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General Power
Supply
Application Notes
The following guidelines, arranged
topically in alphabetical order, will help system designers and application engineers
avoid many of the problem situations that could otherwise interfere with the successful
application of Martek Power Abbott power supply modules.
Topics include: |
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| AC Input Line Requirements |
| Martek Power
Abbott's AC-DCpower supply modules contain two or three AC input
circuit terminals designated as: |
- AC Hot (pin 1 if not designated)
- AC Neutral (pin 2 if not designated)
- Case or chassis (safety) ground
|
| Figure
1 shows the correct way to wire these terminals. Take care to
connect the AC Hot and Neutral lines (if designated) as shown. Interchanging them can
result in circulating ground current. Connect the safety ground line to both the power
supply chassis or case ground connector (if present) and to your system chassis. If the
power supply chassis or case ground connector is not present, connect the safety ground
directly to your system case or chassis. When connecting several power supplies to the
same AC source, connect all AC Hot terminals together and connect all AC Neutral terminals
together. |
| [Figure 1.] AC Input Line Circuit Requirements |
Switches - As a minimum safety precaution, insert a
single-pole switch or fuse in series with the AC Hot line or, for additional safety, a
double-pole switch in series with the AC Hot and Neutral lines as shown in Figure 1.
Fuses - For maximum protection and
safety, always insert a fuse in series with the AC Hot line of the power supply module.
Use Buss MDX type or equivalent slow-blow fuses rated as listed in the operating
instructions accompanying each unit. Alternatively, use a delayed magnetic circuit
breaker, such as the Heinemann Model CF (characteristic 1). |
[Back to the Top] |
Bypass
and Decoupling Circuits
To minimize the effects
of the power supply's output resistance (Rs) and the resistance (Ro) of the power
distribution leads on fast analog or digital circuits, use one or more local decoupling
capacitors (C1 and C2) in parallel with the load as shown in Figure 2. Decoupling circuits reduces the
effects of resonance created by stray capacitance (Cs) and the supply and line inductance
(Ls and Lo), and dampen any voltage spikes that are generated by load current transients.
The two-capacitor decoupling circuit shown in Figure
2 bypasses both medium-frequency (C2) and high-frequency (C1) signals and reduces
cross-talk between multiple loads. |
[Figure 2.] Load Decoupling |
| Tobypass individual analog or digital circuits, always
connect each bypass capacitor to prevent the AC signal from returning through the power
supply module as shown in Figure 3. Keep
signal current paths and lead lengths as short as possible. |
[Figure 3.] Bypassing By Shortest Signal Path |
[Back to the Top] |
Conductor Size and Resistance
As shownin Figure 4,
the resistance (Ro) of the load conductors and the contact resistance (Rc) at the power
supply and load connection points can seriously degrade regulation and significantly
reduce the load voltage. By neglecting these losses, a system designer could expect the
output of a 5-V power supply rated at 0.1% regulation to vary by 5mV from no load to full
load. However, if the losses represented, say 1.2%, of the available output, the same
supply would degrade to 1.3% regulation at the load, and the load voltage would vary by 65
mV from no load to full load- an increase of 1300% over the ideal situation. |
[Figure 4.] Load Connection Resistance Factors |
- To avoidthis problem, always consider the effects of conductor
and connector resistances on the application and observe the following guidelines:
- Forhigh-current applications, keep
conductor lengths as short as possible and ue larger wire sizes (Table 1) as
necessary to minimize resistive losses. When choosing wire sizes, use 1,000 circular mils
per ampere as a general guideline.
- Ensurethat there is adequate conductivity
for high-current conduction paths on printed circuit boards to minimize the voltage drop
along the conductor run. Use the cross-sectional area and length of the conductor run to
calculate its resistance.
- Wrapand solder all connections wherever
possible.
- Carefully clean banana plugs, spade lugs,
and other friction type connectors to remove oxidation and corrosion.
- Avoid using edge-type circuit board
connectors in parallel for carrying high load currents. Variations in contact resistance
can cause severe current imbalances and damage contacts.
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| Table 1. Maximum Resistance of
Annealed Copper Wire (100°C) |
Size AWG No. |
Area Circular Mils |
Resistance Ohms/1000 Feet |
Size AWG No. |
Area Circular Mils |
Resistance Ohms/1000 Feet |
| 0 |
105,500 |
0.14 |
12 |
6,530 |
2.23 |
| 1 |
83,690 |
0.17 |
13 |
5,178 |
2.80 |
| 2 |
66,370 |
0.22 |
14 |
4,107 |
3.54 |
| 3 |
52,640 |
0.28 |
15 |
3,257 |
4.45 |
| 4 |
41,470 |
0.35 |
16 |
2,583 |
5.63 |
| 5 |
33,100 |
0.43 |
17 |
2,048 |
7.08 |
| 6 |
26,250 |
0.56 |
18 |
1,624 |
8.95 |
| 7 |
20,820 |
0.70 |
19 |
1,288 |
11.27 |
| 8 |
16,510 |
0.88 |
20 |
1,022 |
14.21 |
| 9 |
13,090 |
1.11 |
21 |
810 |
17.92 |
| 10 |
10,380 |
1.40 |
22 |
642 |
22.60 |
| 11 |
8,234 |
1.76 |
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[Back to the Top] |
Connection Arrangements
In many cases, as single power
supply module may not provide the exact voltage, current, or reliability required for your
application. For these a series or parallel combination of two or modules may satisfy your
needs. Series aiding (voltage adding) or parallel arrangements can increase the load
voltage, load current, or reliability specifications of single modules subject to the
following limitations.
SERIES CONNECTED SUPPLIES
To achieve the output voltage specification required by your application, connect
power supply modules in series as shown in Figure 5
and described in the following guidelines:
- Use the fewest number of modules to
achieve the required voltage specification.
- Select power supply modules with
similar ratings.
- Do not exceed the output-to-case
breakdown ratings for the models in question.
- Never connect modules in series-opposing
(voltage subtracting) configurations, since they can not sink sufficient current to
operate safely in this way.
- Protect each module from reverse
voltage caused by a load short-circuit by connecting reverse-biased diodes across the
output of each series-connected supply as shown in Figure
5. Choose diodes with peak inverse voltage (PIV) ratings equal to at least twice the
load voltage and with forward current ratings greater than the maximum load current.
|
[Figure 5.] Load Connection Resistance Factors |
| Note that for modules connected in series, the total
line and load regulation specification meets the percentage but not necessarily the
absolute voltage specification of that for the individual modules. Furthermore, the
total ripple specification is the sum of the ripple specifications for each module. Note
also that because their internal switching circuits are not synchronized, switching
regulators typically generate beat frequency components of about 1 kHz that can interfere
with the operation of other circuits. PARALLEL CONNECTED SUPPLIES
To achieve the output current required by your application, connect power supply modules
in direct parallel or master/slave parallel configurations as described in the following
guidelines:
- Select power supply modules with similar
voltage ratings.
- Usethe fewest number of modules to achieve the required
current specification.
- Makethe output leads from each module of equal length and as
short as possible.
- Provideadequate sinking for each module to dissipate the heat
resulting from full-load operation.
- Forlow-current applications, connect the power supply modules
in a direct parallel arrangement as shown in Figure 6a. Set the output voltage of
each module individually by turning on one module at a time and adjusting its output
voltage to within 1% of the required value. Balance less-than- full -load currents by
placing a small amount of resistance (R1 and R2) in series with the load as shown.
Typically, R1 and R2 should approximately equal the output impedance of the power supply,
which you can approximate by dividing the load regulation voltage by full load current.
Note that balancing is extremely difficult, even with two equal resistors, and can
seriously degrade output regulation as described in the topic "Conductor Size and
Resistance." For tighter regulation, use the direct parallel arrangement with remote
error sensing as shown in Figure 6b.
This arrangement generally provides better regulation but poorer load sharing than the
direct parallel arrangement with local error sensing. However, by adding a small amount of
resistance (Rs) in series with the sensing leads as shown, you can modify the circuit
characteristics to provide about the same load sharing as the direct parallel arrangement
with local error sensing while improving load regulation by a factor of 2. Refer to
"Remote Error Sensing" for additional information.
|
[Figure 6.] Direct Parallel-Connected Power Supplies |
| Forhigh-current applications, connect the power
supply modules in a master/slave parallel arrangement as shown in Figure 7. This
circuit uses resistors R1 and R2 to ensure load sharing as in the direct parallel
arrangement with local error sensing. However, it also has an external error amplifier,
which monitors the negative output of the master supply and greatly improves regulation.
As the master supply remotely senses and regulates the load, the amplifier feeds and
error-compensating signal to the sensing input of the slave supply to bring the output
voltage of that supply into line with the master. |
[Figure 7.] Master/Slave Parallel-Connected Power
Supplies |
| Forapplications requiring 100% power redundancy,
ensure that each power supply module has sufficient capacity and heat sinking to supply
100% of the load alone and connect them in a modified direct parallel arrangement as shown
in Figure 8. In this circuit, diodes replace the resistors of Figure 6
and enable one module to fail without affecting the operation of the other. Note that the
power supply output voltage must be specified 0.5 to 1 volt higher than the load voltage
to compensate for the additional diode drops. Choose diodes with PIV ratings greater than
the load voltage and forward current ratings greater that full load current. These diodes
may dissipate high power, so adequate heat sinking is essential. |
[Figure 8.] Input Protection Circuits |
[Back to the Top] |
Line Noise and Transients
In manymilitary
applications, input line noise and transients form motors, generator, ignition systems,
high-current switching circuits, and similar systems pose potential problems for power
supply and power converter modules. Although Pi filters in the input circuits of the power
supply modules prevent noise generated by the supply from affecting the power source, they
also prevent power line noise from entering. For applications where these filters are not
sufficient, you may require separate isolation transformers in the input circuit of the
supply.Power line
transients are usually more serious than noise, because they can damage a power supply
module. If you expect transients in your application to exceed the maximum rated input
voltage of the supply module, use either of the two commonly used transient protection
methods shown in Figure 9.
The first circuit in Figure 9 shows a power
supply input that is protected by a fuse and metal oxide varistor (MOV). The MOV protects
the supply by absorbing and dissipating transient voltages. The fuse is required to
protect the input line from the most common failure mode of MOVs- a short circuit. MOVs
are typically available in breakdown voltages of 25 V and higher.
The second circuit in Figure 9 shows a power
converter input that is protected by a transient suppressor. Transient suppressors are
sold by most semiconductor manufacturers and will protect the supply from short-duration
transients. |
[Figure 9.] Input Protection Circuits |
[Back to the Top] |
Overload Protection
Martek Power Abbott powersupply and converter
modules employ two types of output overload protection, depending on the type of module:
- Foldback current limiting
- Foldback current limiting
Foldback current limiting protects
switching regulator models from damage when an overload occurs by reducing, or folding
back, both the output voltage and current as the load resistance ranges from maximum to
short circuit as shown in Figure 10a. Figure 10a applies to Models BN, C, CC, LV, LLV, MH,
V, W, WW, WL, and WWL. These modules resume normal operation automatically after the
removal of an overload. Models B, S, SN and U turn off when an overload occurs. After
removal of the overload, Model B recovers automatically, but Models S, SN and U can not
resume normal operation until their inputs have been recycled. Foldback current limiting
protects all of these modules by reducing input power under an overload or short circuit.
For the overload protection characteristics of other series, please consult an Abbott
sales engineer. |
[Figure 10.] Overload Protection |
Overload Trip
Points
|
| MODEL |
TRIP POINT |
| B |
Typically 200 to 300% of
rated current, depending on the saturation point of the output transformer. |
| BA, M, MH, M300 |
110 to 130% of rated
current. |
| BN, C, CC, LV, LLV, V, W,
WW, WL, WWL |
135 to 175% of rated
current. |
| S, SN |
150 to 250% of rated
current, depending on the output surge current. |
| BC, MB, WM |
110 to 120% for single
output or main output of rated current. 120 to 140% for auxiliary of triple output
modules of rated current. |
| AB, AW, AM |
110 to 130% of rated
current. |
Constant current limiting protects Models BA, M, and UO2 from
damage when an overload occurs by holding the output current constant as shown in Figure 10b. These modules resume normal operation
after the overload is removed. However, unlike the protection provided by foldback current
limiting, constant current limiting does not reduce the internal power dissipation of the
module. Although each module can withstand short circuits of unlimited duration, the heat
from long-term overloads can deteriorate components within the supply and seriously
degrade the operating life of the module.
Please consult a Martek Power Abbott sales
engineer for further information. |
[Back to the Top] |
Power Distribution
Designing anefficient power
routing system is much more difficult than simply selecting a power supply with the proper
voltage and current rating. For truly effective power distribution, the system designer
must:
- Decide whether one central or several
distributed supplies are appropriate. For additional information on this topic, refer to
"Connection Arrangements."
- Paycareful attention to error-sensing and system grounding
techniques.
- Determineif or how much power
redundancy is required. For additional information on this topic, refer to
"Connection Arrangements."
Parallel
Distribution - The parallel distribution scheme depicted in Figure 11, although simple, is satisfactory
only for low-current applications where the voltage drop in the load conductors is
negligible. In this network the voltage at each load depends on the current drawn by the
other loads, and the possibilities for ground loops, load interaction, and poor regulation
are prevalent. |
[Figure 11.] Simple Parallel Distribution |
Single Point Distribution - The single-point, or radial, distribution network
shown in Figure 12 provides much better
performance than simple parallel distribution, because it eliminates the opportunities for
ground loops and interaction among loads. The success of this technique requires a single
pair of terminal connection points, a pair of heavy (low resistance) wires connecting the
power supply module to each terminal point, and separate leads connecting the terminal
points to each individual load. |
[Figure 12.] Single Point Distribution |
Combination Distribution -
A combination system, incorporating both parallel and single-point distribution offers
some advantages in applications where there are both heavy and light loads. As shown in Figure 13, the heavy loads connect to the power
supply output in a single-point arrangement, and the light loads in a parallel
arrangement.
For best performance, always connect the heavy loads nearest the power supply output, and
the light loads farthest away. |
[Figure 13.] Combination Distribution |
Power distribution for analog and digital
circuits requires separating and grounding each type of circuit as shown in Figure 14.
To ensure that unwanted signals from one do not interfere with the other, observe the
following guidelines:
- Use separate power supplies for analog and
digital circuits.
- Do notallow low-level analog signals to share conduction paths
with digital or power returns. Notice that there is only one ground point and that the
paths between the analog and digital supplies are shared.
Analyzeeach ground path
separately to make sure that it goes directly to the single point ground with no shared
path.
- Workclosely with the layout draftsman to assure the correct
physical layout of signal and power paths on each circuit board.
|
[Figure 14.] Analog and Digital Circuit Distribution |
[Back to the Top] |
Remote Error Sensing
The remote error-sensing
feature of Abbott power supply modules enables them to maintain the load regulation
specification of a power supply right at the load by compensating for voltage drops across
the load conductors. Remote error sensing is especially suitable for high-current
applications, where the load conductor voltage drop could seriously degrade the regulation
specification.
Figure
15 shows a typical remote
error-sensing application. Each output terminal has its own error-sensing connection
points (designated +s and -s), which connect to the load through shielded or twisted pair
leads (24 AWG size wire recommended). For optimal performance the error-sensing leads
carry much less current than the load conductors, and the shielded or twisted pair wiring
prevents noise from entering and interfering with the regulation circuits. In operation,
the error-sensing circuit detects any deviations from the nominal load voltage and
compensates by automatically adjusting the voltage available at the output terminals. If
the remote error-sensing terminals are not used, the supply operates normally with local
error-sensing when the sensing terminals are connected to the appropriate output terminal. |
[Figure 15.] Remote Error Sensing |
[Back to the Top] |
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