Magnetic devices addressed here include
transformers, inductors, coils, and RF chokes.
Although there are several types of magnetic devices, many properties and
failure mechanisms are common; therefore, for the purpose of derating,
requirements are subdivided based on frequency.
Two sets of derating requirements are presented, one for coils and chokes
designed to operate in the RF range, and the other for transformers and
inductors operating at power rectification and audio frequencies.
This information is intended to be a general
guide to aid the designer in the selection of an appropriate inductor or
magnetic device for his intended application.
More detailed information is available from vendors, MIL-STD-1286,
MIL-T-27, and other publications.
MIL-STD-1286 is particularly useful to military users because it details
the various types of military-grade transformers, the advantages and
disadvantages of each, and the behavior of each type under different operating
conditions. A further source of
information on the design of power supply-related magnetics is NAVMAT P-4855-1A,
“Navy Power Supply Reliability Guidelines”. Complete listings of the military
specifications are available from DSCC
(Defense Supply Center Columbus) 3990 E. Broad St., Columbus, Ohio
43216-5000.
PACKAGING
Magnetic devices are available both in
standardized and customized package styles.
The standardized styles include chip, axial leaded, and radial leaded
package styles. Chip style inductors
typically range in value from 0.002 to 270 mH, and tolerances of
±5, 10, or 20%. Axial leaded coils typically carry higher
inductance values (between 0.10 and 10000 mH), with package size greatly
dependent on inductance value. Radial
leaded packages are likewise available with a large variety of inductance values
and sizes. Most variable inductors are
manufactured in the radial leaded packages.
Magnetic devices can also be purchased in
customized package styles. They are
often designed for a specific application rather than manufactured in large
quantities and purchased as off-the-shelf items. This is especially true for power
transformers. Due to the high power
density requirements in many power supplies, the designer cannot afford to waste
real estate with unnecessary tapes or other
features.
Unlike other passive devices, many types of
magnetics (particularly transformers) are available at various levels of
assembly. For example, many transformers
can be purchased in potted packages with metal shields or as an individual
transformer intended to be potted and shielded by the
user.
The packages of magnetic devices are among the
most massive of any component. This
demands special mounting considerations to assure excessive forces are not
exerted onto leads, especially in high shock and vibration environments, both in
operation and while in storage. A
general rule-of-thumb is to limit stress to 1/4 ounce per lead for small sized
inductors, coils, and chokes. Stress
must also be limited for large sized transformers, but these are usually
designed to handle much higher stresses.
Packages are also available with varying amounts of shielding. This is an important consideration where
there is concern about the affects of electromagnetic
fields.
FAILURE MECHANISMS &
ANOMALIES
Relative failure modes of transformers and coils are
shown in Table 1. Data shows they are equally
likely to fail in either the open or short mode, but are about 3 times less
likely to fail due to a shift in value.
Most failures are related to heat.
This document will restrict itself to a discussion of internal heating
effects.
|
Table
1. Normalized
Failure Mode
Distributions for Transformers and
Coils.[1]
|
| Failure Mode |
Relative
Probability |
| Open |
42% |
| Short |
42% |
| Parameter Change |
16% |
The most common cause of internal heat is from
a lack of understanding of the frequency performance limitations of the
device. The wire coil of a magnetic
device is wound to give it the best performance over a specific frequency
range. Operation outside this range
results in overheating and eventual performance degradation. Operation at frequencies lower than the
designed frequency range tends to saturate the core, while operation at higher
frequencies increase core losses.
Heat can also be introduced from capacitive
loads. Transformers that drive rectifier
circuits with capacitive input filters require special consideration. Capacitive loads cause the current through
the transformer to be very non-linear (since the current used to keep the
capacitor input filter charged is much greater than the average current
delivered to the load). As a result,
current is initially delivered as a series of spikes of much higher amplitude
than the steady state current. Since
transformer heating is a function of the square of the current, internal power
dissipation increases more rapidly with a capacitive load over an inductive
load.
Heating can also be caused by introducing DC
current into a device that is not designed to handle DC. Small amounts of DC into an audio-type
transformer, for instance, causes core saturation. This leads to a subsequent degradation in
performance, particularly at low
frequencies.
RELIABILITY
The failure rate of a magnetic device is most
strongly affected by temperature.
Specifically, it is dependent on how close the operating hot spot
temperature is to the maximum rated temperature of its insulation material. For this reason, reliability prediction
models base failure rate primarily on temperature and to a lesser extent
insulating material. A comparison
between the six most common insulating materials is shown in Figure 1. The failure rate models
show there is relatively little change in reliability between most insulation
materials, provided temperature is kept sufficiently below the maximum rated
operating. The source of internal heat
for a magnetic device is discussed under the section Failure Mechanisms &
Anomalies. External heating sources can
be caused from a large variety of sources and are considered outside the scope
of this document.
A further reliability concern is inherent
design and manufacturing deficiencies of customized magnetic devices. As a general rule, one-of-a-kind devices have
higher failure rates than off-the-shelf items.
This is because customized devices are typically manufactured in small
lot sizes and there is less of an opportunity for reliability growth. Magnetics are therefore more likely than
other passive elements to be tested and receive a power burn-in prior to
assembly.
DERATING
Derating requirements are divided into two
categories, dependent on frequency.
Requirements for low frequency (audio), power, and pulse magnetic devices
is shown in Table 2. High frequency RF coils are shown in Table 3.
| Table 2. Derating Requirements for Transformers and Inductors (low
frequency) |
| Parameter |
Derating
Requirement |
|
Category 1 Protected |
Category 2 Normal |
Category 3 Severe |
| Hot Spot Temperature |
30oC Below Specified Maximum |
30oC Below Specified Maximum |
30oC Below Specified Maximum |
| Current (Surge) |
90% of Max |
90% of Max |
80% of Max |
| Voltage (Surge) |
90% of Max |
90% of Max |
80% of Max |
| Additional Derating Requirements if Hot Spot Temperature is Not Known
to High Level of Confidence: |
| Current (Continuous) |
70% of Max |
70% of Max |
60% of Max |
| Voltage (Continuous) |
70% of Max |
70% of Max |
60% of Rated Max |
| Table 3. Derating Requirements for Coils and Chokes (High
Frequency) |
| Parameter |
Derating
Requirement |
|
Category 1 Protected |
Category 2 Normal |
Category 3 Severe |
| Current (DC) |
90% |
90% |
80% |
| Hot Spot Temperature (Operating) |
30oC Below Specified Maximum |
30oC Below Specified Maximum |
30oC Below Specified
Maximum |
Derating requirements for low frequency devices are further
broken down into two categories, dependent on the level of confidence the user
has in the hot spot temperature estimate of the device. The most important
parameter to derate is hot spot temperature, but it is also the most difficult
to determine. It usually needs to be
estimated from a DT temperature rise from ambient
based on power dissipation and radiating surface area of the device. Three methods of performing an empirical
estimate of hot spot temperature are shown in Table 4.
From top to bottom, they are listed in order of highest confidence to
lowest (consequently, they are also listed in order of most to least difficult
to compute). High quality suppliers or
suppliers conforming to established specifications will supply specification
sheets with additional information to determine the DT temperature rise (e.g., a
DT rise at full rated power).[2]
Note:
DT Represents
the Temperature Rise from Ambient (TA).
If there is a high degree of confidence in the hot spot
temperature estimate for a magnetic device, it is acceptable to derate to hot
spot temperature and surge current/voltage only, as shown in Table 2. If hot spot temperature is not known or the
level of confidence is low, perform additional derating on continuous current
and voltage, also shown in Table 2. This manual does not define an acceptable
level of confidence because it is dependent on the particular application and
device type. For example, there would be
little concern about using the transformer mass method of Table 4 for a low power
magnetic device intended for a room temperature application. In contrast, a transformer used in a three
phase rectifying circuit for a shipboard application needs a much better
estimate of hot spot temperature. It
also depends on how close the temperature estimate is to the maximum permissible
derated temperature.
Consideration should be given to imposing additional voltage
derating on equipment intended to be used at low barometric pressure (high
altitude or space) to protect against degraded dielectric properties and
potential corona effects.
DESIGN & MATERIAL
If it is necessary to know the individual materials or design
controls used in the manufacturing operation, consult the individual vendor.
FACILITY ASSESSMENT & QUALITY
Magnetic devices can be purchased with known quality and failure
rate levels. These are determined by the
vendor, based on testing. Magnetic
devices conforming to the established reliability requirements of the Military
specifications are identified by a failure rate letter designator, shown in Table 5. These carry a 60% consumer’s confidence level
and are maintained at a 10% producer's risk.
The confidence level of commercial magnetic devices may
vary.
|
Table 5. Failure Rate Designator for Magnetic Devices
Conforming to Military Specification Requirements
|
Failure Rate Letter
Designator |
Failure Rate (failures per 1000 hrs) |
| C |
Non-ER |
| M |
1.0 |
| P |
0.1 |
| R |
0.01 |
| S |
0.001 |
SAMPLING
For sample lot sizes used by the vendor, consult the individual
specification requirements.
PROCESS CONTROL
Vendors manufacturing magnetic devices, conforming to the
Military performance specifications, are required to implement an SPC system
that meets the requirements of EIA-557.
Typical processes applicable for SPC are coil windings, molding
dimensions, final electrical test, final inspection, and production
monitoring.
PART ASSESSMENT
Off-the-shelf magnetic devices are seldom subjected to
additional testing or screening by the user.
Customized magnetics and transformers used in high reliability
applications are often tested, especially on a first piece basis. The most popular method of propagating early
life failures and latent failure mechanisms without damaging inherent
reliability is with temperature shock.
HANDLING & STORAGE PRECAUTIONS
No special handling precautions are necessary beyond what would
be considered normal handling precautions to prevent damage during the assembly
operation. Transformers and inductors are not considered ESD
sensitive.
Closing
Comments
Stress Screening: If there is a need for
testing/screening to surface workmanship defects and infant mortality failures,
thermal shock is recommended. Normal
temperature cycling produces relatively benign benefits due to the high thermal
mass of magnetic devices and subsequent slow temperature rate of
change.
High Altitude/Space
Applications:
Recommend additional derating limitations be applied to voltage for applications
that will experience low barometric pressure.
Traditionally, derating guidelines for airborne applications (Air Force,
NAVAIR, and NASA) recommend more restrictive voltage limitations, usually in the
form of a 50% derating to both continuous and surge voltage. The rationale is to protect against the
negative effects of low barometric pressures, such as lower dielectric voltages,
increased corona effects, and the effects from condensation due to
altitude/temperature changes.
Drawings: Assembly drawings for
specialized magnetic devices are often complex and difficult to interpret. Companies do not use consistent formats, and
some companies/suppliers use unique processes that are not reflected on assembly
drawings. Therefore, first piece
inspection and testing is recommended when switching vendors or when the build
of a specialized magnetic device is sent to an outside
source.
Mounting: Special considerations need to
be given to the mounting of high power magnetic devices due to their relatively
large mass in comparison to most other parts.
Designers should attempt to mount massive magnetic devices on resilient
mounting plates or to the edges of circuit boards. If solder connections are used to secure
devices to a printed wiring board, limit stress to the solder connection to less
than 8 grams (1/4 ounce) per lead.
Heat Sinks: Specifications for temperature
rise from ambient (DT) of large power magnetic
devices are often given under the assumption that the device will be mounted to
a heat sink of a specific size. The size
of the heat sink in some device specification is sometimes overly optimistic and
may not be practical for many applications.
Assure heat sinks have adequate capability to keep hot spot temperature
below derating guidelines.
Adjacent
Components:
Power transformers can dissipate substantial amounts of heat. Attention must be paid to the mounting of
adjacent components, especially capacitor filters and voltage
regulators.