nd reliability of the internal
implementation of power converters because the proven standard designs
result in a robust and reliable power system with minimal technical risk. The
power system designer is thus freed up to concentrate on the system
aspects of the design. It is assumed that the power architecture has been
selected based upon the principles outlined in Chapter 3 and appropriate
power converters have been specified based upon the information presented
in Chapters 1, 2 and 4.
We now turn to the aspects of the system design that will insure the success
of the product. These areas of design will tend to focus on the interfaces
between the power converter(s) and the remainder of the system - input
circuits, output distribution considerations, controls, diagnostics and
paralleling. Each of these areas will be explored in some detail with the intent
of highlighting the aspects that will have the largest influence on the
performance of the overall system. Hopefully the priorities and design tips
presented here will help to demystify the process of designing a reliable and
cost-effective high performance power system.
5.2 Input Design Considerations
Certainly one vital interface to a power converter is the connection to the
input power! Compared to the output connection, the input interface tends to
be much more standardized and predictable because the converter is usually
operating from either the AC power mains or from some kind of Telecom DC
bus. These sources are controlled by many standards and industry
conventions so that the input circuitry of equipment power systems is often
very similar. The main system design issues will be safety (conductor sizing
The use of standard power modules has
made system design much easier.
However, the designer must still
understand how the power converter
interacts with external circuitry such as
input filters and diagnostic circuitry.
Paralleling of power converters is now
becoming important due to increased
redundancy requirements and the system
designer must understand how this is
achieved. This chapter sets out a
methodology whereby a system designer
can implement the power solution in the
system.
Chapter 5 - System Design using Standard
Converters
1 and fusing) and immunity from input power transients.
We will discuss these considerations for both AC input
and DC input power converters. We will also be
addressing som other system requirements including
reflected ripple, input voltage operating range, the need
for and selection of input capacitors, hot plugging and
primary-side control functions.
AC Input Converters - Artesyn's line of standard
AC/DC converters are designed to accommodate
worldwide AC mains standards, and will provide reliable
performance when properly installed in the end use
equipment. This installation only needs attention to the
most normal considerations, such as safety and
immunity from powerline transients.
The most important safety related design items are
grounding, fusing, and proper conductor selection. An
AC input power converter will have a chassis safety
ground connection included on the input connector. It is
imperative, for purposes of safety, that this pin is
connected to the "green wire" ground of the AC power
mains. This requirement is in addition to the chassis
bonding requirements that may apply. Most AC/DC
converters will contain either a fuse or circuit breaker
internal to the unit which is rated so that it will protect
the converter itself from creating a fire hazard in the
event of an internal fault. Note that this internal current
limiting will not protect the wiring that connects the
converter to the AC powerline. In the event of a short
circuit between the input conductors, this fuse or circuit
breaker will not activate, and the current in the
conductors will be limited only by the line impedance or
an external fusing device. This external fuse or circuit
breaker is located at the front end of the equipment box,
often in the area of the input EMI filter. The sizing of the
System Design using Standard Converters
AC conductors to the power converter and the trip rating
of the fuse or circuit breaker must be coordinated so that
the circuit will open before the conductors reach a
dangerous temperature. Note that, in general, these
conductors must support the current for more than one
converter. Figure 5.1 summarizes the safety-related
issues of connecting the input power to an AC/DC
converter. It depicts input wiring for a 208/240V line-to-
line application. The input connection will be similar for a
line-to-neutral 100/120V connection, except that no fuse
will be required in the neutral conductor. Many, if not
most, high quality AC/DC converters now include an
automatic input voltage ranging function as part of the
PFC circuitry, allowing for operation from below 100V to
over 250V. This simplifies the power system design
because jumpers or different connection points to the
converter are no longer required to configure it for either
line-to-neutral or line-to-line applications.
Figure 5.1 - Safety Consideration of AC Input Connection
Keep in mind that the AC/DC converter will have an input
current surge each time that the AC power is applied.
This current will be higher than the normal operating
current, and is caused by the need to charge up the
POWER
APPS
2
AC/DC Converter
L1
L2
To other Converters
Equipment
Powerline
Interface
External Fuse or CB
Chassis Bonding Connection
Internal Fuse (if used)
Conductor Sizes and
External Fuse Selected
to Protect against
Short Circuit Fault
Earth Ground open circuit output from the test signal generator and
that when it is coupled through capacitors to the low
input impedance of the actual powerline, the voltage
values will be substantially reduced. If you select high
quality AC/DC converters that are designed to meet the
EFT requirement and pay attention to normal AC
distribution design practices you should not have a
problem with the EFT requirements.
Figure 5.2 - Electrical Fast Transient Immunity
The least complex input circuit design will occur when
using Artesyn's AFE family of standard Front-End
products, shown in Figure 5.3. These AC/DC converters
are supported by a series of standard power shelves that
perform several functions while relieving the power
system designer of the corresponding design tasks. The
power shelf contains circuit breakers and an input filter to
support the Front-End converters so that these devices
do not need to be included elsewhere in the system.
(Note that the system may still require another circuit
breaker and/or EMI filter if there are loads not powered
by the Front-End converters) This system will also
support "hot plugging" of the converters so that
concurrent maintenance can be provided for high-
availability systems with redundant AC/DC converters. It
capacitors on the output of the off-line rectifier circuit.
The magnitude and duration of this surge should be
specified by the converter manufacturer. With well-
designed converters, the current will be manageable.
Make sure that whatever circuitry is located in front of the
power converter can handle this surge current. Pay
special attention to the current vs. time profile of the fuse
or circuit breaker to insure that you will not experience
any nuisance tripping when the AC power is
simultaneously applied to all the power converters.
In general, the system will require some sort of EMI filter
near its input, the primary function of which is to reduce
the conducted emissions from the equipment that are
imposed on the powerline. The need for and selection of
the EMI filter will be covered in more detail in Chapter 9.
Many systems are designed to comply with the so-called
Electric Fast Transient (EFT) or Burst Immunity
requirement as defined in specification EN61000-4-4.
This requirement attempts to simulate the type of
impulses that can result from contactors, relays and
switches being activated elsewhere in the system. The
bounce from such contacts opening and closing can
create high frequency but relatively low energy voltage
transients on the AC input lines. Meeting EFT is a
system-level requirement, but some AC/DC converters,
including Artesyn's, will meet this specification as a
stand-alone device. The addition of the system EMI filter
and other components in the system's AC Front-End
generally makes adherence to this requirement even
easier. There are several versions of the EFT with varying
severity for different system environments, but the values
most commonly encountered by Telecom and Datacom
power converters, level 3 and level 4, are shown in Figure
5.2. Note that the values shown in the diagram are the
3
Level 3
Level 4
300 ms
15 ms
2 kV
300 ms
400
µ
s
200
µ
s
15 ms
4 kV
50 ns
Each Pulse
Waveforms not to Scale also provides a self-contained low-powered source of
12V and convenient system level diagnostic and control
signals to and from the AC/DC converters to simplify the
system power interface and monitoring.
Figure 5.3 - Artesyn AFE Standard Front-Ends with Power Shelf
DC Input Converters - DPA systems have only become
popular for other than Telecom usage in recent years. As
a consequence, many power system designers are not
as familiar with the proper configuration of input circuits
for DC/DC converters as they are with the AC interface to
off-line AC/DC converters. It is a very different
environment, since there is either a battery bank or an
AC/DC converter in front of the DC/DC rather than an AC
powerline. Each system will be slightly different, and the
resulting DC source will not be as standardized as the
power utility. Nevertheless, there are some general
principles that will lead to a successful input circuit
design. As we discuss them below, it will be seen that
they are sensible and easily understood. Fo