Introduction

Although automotive manufacturers are working to provide users with electric vehicles such as EVs and hybrid vehicles, market growth has not been as rapid as expected. Meanwhile, it has become clear that some of the technologies used in these electric vehicles can also be applied to conventional vehicles (combustion engine vehicles). One new technology that has attracted attention in recent years is the "Start-Stop" function. This technology aims to improve the fuel efficiency of engine vehicles, but introducing this function affects the entire automotive electronics system and presents several development challenges.

In particular, voltage conversion using a DC-DC converter is complicated by the fact that the voltage is not constant during idle stop. While surge resistance and EMI (electromagnetic interference) requirements vary depending on the vehicle model, meeting these requirements is essential. The quality, structure, and characteristics at the electronic component level are key to the success of such a design. Advances in high-current inductor technology, in particular, are attracting attention as a solution for realizing compact, high-performance components in power conversion and power supply circuits.

This article explains the basic mechanisms of idle reduction systems in internal combustion engine vehicles and the evaluation tests for related in-vehicle electronic devices. It also discusses various design considerations for such systems and the evolution of inductor technology. In particular, it explores the advantages of utilizing high-conductivity inductors that use rectangular wire rather than round wire, and introduces specific solutions.

Bourns offers advanced solutions for the high-frequency, high-current automotive applications of stop-start systems through a portfolio of approximately 2,000 inductors featuring rectangular conductors, molded powdered iron cores, and a wide range of inductance values.

Differences between hybrid/combustion vehicles and idling stop compatible designs

Automotive designers are using various methods to improve fuel efficiency in electrified vehicles. For example, hybrid vehicles are controlled to switch between a state in which they consume fuel to charge the battery and a state in which they run on the battery to reduce fuel consumption, depending on specific conditions. On the other hand, conventional engine vehicles consume fuel as long as the engine is running.

In contrast, gasoline-powered vehicles with an idle-stop system automatically shut off the engine while the vehicle is stationary and restart it only when necessary to reduce fuel consumption. This system requires the engine to be shut down every time the vehicle comes to a stop and then restarted when the vehicle starts moving again.

Basics of idling stop mechanisms in engine vehicles

Automotive electronic devices are designed to operate using a nominal 12V to 14V battery as a power source. However, in internal combustion engine vehicles, the battery voltage can temporarily drop to around 7V when the engine starts. Even a momentary voltage drop like this can cause electronic circuits that rely on a stable 12V to 14V supply to malfunction, so an auxiliary battery is required in the design.

Figure 1 shows a high-level circuit diagram of a vehicle with a stop-start system. While the engine is running, the main battery powers the electronics, and the auxiliary battery is isolated from the rest of the vehicle by a switch. When the vehicle is stopped or when the engine is restarted, the switch is opened and the auxiliary battery powers the electronics.

Figure 1: Example of an idle-stop circuit configuration that stabilizes power supply to electronic devices by switching to an auxiliary battery when the engine is stopped. (Provided by Bourns)

DC-DC conversion is performed between the battery and the onboard electronics. If the backup battery voltage is lower than the main battery, a boost DC-DC converter is required between the backup battery and the onboard conversion circuit. A typical DC-DC converter design uses an output capacitor and a switching inductor. As shown in Figure 2, it is configured with a power supply, a voltage control switch, and a load.

When the switch is on, the inductor converts current into magnetic flux, storing it and charging it. When the switch is off, the magnetic flux collapses and is converted back into current to power the load. Conductors are always in use, and efficient conductor design is essential for high power and high frequency applications.

Figure 2: Components of a DC-DC converter - power inductor, control chip/switch, diode, bulk capacitor (Courtesy of Bourns)

Meets surge and EMI requirements

Another key parameter that makes start-stop design challenging is the surge voltage that all on-board electronics must withstand. Well-defined automotive tests are performed to ensure engine operation can be maintained during surge events. All on-board circuits (including power circuits) must be pulse tested for transients as part of the design process, according to ISO7637 and ISO16750 standards. Defined test pulses are applied at regular intervals, and the electronics must be able to operate within those parameters.

Additionally, ISO 7637 specifies bench tests to assess electrical transient compliance for equipment installed in passenger cars and light commercial vehicles with 12V power systems or commercial vehicles with 24V power systems. The characteristics of the test pulses are shown in Figure 3 and listed in Table 1.

Figure 3: Automotive Test Pulse 1 — Source: ISO 7637 Manual. Image courtesy of Bourns.

* a. t1 must be set appropriately so that the DUT (device under test) is properly initialized before the next pulse is applied.
*b. t3 is the shortest time required between power-off and pulse application.

Automotive manufacturers have their own specifications for EMI (electromagnetic interference) emissions. The most common international standard is EN55022 Class B, which requires a minimum attenuation of 37dBμV/m for frequencies above 230MHz. Controller boards for electric motors that drive various functions within a vehicle are a prime example of where EMI emissions requirements place a significant burden on the design. Motor controllers use PWM (pulse-width modulation) drive, and the switching action of their power transistors makes them susceptible to high-frequency noise.

To attenuate this high-frequency noise, power inductors are used as part of a filter. Figure 4 shows a pair of inductors used as filters between the battery and the inverter, which is connected to a three-phase AC motor. Electric motors draw large currents, which requires physically large inductors.

Figure 4: Inductor used as a filter in an electric motor controller (Courtesy of Bourns)

Important Design Considerations

When selecting the type of inductor to use in an automotive design, several important factors must be considered, including core type, frequency rating, current, saturation characteristics, temperature, EMI (electromagnetic interference), and conductor type. While ferrite core-based inductors may be sufficient for some designs, today's high current, high frequency automotive designs favor the enhanced saturation characteristics of Bourns® SRP series flat wire power inductors. Bourns provides detailed test results and datasheets to help you select the best inductor component for your application.

One important consideration for power supply designers is to avoid selecting an inductor that will saturate for the application's specifications. Motor drives can have high DC currents, requiring large inductance values to effectively filter radiated noise. Similarly, DC-DC converters, such as those used in start-stop systems, can handle very high DC currents, requiring physically large components to avoid overheating and saturation.

To determine if a ferrite inductor meets application specifications and to test for core saturation, designers can use the high transient pulse test specified in ISO16750-2:2012, Load Dump Tests A and B. This test uses very high pulses of up to 174V to simulate a discharged battery being disconnected while the alternator is still producing charging current (load dump).

All automotive powertrain electronics are required to operate at high temperatures typical of automotive environments, typically reaching up to 150°C. The operating temperature of each component must be verified to meet the vehicle manufacturer's specifications. While ferrite cores require derating of their saturation flux density according to temperature, iron powder cores have an unaffected saturation point. This means that iron powder inductors can operate at higher currents than ferrite inductors in high-temperature environments.

Low EMI (electromagnetic interference) and low ripple voltage are important considerations in DC-DC converters. The magnitude of the ripple voltage is determined by the ripple current in the inductor and the equivalent series resistance (ESR) of the capacitor. The ripple voltage ΔVout can be calculated using the following formula:

where Iout is the output current, ΔIL is the inductor current ripple, D is the duty cycle, and ESR is the equivalent series resistance of the output capacitor. These calculations are necessary to ensure that the selected components meet the required specifications.

The Bourns® SRP Series offers features that make it 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 and powdered iron core reduces wasted space within the package and improves conductor conductivity during high frequency operation.

The Importance of Selecting the Right Inductor

By applying technologies such as hybrid vehicles, start-stop powertrain configurations in conventional (combustion) vehicles can significantly improve fuel economy. However, design presents several challenges, as the entire vehicle's electronics must be considered. Successful design requires careful consideration of the components used in the DC-DC converter. It is crucial for designers to select robust, high-quality components that can handle the high current, high frequency, and high temperature environments of the entire start-stop system.

Additionally, start-stop designs must withstand current surges to meet various OEM (original equipment manufacturer) requirements. Bourns® SRP Series Inductors provide an ideal solution for start-stop powertrain applications. Their flat wire construction, powdered iron core, and high frequency capability provide high DC current bias and temperature stability. This allows them to operate without saturation even at high temperatures and currents, meeting the stringent requirements of automotive designs.

Recommended related articles

• AEC-Q200 Automotive Standard Compliant Power Inductor
• SMD High Current, Shielded Power Inductor

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