Download Fundamentals of Linear Electronics Integrated & Discrete

CHAPTER 18
Power
Supplies
Objectives
Describe and Analyze:
• Power Supply Systems
• Regulation
• Buck & Boost Regulators
• Flyback Regulators
• Off-Line Power Supplies
• Troubleshooting
Introduction
• 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
• 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.
VNL = DC output voltage with no load current.
VFL = DC output voltage with maximum load current.
Load Regulation = ([VNL – VFL] / VFL)  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.
3-Terminal Regulators
• Fixed-voltage 3-terminal regulator ICs allow simple
linear supplies at 1 Amp DC or less.
• 78XX are positive voltage regulators (7805 = 5 Volts,
7812 = 12 Volts, etc.).
• 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.
Troubleshooting
• 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.