AC Direct Off-Line Power Supplies
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Introduction
Many DC power supplies found in electronic systems, including those
in this Tech School, rectify the 120 volts available at an electric outlet.
The initial input circuitry of the power supply rectifies and filters the AC
voltage to a filtered DC voltage. The initial rectifier/filter circuits may be
referred to as an AC line-derived DC power supply or simply a raw DC
power supply. The output DC voltage from the AC line-derived DC
power supply feeds the regulator circuits or remaining circuits of a
switching power supply. This section covers the theory, precautions, and
measurements related to the AC line-derived DC power supply section.
AC Line-Derived Half-wave Rectifier
A simple half-wave rectifier and filter circuit covered earlier in this course
can be supplied with AC voltage from an electric outlet. On the positive
AC cycle the diode conducts, charging the filter capacitor to the AC line
voltage peak. On the negative cycle of the AC voltage the diode remains
open and the capacitor remains charged. With no load resistor or discharge
path for the capacitor, the output is a steady voltage of approximately 169
volts. This assumes the electric outlet voltage is near the nominal value of
120 V RMS with a normal peak voltage. By changing the polarity of the
diode and filter capacitor in the half-wave supply shown in figure 15, a
negative output voltage may be produced.
In a half-wave rectifier, the filter capacitor's charge is only replenished on
one alternation of the AC cycle. Therefore, a large and expensive filter
capacitor is required for high load currents. For this reason, a half-wave
rectifier/filter is commonly used in low power circuit applications.
Fig. 15: An AC line-derived DC power supply using a half-wave rectifier with a filler
capacitor.
AC Direct Off-Line Power Supplies 27
AC Line-Derived Full Wave Rectifier
Both alternations of an input AC voltage can be rectified by using a circuit
arrangement shown in figure 16. When both the positive and negative AC
alternations are rectified it is called full-wave rectification. A full-wave
rectifier requires 2 or more diodes.
The full-wave rectifier in figure I6 uses two diodes and requires
a center-tapped transformer secondary. The center-tap is the common
reference point or circuit ground for the load. The voltages at the
opposite ends of the secondary are 180 degrees out-of-phase with each
other. When the voltage at the top of the secondary swings positive with
respect to the center-tap, the voltage at the bottom of the transformer
secondary is negative with respect to the center-tap. On the next AC
alternation the voltage at the top is negative and the voltage on the
bottom positive with respect to the center-tap.
D1
Load
D1
Load
Fig. 16: Full wave rectifier with center
tapped transformer.
-
28AC Direct Off-Line Power Supplies
Consider the circuit operation of the full-wave rectifier in figure 16.
When the voltage at the top of the secondary goes positive, D1 is forward
biased. Current flows from the transformer center-tap charging Cl and
through D1 to the top of the secondary. Current also flows through the
load resistor. During the next alternation the voltage at the bottom of the
transformer secondary becomes positive forward biasing D2. Current
flows from the center-tap recharging CI and through D2 to the bottom of
the secondary. Current also flows through the load resistor.
The filter capacitor in the full-wave rectifier smoothes the pulsating output voltage. Without the filter capacitor the voltage output of a full-wave
rectifier would be DC pulses during the negative and positive alternation
as shown in figure 17A. The filter capacitor holds a charge and opposes
a voltage change to smooth the output voltage as shown in figure 17B.
The output voltage of the supply shown in figure 16 depends on the turn
ratio of the transformer. For example a 1:1 turns ratio or full secondary
voltage peak of approximately 169 volts results in a voltage output of
approximately 85 volts (1/2 of the 169 V peak).
A.
Full-wave output
( No Capacitor)
B.
Full-wave output
( With Capacitor)
Fig. 17: Output voltage waveforms of a full-wave rectifier without a filter capacitor
and with a filter capacitor (B).
(A)
The full-wave rectifier output voltage requires less capacitance because it
is much easier to filter than the half-wave rectifier. However, the full-wave
rectifier shown in figure 16 requires a center tapped transformer. This
makes it undesirable for most modern day cost-sensitive and efficient
operating electronic systems.
Full-wave rectification without a transformer is possible with a bridge
rectifier circuit arrangement shown in figure 18. A full-wave bridge
consists of 4 diodes arranged so current flows in the load in only one
direction during both alternations of the AC input voltage. A ground
or common circuit reference is designated at the junction of diodes
D3 and D4.
AC Direct Off-Line Power Supplies29
A.
(
Load
B.
Load
Fig. 18: Direct AC line-derived full wave bridge rectifier and filter.
Figure ISA shows the current flow during the positive half cycle of the
input AC sine wave. The voltage potential forward biased diodes DI and
D2. This places the full voltage of the AC line across capacitor Cl and the
load. Current flows from the bottom side of the AC plug through D I to
charge capacitor Cl. Current flows from the top plate of Cl through diode
D2 to the positive side of the AC plug. At the same time current flows
through the load.
During the negative half cycle of the input AC sine wave, diodes D3 and
D4 are forward biased. Current flows from the top side of the AC plug
through D3, charging C1 in the same polarity as the previous half cycle.
Current flows from the top plate of Cl through D4 to the bottom side of
the AC plug. Current also flows through the load as shown. The voltage
output of an AC line-derived bridge rectifier is approximately 169 volts
with no resistor load or C1 discharge path.
The bridge rectifier provides direct AC line-derived full-wave rectification
of the AC line voltage without the added cost, weight, and energy losses
associated with a power transformer. The bridge rectifier is ideal for most
applications and is by far the most popular raw DC power supply found in
electronic systems.
30AC Direct Off-Line Power Supplies
HOT Grounds
A direct AC line-derived supply commonly results in a "HOT" ground or
circuit common point in the electronic system. The electronic system may
be said to have a "HOT" chassis. A "HOT" chassis or "HOT" circuit
ground or common reference results from a direct path to one or both sides
of the electrical outlet. This creates a shock hazard for service technicians
when servicing and poses a test equipment hazard.
To understand what is meant by a "hot" ground in a electronic system,
consider an AC electric outlet and the building's electric box shown in
figure 19. The 120 volts AC power line voltage potential exists between
neutral or earth ground and a hot wire input to the electric box. A white
wire is connected from neutral or earth ground to the wide plug of the
electric outlet. The hot wire is connected through a circuit breaker in
the electric box with a black wire to the narrow or "hot" side of the
electric outlet.
Electric
Box
HOT
Electronic System
DC Power Supply
L
System
Ground
Fig. 19: Wiring of an electric outlet and electric box to an electronic systems power
supply.
Figure 19 shows that when the enclosures of an AC powered electronic
system are removed, circuit points in thesystem are connected directly to
the hot side of the AC outlet. Touching hot circuit points in the electronic
system would be essentially that same as removing the AC outlet cover
AC Direct Off-Line Power Supplies 3
and touching the black wire connection. An alternating voltage potential
exists between the hot side (narrow plug) and earth ground. Current flows
through any resistance path between these points of potential.
An electronic system with a half-wave rectifier uses a polarized AC plug.
The polarized plug insures connection from the wide side of the outlet,
neutral or earth ground, to common or ground points of the system.
(See figure 20.) This arrangement presents no voltage potential in respect
to earth ground if proper electric wiring and earth ground connections
have been made. You can safely touch the circuit ground in the system or
connect test equipment grounds.
But what happens if the AC line cord is plugged into the receptacle
backwards with a "cheater" adapter? Now the circuit common or ground
is "hot" or at a 120 volt AC potential to earth ground. This creates a
potentially lethal shock hazard for a service technician if they touch this
point and complete a conduction path to earth ground.
A greater servicing problem exists with a full-wave AC line-derived bridge
power supply. A direct AC line-derived bridge rectifier uses a common
ground reference at the anode junction of two bridge rectifier diodes.
During the operation of the bridge rectifier, these diodes are alternately
turned on and off, each conducting during the opposite half of each AC
cycle. During each negative AC alternation, the common ground in the
electronic system is electrically connected to the hot side of the AC plug.
A voltage difference of half the AC line potential exists to earth ground no
matter how the AC plug is connected. A voltmeter connected from the
bridge common ground to earth ground reads approximately 60 volts AC.
The voltage created on the "HOT" ground in a line-derived bridge supply
is capable of delivering current through any conductive path to earth
ground. This includes current through
you are unfortunate enough
if you
to touch the ground of the chassis while your shoes conduct through a
damp cement floor.
A "HOT" ground can pose a hazard for the power supply and other test
equipment when connected. Most test instrument grounds or shields are
connected to earth ground for shielding and user safety. Connecting a
ground from a test instrument to the "Hot" ground of the power supply
causes high current to flow through the bridge rectifier diode as shown
in figure 3. The high current may cause damage to the power supply
components and test equipment ground paths.
Note: The term "hot ground" is really a misnomer, since the circuit
ground is not at any ground potential. Instead, think of the "hot ground"
as "hot common" meaning that it is the common reference point for
making voltage measurements in the electronic system.
32AC
Direct Off-Line Power Supplies
digital display indicating the potential safety leakage current that may be
produced from each exposed metal part. The reading should be less than
500 microamps on each part. A readings considerably higher than this
indicates a high leakage current potential and shock hazard.
Note: Any metallic part that reads overrange on the CURRENT/POWER
LCD (blank display with a digit "1" on the left) is connected directly to
one side of the AC line.
AC Line-Derived Power Supply Voltage & Waveform
Measurements
Determining if the AC line-derived power supply is functioning properly
requires voltage measurements at key input and output circuit points.
To analyze an AC li ne-derived power supply you must test the input AC
voltage and the output DC voltage.
The input AC voltage to the bridge rectifier is applied to the bridge
rectifier at the junction of D2 and D3 and junction of D4 and Dl as shown
in figure 24. At these junctions the anode of a diode connects with a cathode of another. These points may be identified with a sine wave symbol on
a bridge rectifier module or IC. On the power supplies used in this course,
the input ACV test points are identified as "ACV H" (hot) and "ACV N"
(neutral) as shown in figure 24.
Test Equipment
Input ACV
Measurement
To isolation
Transformer
D4
ACV N
Cl
HOT
Ground
Test Equipment
Output DCV, VPP
Measurement
0
Fig. 24: Test lead connections to measure input and output voltages and waveforms.
To measure the input AC volts to a power supply, connect the test
instrument leads as shown in figure 24. Note that when connecting a test
instrument for input AC voltage measurements you do not connect to the
common or hot ground of the power supply. Therefore, connecting a
second ground shared by the same instrument or another instrument shunts
DI producing high input currents.
AC Direct Off-Line Power Supplies37
CAUTION
When connected for AC input voltage measurements, as in
figure 24, do not connect a second test instrument ground or
ground lead from the same instrument. Doing so shunts
D1 causing high AC line current that may damage power
supply components.
Figure 25 shows the typical scope waveform you should observe at the AC
input voltage test points to an AC line-derived power supply. The waveform peaks may be flattened slightly due to inductance of the isolation
transformer and input AC filter coils. This may lower the measurements
from the nominal values. Some typical voltage readings with the PR570
set to output 120 V are shown in figure 25. The time of one AC waveform
cycle is approximately 16.67 milliseconds and the frequency is 60 Hz.
Volts/DIV = 50
Time/DIV = 2 mS
VRMS = Approx. 115 V
Vpeak = Approx. 167 V
VPP = Approx. 335 VPP
1 6.67 milliseconds
1
ti me = 60 Hz
Fig. 25: Input AC voltage waveform and approximate measurement values.
The output voltage of the AC line-derived power supply is developed at the
positive terminal of the filter capacitor in respect to the common or hot
ground of the circuit. The negative terminal of the capacitor connects to
the common or hot ground. Connecting to the terminals of the large filter
capacitor near the bridge rectifier provides the proper circuit points for
outputmeasurement. In the power supplies in this course. TP1 corresponds
to the positive capacitor terminal and HOT GND to the negative capacitor
terminal of Cl as shown in figure 24.
The positive terminal of the capacitor feeds the regulator or remainder of
the switch mode power supply circuitry. Initially these circuits are the
load on the filter capacitor. When the capacitor is loaded it discharges
between AC alternations resulting in a variation to the output DC voltage.
38AC Direct Off-Line Power Supplies
This voltage variation appears as a repeating sawtooth waveform of
approximately 120 Hz when viewed on a scope. The sawtooth waveform
is commonly referred to as "AC ripple." Analyzing the output should
include a measurement of the DC voltage and the peak-to-peak amplitude
of the AC ripple.
When the filter capacitor is unloaded or has no discharge path, the output
DC voltage has little if any ripple. The DC output voltage reads near the
peak of the AC voltage or approximately 169 volts. A voltage near 169
volts with little or no ripple indicates the raw DC supply is not loaded. As
the capacitor is loaded by the remainder of the circuits the AC ripple
increases. The increased load and ripple peak-to-peak volts results in a
reduced DCV measurement. The ripple peak-to-peak volts rarely exceeds
10% of the DC value. For example, if a ripple greater than 16VPP is measured, the load may be excessive, the filter capacitor defective, or a bridge
diode open. To determine if a bridge rectifier diode has opened, measure
the frequency of the ripple waveform. A frequency of 60 Hz indicates an
open diode.
Volts/DIV = 1 or .5
Time/DIV = 5 mS
No Load = Approx 169 V
0V
Ripplet
VPP
0V
Time
- - Med. Load = Approx 160 V
)1.-
Ripple
VPP
0V
Time
Heavy Load = Approx 150 V
)
Time
Fig. 26: AC ripple waveform at output of AC line-derived power supply to hot ground.
AC Direct Off-Line Power Supplies39