Application Note

VISHAY DALE
Magnetics
Application Note
Using IHLP’s in Automotive Applications
Introduction
Most innovation in automobiles is driven by electronics and
these innovations are becoming a steadily growing portion
of automotive production costs. There is an industry push to
reduce size, increase dc-to-dc converter switching
frequency and power density. Add to this; harsh
environments and strict qualification procedures mean
designers have a difficult job ahead of them.
There are several technology drivers in the automotive
industry such as passenger comfort and safety as well as
environmental considerations. All of this requires
expanding electronics in decreasing available space.
Vishay’s low-profile, surface-mounted, fully shielded IHLP
composite inductors were designed to address this issue.
The IHLP was created for, among other things, two major
applications: EMI filtering for the high power line and
energy storage for high frequency dc-to-dc converters.
To meet the requested PC board real estate, available height
and power densities the designers require a low profile, high
current fully shielded power inductor. The IHLP series of
inductors from Vishay meets these needs and has been used
in numerous automotive applications since 2002.
IHLP Basics
The IHLP inductor is constructed using a wound copper coil
that is ultra-sonic welded to a lead frame. Iron powder
combined with an epoxy binder is then pressed around the
inductor coil, giving the inductor its final shape or footprint.
Fig. 1 illustrates how the IHLP inductor is constructed.
200 °C
Polyester/Polyamide
Insulation
Copper Wire
High Reliability
Ultra-Sonic Weld
Proprietary Blend Low
Loss Iron Powder
Figure 1.
The enameled copper winding inside of the IHLP inductor
is isolated and can withstand operating temperatures up to
200 °C.
The powder materials used in the IHLP series deliver stable
performance under worstcase overload conditions up to
125 °C operating temperature. The operating temperature is
defined as the self-heating temperature of the inductor plus
Document Number: 34256
Revision: 14-Aug-08
ambient temperature. At present, the IHLP has a maximum
operating temperature of 125 °C and are qualified according
to AEC-Q200. The IHLP can be operated above 125 °C
provided the effects of thermal aging are taken into
consideration (see below). Vishay is developing an IHLP
material that will operate at 155 °C with no aging and this
product will be announced when it is available.
For technical questions contact: magnetics@vishay.com
www.vishay.com
1
APPLICATION NOTE
Copper Leadframe Plated
with 50 to 125 microinches Ni
and 180 to 250 microinches Sn
Application Note
Vishay Dale
Using IHLP’s in Automotive Applications
EMI Filtering
An EMI (Electro Magnetic Interference) filter is a specific
type of filter used to reduce electro magnetic interference
generated by power electronic equipment. This is an
important circuit element as EMI will negatively impact
other electronic devices within the automobile. Important
characteristics for EMI filter designers are attenuation,
insertion loss, voltage drop, and the number of filter sections
required. The IHLP’s advantage of lower DCR for its
package size will be of great assistance to the filter designer
as the above concerns of filter design are all, in one form or
another, linked to DCR. If multiple filter sections are
required the voltage drop savings will be further
compounded.
A key requirement of automotive electronics is avoiding or
eliminating EMI and electromagnetic radiation and its
negative effects on electrical circuitry without excessive
reduction in operating voltages. Current often has to travel
over cables of appreciable length to the car’s electrical
control units (ECUs) so voltage drop in the cable and the
EMI filter can be a concern. An EMI filter and cable with
combined resistance of 200 mΩ with 10 A of current would
cause a voltage drop of 2 V.
DC/DC Converters
A Discussion on rated current
The most prevalent use for the IHLP style of inductor is in
non-isolated dc-to-dc converters. In today’s and tomorrow’s
power supplies, power handling capability and size are
becoming the driving forces. To meet these requirements
designers need to increase the operating frequency.
Increasing frequency allows the use of smaller components,
but the downside to this strategy is an increase in losses.
Dc-to-dc converters are also being asked to operate at higher
ambient temperatures. This in turn requires the inductor to
operate at the higher temperature in addition to its own
temperature rise incurred due to power losses. It is known
that iron powder exhibits the effects of aging at higher
temperatures in the form of increased core losses. These
losses must be accounted for during the design process in
order for a composite inductor to be used at component
temperatures in excess of 125 °C. The effects of thermal
aging can be minimized by simply limiting the maximum
inductor temperature to 125 °C or less. This does not mean,
however, they cannot be used in excess of 125 °C, all that is
required is proper care in the design process.
Thermal Aging
Based on experimental observations by Vishay, it has been
determined that aging, in the form of increased core loss,
occurs above 125 °C as a function of time and temperature.
The higher the temperature above 125 °C, the shorter time it
takes for the core loss to increase to a given level. This aging
occurs due to the fact that the electrical insulation between
powder particles becomes more conductive. Testing has
shown that a pure iron component with no binder or
insulation has roughly the same resistivity as an aged fully
insulated part with binders. The IHLP product line exhibits
a plateau (see Fig. 2) after 2500 h to 9000 h in core loss at
higher temperatures. The higher the temperature, the
quicker this plateau is reached.
Core Loss vs. Hours Aging at Various Temperature
3500
125 °C
3000
APPLICATION NOTE
105 °C
2500
85 °C
mW/cc
2000
1500
1000
500
0
0
4000
8000
Hours
12 000
16 000
Figure 2.
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For technical questions contact: magnetics@vishay.com
Document Number: 34256
Revision: 14-Aug-08
Application Note
Vishay Dale
Using IHLP’s in Automotive Applications
Thermal aging discussion
There is evidence that suggests that the insulating coating of
the iron particles reacts in the presence of oxygen at elevated
temperatures reducing its resistivity resulting in increased
eddy currents and ultimately higher core losses.
Once this coating is used up, the degradation of the
insulation layers ceases and the core loss stabilizes resulting
in the “plateau” in the aging curve shown in Fig 2.
There has been much industry concern over the
representation illustrated in Fig. 3 which would indicate a
thermal runaway condition in a power inductor. In order for
a thermal runaway condition to occur, an inductor
application would have to be dominated by core loss. Vishay
recommends that the core losses be roughly a third of the
total losses, and that the combined ambient temperature, and
temperature rise of the inductor due to core loss and copper
loss be less than 125 °C. If operation in excess of 125 °C is
required it is recommended that core losses be kept to 1/6 of
total losses to mitigate the effects of thermal aging. Vishay
further suggests that the total temperature rise of the
inductor due to all loss factors (core loss, copper loss,
proximity loss, skin effect) be kept to 40 °C or less
regardless of the ambient temperature. If the core losses are
a fraction of the total loss of inductor, any increases in core
loss due to aging will have only a small effect on the final
operating temperature of the inductor. To determine IHLP
core losses please reference Vishay document number
34250, Selecting IHLP Composite Inductors for
Non-Isolated Converters Utilizing Vishay’s Application
Sheet [2].
Core Loss
Core Loss vs. Time
Time at Elevated Temperature
Figure 3.
However, if the circuit has current limiting on the switching
transistor, then the regulator will shut down and thermal
runaway will not occur. The inductor will still not support
the energy storage due to aging, but a catastrophic heating
will not occur if the current is limited. In addition, the losses
of the powdered iron material cannot increase exponentially
forever as the graph suggests. The losses can only increase
Document Number: 34256
Revision: 14-Aug-08
to a point equivalent to that of the powdered iron since the
magnetic characteristics of the base iron are not affected at
these temperatures.
PCB Trace Size Concerns
In 1956 the National Bureau of Standards attempted to
quantify the current carrying capacity of printed circuit
board (PCB) traces. Their attempt fell short in being able to
predict the temperature rise of PCB traces consistently. It
was replaced in 1973 by MILSTD-275, since superseded in
1999 by IPC-2221, which also fell short in its attempt to set
accurate standards for PCB trace design. While these
documents were somewhat successful in predicting current
carrying capacity they did not address surface mount
components that utilize the PCB traces as heatsinks.
For technical questions contact: magnetics@vishay.com
www.vishay.com
3
APPLICATION NOTE
As stated earlier, a thermal runaway condition will occur if
the core loss is the dominant loss factor and only if the
impedance of the inductor in the circuit controls the current
limit in the inductor. As the core loss increases the effective
inductance and energy storage (1/2LI2) decreases. The
scenario for thermal runaway would occur if the pulse width
of the converter is increased in order to maintain regulation
under load, this will increase current in the inductor. More
current, more heat, etc. until runaway occurs.
Application Note
Vishay Dale
Using IHLP’s in Automotive Applications
Designers are increasingly being asked to design smaller
and cheaper power supplies to meet market demand. To
accomplish this task designers are often skimping on printed
circuit board trace width, thickness or both. The end result
of this practice is overheated parts, reduced efficiency,
costly redesigns or product recalls. Contributing to the
problem are power losses from surface mount components,
in this case inductors, using the PCB traces as heatsinks.
Each power inductor manufacturer provides “Rated
Current” numbers for their products. These ratings are
usually based on temperature rise or saturation. In most
cases, manufactures have adopted temperature rise as the
deciding factor for rated current. Many times, the rated
current is the amount of DC current that results in a
temperature rise of 40 °C due to the DCR or self heating due
to the resistance of the copper coil in the inductor. This
current rating is performed under DC conditions only and
does not take into account the self heating due to core losses.
This is a topic for another paper. However, the rated current
numbers do assume that the termination pads and copper
traces on the designer’s printed circuit board are adequate to
carry the rated current and to carry away the heat produced
by the copper winding. Many designers do not take into
account the amount of copper necessary to handle the high
currents that flow into the inductor and will experience more
heat rise is indicated by the manufacturer’s datasheet. While
there are many factors that will affect a printed circuit
board’s ability to transmit heat, guidelines have been
established to insure proper trace width to handle high
currents.
TABLE 1 - RECOMMENDED EXTERNAL TRACE (1) CARRYING CAPACITY BASED ON TEMP. RISE
Temperature Rise
30 °C
Trace Thickness (mm) 0.0175 0.035
40 °C
0.07
0.0175 0.035
50 °C
0.07
APPLICATION NOTE
Trace Width (mm)
0.0175 0.035
60 °C
0.07
0.0175 0.035
70 °C
0.07
0.0175 0.035
0.07
Maximum Current (A)
0.250
0.6
0.8
1.2
0.7
0.9
1.4
0.7
1.0
1.5
0.8
1.1
1.6
0.8
1.2
1.8
0.500
1.0
1.4
2.1
1.1
1.6
2.4
1.3
1.8
2.6
1.4
2.0
2.8
1.5
2.1
3.0
0.750
1.4
2.0
2.9
1.6
2.3
3.3
1.7
2.5
3.6
1.9
2.7
3.9
2.0
2.9
4.2
1.000
1.7
2.5
3.6
2.0
2.8
4.1
2.2
3.1
4.5
2.4
3.4
4.9
2.5
3.6
5.3
1.250
2.1
3.0
4.3
2.3
3.4
4.9
2.6
3.7
5.4
2.8
4.1
5.9
3.0
4.4
6.3
1.500
2.4
3.4
5.0
2.7
3.9
5.6
3.0
4.3
6.2
3.2
4.7
6.8
3.5
5.0
7.3
1.750
2.7
3.9
5.6
3.1
4.4
6.4
3.4
4.9
7.0
3.7
5.3
7.6
3.9
5.7
8.2
2.000
3.0
4.3
6.2
3.4
4.9
7.1
3.8
5.4
7.8
4.1
5.9
8.5
4.4
6.3
9.1
2.250
3.3
4.7
6.8
3.7
5.4
7.8
4.1
6.0
8.6
4.5
6.5
9.3
4.8
6.9
10.0
2.500
3.6
5.1
7.4
4.1
5.9
8.4
4.5
6.5
9.3
4.9
7.0
10.1
5.2
7.5
10.9
2.750
3.8
5.5
8.0
4.4
6.3
9.1
4.8
7.0
10.1
5.2
7.6
10.9
5.6
8.1
11.7
3.000
4.1
5.9
8.6
4.7
6.8
9.8
5.2
7.5
10.8
5.6
8.1
11.7
6.0
8.7
12.5
3.250
4.4
6.3
9.1
5.0
7.2
10.4
5.5
8.0
11.5
6.0
8.6
12.5
6.4
9.3
13.4
3.500
4.6
6.7
9.7
5.3
7.6
11.0
5.8
8.4
12.2
6.3
9.2
13.2
6.8
9.8
14.2
3.750
4.9
7.1
10.2
5.6
8.1
11.6
6.2
8.9
12.9
6.7
9.7
14.0
7.2
10.4
15.0
4.000
5.2
7.5
10.8
5.9
8.5
12.2
6.5
9.4
13.5
7.0
10.2
14.7
7.6
10.9
15.8
4.250
5.4
7.8
11.3
6.2
8.9
12.8
6.8
9.8
14.2
7.4
10.7
15.4
7.9
11.4
16.5
4.500
5.7
8.2
11.8
6.4
9.3
13.4
7.1
10.3
14.9
7.7
11.2
16.1
8.3
12.0
17.3
4.750
5.9
8.5
12.3
6.7
9.7
14.0
7.4
10.7
15.5
8.1
11.7
16.8
8.7
12.5
18.0
5.000
6.2
8.9
12.8
7.0
10.1
14.6
7.7
11.2
16.1
8.4
12.1
17.5
9.0
13.0
18.8
Note
1. Derate maximum current by 50 % for internal traces
Table 1 summarizes the recommended maximum current for
a PCB trace based on trace width and thickness. These
recommendations were developed using the following
model: (1)
I = 3.188 × ΔT
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0.45
×W
0.79
× Th
0.53
(1)
The model uses ΔT in °C and width (W) and thickness (Th)
in millimeters. The 3.188 is a constant based on a common
thermodynamics model that has been converted to SI units.
If these guidelines for circuit board traces are followed, the
designer should experience thermal performance very
similar to that listed on the IHLP datasheet.
For technical questions contact: magnetics@vishay.com
Document Number: 34256
Revision: 14-Aug-08
Application Note
Vishay Dale
Using IHLP’s in Automotive Applications
Moving Forward
References
As the electronics content increases in automobiles and
more systems become electrical instead of mechanical, large
amounts of current need to be properly regulated and
filtered. The advantages of the IHLP including its low DCR
and high current handling capacity make it an excellent
choice for dc-to-dc converters or EMI filters. With proper
design techniques the IHLP style of inductor can safely be
used by and be of great benefit to automotive electronic
designers now and in the future.
[1] John
Vandersleen,
“Printed
Wiring
Board
Manufacturing
Advances”,
Power
Electronics
Technology, September 2004, pp. 40 to 43.
[2] Selecting IHLP Composite Inductors for Non-Isolated
Converters Utilizing Vishay’s Application Sheet,
available on the Vishay Intertechnology, Inc. website:
http://www.vishay.com
APPLICATION NOTE
Document Number: 34256
Revision: 14-Aug-08
For technical questions contact: magnetics@vishay.com
www.vishay.com
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