Introduction

CO ₂ To reduce emissions, automotive manufacturers are introducing innovative methods. One promising vehicle type in this field is the micro-hybrid. Micro-hybrids use stop-start systems and regenerative braking to reduce CO ₂ It helps achieve reductions in emissions.

In micro-hybrid applications, 48V The starting motor of 12V This allows the engine to be restarted more quickly than with a conventional starter. Therefore, many micro-hybrid vehicles use 48V Starter / A generator is used in combination with the generator. 48V from 12V for efficient conversion to charge the battery DC/DC A converter is required to convert battery voltage to drive the starter motor. 12V from 48V For conversion to DC/DC You will also need a converter.

High power inductors and current sense resistors are required for the very low power conversion and supply requirements. DC resistance (R _dc) This provides efficient DC/DC This article focuses on low-power MOSFETs designed to handle high current levels in a compact form factor. Bourns® PQ We will introduce a micro-hybrid design using the high-power inductors of the series. Bourns® CSS2H An optimal current measurement solution using a series current sense resistor is also included.

Basics of the Stop-Start System

A regenerative braking system charges the battery only when the vehicle is coasting, braking, or decelerating. During acceleration, the engine is disconnected from the driveline and all engine power drives the wheels, improving fuel economy and acceleration. A conventional engine charges the battery whenever the engine is running.

Stop-Start systems are designed to reduce fuel consumption and associated emissions by reducing engine idling time, which is particularly effective in vehicles operating in urban areas with busy roads and many traffic lights.

Figure 1: Micro Hybrid *Provided by Bourns

High Power Converter Description

​DC/DC converters in these applications are high power, rated at 1 - 3kW, and are best implemented using multiphase converters, due to their high efficiency (> 90%) with low switching losses (< 500 kHz), low input and output ripple currents, reduced hot spots on the PCB and on the individual components, and allowing the use of lower power rated inductors and field effect transistors (FETs).

Figure 2: Six-phase synchronous buck-boost converter with one inductor and current-sense resistor per phase. (Courtesy of Bourns)

Inductors are the energy storage in all switching converters. The stored energy is calculated as:

The variations in inductor current during switcher operation are known as ripple current and their amplitude depends on the inductance value. The ripple current is typically kept below 30% of the output current, which reduces AC and core losses.

Major inductor specifications

Key specifications for Bourns® inductor products are I _rms and I _sat . I _rms is the current at which the inductor temperature rises by 40°C and I _sat is the current at which the inductance decreases by 10 - 30%. Currents in switching converters are rarely stable, their variations are due to transients, spikes or sudden load or supply changes. An important characteristic is that inductance drops as current increases.

​The Bourns® PQ Series helps solve these problems by using ferrite and featuring an air gap that allows the inductance to remain relatively stable as the current increases until the core saturates and the inductance collapses rapidly. Designed with flat windings, the AEC-Q200 compliant PQ Series inductors offer a compact solution with very low DC and AC resistance and high saturation current handling capability of over 100A.

Figure 3: PQ series products offer a high operating temperature of 150°C. *Courtesy of Bourns

Voltage Measurement Detection

One of the key elements in the application design of a micro hybrid DC/DC converter is the measurement of the sense voltage (V _sense). By measuring the voltage across a dedicated current sense resistor, the device continuously senses the input current. Current sensing provides the required accuracy of channel current balancing and per-phase overcurrent protection. Current sense resistors are becoming increasingly popular due to their high measurement accuracy and relatively low cost compared to other technologies.

To calculate the inductor current, we use the following formula:

Typically in this type of application, V _sense is set to 100 mV to conserve power and at the same time maintain a sufficient voltage level above noise. For example, to sense an average output current of 250 A, R _sense should be 100 mV/250 A = 0.4 mΩ.

Because the voltage drop produced by the current is very low, it is important to minimize the error voltage. Sources of error voltage include thermo-electromotive forces (EMF), lead voltage drops and induced voltages due to inductance. Careful selection of resistor leadframe material and strict process controls can minimize error voltages.

The perfect sense voltage solution for micro-hybrid DC/DC applications is the Bourns® CSS2H current sense resistor. To maximize sense performance while saving energy in your design, the CSS series features a low temperature coefficient of resistance (TCR)​ ​(down to 0.0002Ω​) that provides operating accuracy over a wide temperature range and excellent long-term stability. Addressing extended automotive temperature requirements, they offer a wide operating temperature range (-55°C to +170°C), low thermal electromotive force (EMF) and high current handling capability up to 120A.

Advantages of Micro-Hybrid Design

To ensure a more efficient converter design, Bourns® PQ Series inductors are DC Very low losses required to reduce R _dc It also offers the high operating temperatures required for many automotive applications. (150℃) It provides stable inductance over a wide range of currents while meeting the requirements. Its flat winding design provides a compact inductance solution that is especially suitable for automotive power conversion applications such as micro-hybrid vehicles. DC/DC To improve the overall efficiency of the converter, PQ The series inductors feature low power consumption combined with the low core loss characteristics of a ferrite core construction. Additionally, these devices are mounted on a plastic base with additional solder pads for increased mechanical stability.

Bourns® CSS2H current sense resistors represent a new generation of precision devices answering the needs of designers in high power applications. In micro-hybrid vehicle designs, they are a critical component as they monitor the voltage drop across a sense resistor and convert it into a current reading to help the circuit operate efficiently.

Bourns has approximately 2,000 We continue to be a leader in providing enhanced solutions for high frequency, high current automotive start-stop applications through our broad portfolio of inductors, including: 1000V, ...

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