Selecting Inductors for DC-DC Converters andFilters in Automotive Applications

APPLICATION NOTEINTRODUCTION

SRP Series
Shielded Power Inductors

While automotive manufacturers are doing their part to offer alternative powered vehicles toconsumers, the market for them has not grown as rapidly as hoped. It has been discoveredthat some of the advancements employed in hybrid and electric vehicles can be adopted bytheir Internal Combustion Engine (ICE) counterparts. A newer technology that is gaining
momentum is “start-stop” engine technology to improve fuel economy in ICE vehicles.
However, implementing a start-stop design can affect electronics in the entire automotivesystem and introduces several development challenges. Direct Current (DC)-DC conversion
becomes more complicated since the voltage level in start-stop is not inherently consistent.
Though the surge requirements and Electromagnetic Interference (EMI) emissions
requirements may vary based on vehicle model, it is still a mandatory requirement to meetthem. Successful implementation will depend on the quality, construction and characteristics
at the electronic component level. One key solution lies in the advances achieved in highcurrent inductor technologies which provide compact components for power conversion andsupply circuits.
This paper presents the basics of the start-stop powertrain in ICE vehicles along withassociated automotive electronics tests for this type of design. Also presented will be a
discussion of the many design considerations and advancements in inductor technology.
Specifically, one solution will be provided to show the advantages of using a highly conductive
inductor which employs a flat, rather than, a round shape. Bourns offers enhanced solutions
for high frequency, high current start-stop automotive applications through its extensive
portfolio of approximately 2,000 inductors, featuring flat wire conductors, powdered iron cores
molded directly to the wire and a wide range of inductance values.

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Selecting Inductors for DC-DC Converters and Filters in Automotive Applications
DIFFERENTIATING BETWEEN HYBRID, ICE AND START-STOP DESIGNSSRP Series
Shielded Power Inductors

Automotive designers use various methods to conserve fuel in alternatively powered vehicles.
A hybrid vehicle, for example, utilizes specific triggers to transition between charging thebattery while consuming fuel and operating under battery power to conserve fuel. Contrarily, intraditional ICE vehicles, fuel is always being consumed when the engine is on. A start-stop ICE
design differentiates a vehicle’s moving and stopped states to cause the engine to run only whennecessary. Each time the automobile is stopped, the start-stop engine goes into shutdown mode.
When the car prepares to move, the engine must start again.

THE BASICS OF ICE START-STOP POWERTRAINS
Automotive electronics are typically designed to operate from a 12 V battery with a 14 Vnominal voltage. The battery voltage in an ICE vehicle drops to as low as 7 V any time theengine is switched to the on-state. Even this short drop in the voltage level can cause otherelectronics that expect a steady 12-14 V to malfunction, so reserve batteries need to be added
to the design. A schematic that shows a high-level start-stop example circuit is shown in figure
1. Any time the engine is running, the main battery powers electronics, and the reserve battery
is isolated from the rest of the vehicle by a switch. When in the stop-state, and during thetransition to the start-state, the switch is open so the reserve battery can supply the needed
consistent level of power to the electronic circuits, such as those used for infotainment features.

SwitchMain
Battery
12 V

Figure 1.

9/13 • e/IC1338+–Reserve
Battery+–

Infotainment Circuits,Clusters, etc.

This schematic illustrates the start-stop circuit that switches to a reserve battery when an
ICE turns off, in order to keep the power level consistent to run electronics.

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Selecting Inductors for DC-DC Converters and Filters in Automotive Applications
THE BASICS OF ICE START-STOP POWERTRAINS (Continued)

SRP Series
Shielded Power Inductors

DC-DC conversion occurs between the battery and the onboard electronics. If the reserve
battery voltage is lower than the main battery, then a boost DC-DC converter is necessary
between the reserve battery and onboard conversion circuits. A typical DC-DC converter
design employs an output capacitor and a switching inductor as shown with the power source,
voltage-controlled switch and load in figure 2. If the switch is on, the inductor charges while thecurrent is turned into magnetic flux and stored in the inductor. When the switch is turned off,
magnetic flux collapses and is transformed back to electrical current to provide power to theload. The conductor wire is in use at all times, and it is essential to have an efficient conductor
wire design in this high power, high frequency application.

t2

U
t3

t0.1 US

USUA
00.9 UStrtd
t1
Figure 2.

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The components in the DC-DC converter are the power inductor, thecontroller chip/switch, a diode and a bulk capacitor.

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Selecting Inductors for DC-DC Converters and Filters in Automotive Applications
CONTENDING WITH SURGE AND EMI REQUIREMENTS

SRP Series
Shielded Power Inductors

Another crucial parameter that makes a start-stop design challenging is the surge voltage that all
automotive electronics must withstand. Specific and well-defined automotive tests are conductedto ensure engine operation is maintained during surge events. All automotive circuits, including
power supplies, must be pulse-tested with the transients described as part of the design process per
the ISO 7637 and ISO 16750 standards. The specified test pulse is applied at regular intervals, andthe electronics must be capable of operation within its parameters. In addition, ISO 7637 specifies
bench tests for electrical transient compatibility of equipment installed on passenger cars and lightcommercial vehicles fitted with a 12 V electrical system or on commercial vehicles fitted with a 24 V
electrical system. The characteristics of the test pulse are shown in figure 3 and described in table 1.

D1

Automotive Inductor
L1

D

SwitchVout+ C1Low ESRCapacitor

VinFigure 3.

Automotive Test Pulse 1 – Source: ISO 7637 Manual

Table 1.

Parameters for Test Pulse 1 – Source: ISO 7637 ManualParameter

12 V System

24 V System

Us-75 V to -100 V-450 V to -600 VRi

10 Ω50 Ωtd2 ms

1 mstr

1 µs

3 µs

t1

a0.5 s to 5 s

t2200 mst3b<100 µs

a. t1 shall be chosen so that the DUT is correctly initialized before the application of the next pulse.
b. t3 is the shortest possible time necessary between the disconnection of the supply source and the application of the pulse.

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Selecting Inductors for DC-DC Converters and Filters in Automotive Applications
CONTENDING WITH SURGE AND EMI REQUIREMENTS (Continued)

SRP Series
Shielded Power Inductors

Car manufacturers have separate specifications for EMI emissions. The most common international
standard is EN55022 Class B which requires attenuation of frequencies above 230 MHz to be aminimum of 37 dBuV/m. Controller boards used in electric motors to power various features
throughout the vehicle provide an example of the design overhead introduced by EMI emission
requirements. Motor controllers have a Pulse-Width Modulated (PWM) drive, and the switchingaction of its power transistors typically leads to high noise generation. Power inductors are usedas part of a filter to attenuate this high frequency noise. Figure 4 shows a pair of inductors usedas filters between the battery and the inverter connected to the 3-phase AC motor. Since electricmotors can introduce heavy Amp loads, large inductors are needed to carry the current physically.

Automotive Inductor

Car Battery+–Figure 4.

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3-PhaseAC Motor

An inductor is used as a filter in an electric motor controller

5Selecting Inductors for DC-DC Converters and Filters in Automotive Applications
IMPORTANT DESIGN CONSIDERATIONSWhen it comes to selecting the type of inductor to use in an automotive design, there are several
major factors to consider such as core type, frequency rating, current, saturation, temperature, EMIand wire type. While a ferrite core-based inductor may suffice for some designs, today’s high current,high frequency automotive designs would be well-served with the Bourns® Model SRP Series flat wire
power inductor’s higher saturation capabilities. Bourns offers detailed test results and data sheets tohelp designers determine the right inductive component for a given application.
SRP Series
Shielded Power Inductors

A crucial consideration for power supply designers is to avoid selecting an inductor that would
saturate the given specifications of an application. Motor drives can have high DC current values andwould require large inductance values to effectively filter emissions. Similarly, DC-DC converters,
as in start-stop systems, may also have very high DC currents that generally require physicallylarge components to carry the current, without overheating or saturating. To determine if a ferrite
inductor will meet an application’s specifications and to test for core saturation, designers can use ahigh transient pulse test specified by ISO 16750-2:2012 load dump A and B. This test is designed tosimulate disconnecting a discharged battery while the alternator is generating charging current (alsoknown as load dump), using a very high pulse of up to 174 V.
All automotive powertrain electronics are required to perform in the high temperatures associatedwith the automotive environment, typically as high as 150 ˚C. The operating temperature of eachcomponent must be examined to ensure that it is rated to the requirements of the car manufacturer.
Ferrite cores have to derate their saturating flux density based on temperature, while an iron powder
core’s saturation point is not affected by temperature. This means that at high temperatures, ironpowder inductors can operate at even higher currents than ferrite inductors.
In DC-DC converters, low EMI and low-ripple voltages are important considerations. The overall
ripple voltage is a function of the ripple current in the inductor and the Equivalent Series Resistance(ESR) of the capacitor. Ripple voltage, ΔVout, can be calculated by the following equation:+ ΔIL )ΔVout = ESR ( 1Iout-D2
Where Iout is the output current, ΔIL is the ripple in the inductor current, D is the duty cycle, and ESR
is the output capacitor’s equivalent series resistance. These calculations must be done to ensure theselected components meet the requirements.
The Bourns® Model SRP Series offers features that are especially advantageous for start-stop engine
applications:• Flat wire technology allows for a compact footprint• Capable of handling high frequency switching• The combination of flat wire technology with a powdered iron core reduces wasted space in thepackage and increases the conductivity of the conductor when operating at high frequencies

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Selecting Inductors for DC-DC Converters and Filters in Automotive Applications
IMPORTANCE OF SELECTING THE RIGHT INDUCTORSRP Series
Shielded Power Inductors

Borrowing from alternative fuel vehicles, a start-stop powertrain architecture in ICE vehicles
offers a marked improvement in fuel efficiency, but presents several challenges since all of
the vehicle’s electronics must be considered. A successful design requires attention to thecomponents used for the DC-DC converter design. It is vital that designers specify robust,high-quality components that can handle high amounts of current and frequency at extended
operating temperatures, throughout the automotive start-stop system. Furthermore, a startstop design must be able to withstand surges of current in order to pass various OriginalEquipment Manufacturer (OEM) requirements. Bourns® Model SRP Series Inductors offer an
optimal application solution for start-stop powertrain applications. Their flat wire, iron powder
core construction and high frequency capability provides the needed high DC current bias
and temperature stability. This allows them to operate at high temperature and high currents
without saturating, thus meeting many of the stringent requirements of automotive design.

ADDITIONAL RESOURCES
For more information and further technical support, visit Bourns online at:www.bourns.com
COPYRIGHT© 2013 • BOURNS, INC. • 9/13 • e/IC1338“Bourns” is a registered trademark of Bourns, Inc. in the U.S. and other countries.

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