:

– Buck: Vout is lower than Vin

– Boost: Vout is higher than Vin

– Flyback: Vout polarity opposite Vin

• The inductor in a Flyback can be made as a

transformer, allowing Vout to be higher or lower, same or opposite polarity

Boost Regulator

Flyback Regulator

Off-Line Switching Supply

Switching Regulator IC

One of many

• Be careful! If possible, use an isolation

transformer when testing off-line supplies Don’t touch a transistor to see if it is hot

• Replace a bad fuse only once If it blows again, there is a reason

• First check the components that are under stress from high voltage, high current, high temperature That includes filter capacitors, power transistors, rectifiers, and switching diodes

• Look for components that are discolored, swollen, cracked, or show other show signs of damage