Identifying the key components of automotive DC-DC converters

In the vast world of automotive electronics, the powertrain plays a crucial role, especially in cars equipped with a start-stop system. This system is increasingly embedded in cars and allows the engine to be automatically shut down and restarted in all situations such as traffic stops, traffic lights and waiting for travel. During the motor restart phase, high current peaks occur, causing the 12 V supply voltage to drop to half.

The consequence of this is that on-board electronic devices such as car radios, navigation devices, cooling systems, ventilation systems, etc. may experience serious malfunctions or even damage electronic components. The control unit (ECU) of the high-power DC-DC converter is designed to overcome this problem and stabilize the 12 V supply during the motor restart phase.

DC-DC converters play a major role and are also used in different types of applications such as Hybrid Electric Vehicle (HEV). The optimization and selection of DC/DC converters plays a vital role in vehicle power management, which is driven by a set of efficiency requirements that must be met.

Efficiency of DC-DC Converters

When evaluating the efficiency of a DC-DC converter, system losses that significantly reduce the efficiency play an important role. There are two types involved in the system loss analysis: the losses caused by the peak current in the inductor and the so-called switching losses caused by the charging and discharging phases of the converter circuit. Regarding the losses caused by the peak current in the inductor, two leakage components can be determined, one related to the drain-source resistance of the ON-OFF switching FET during conduction and the other to the DC resistance of the inductor. In addition, during the discharge phase of the inductor, power losses proportional to the discharge current occur. The switching losses or dynamic characteristics are mainly caused by the capacitive effects of the circuit. In particular, the drain-source parasitic switching capacity of the FET and the diode must be considered. Another type of loss is the energy loss caused by the inductor core losses, which is proportional to the switching frequency; in fact, an increase in frequency is accompanied by an increase in the inductor core losses. This type of leakage is due to the material and size of the core.

Optimization to reduce energy losses can be achieved by reducing parasitic elements through careful component selection and careful printed circuit board design. One way to improve efficiency is to reduce the current flowing into the inductor by reducing the resistive losses in the active components and inductors.

Many ADAS systems use 5V and 3.3V lines to power the many analog and digital components within them; likewise, manufacturers of these systems prefer to use a single converter in both single- and dual-battery configurations. In today’s ADAS systems, switching regulators must also be able to switch to frequencies of 2MHz or higher, rather than relying on a standardized switching frequency of less than 500kHz. The main reason for this change is the need for a smaller solution while staying above the AM band frequency to avoid potential interference.

Integrated Circuit Solutions

The LT8645S is a synchronous monolithic step-down converter for high input voltages with low EMI emissions. Its 3.4V to 65V input voltage range makes it ideal for automotive applications, including ADAS systems that must maintain regulation during cold-crank and stop-start conditions, with input voltages as low as 3.4V and load dump transients exceeding 60V (Figure 1).

Figure 1: Typical application circuit for the LT8645S [Source: Analog Devices]

ROHM offers the BD9S series (BD9S400MUF-C, BD9S300MUF-C, BD9S200MUF-C, BD9S100NUX-C, BD9S000NUX-C). It is a series of secondary-side synchronous DC/DC step-down converters for the automotive field that feature excellent reliability and low power consumption in a compact form factor.

The BD9S series consists of a very compact power supply circuit that includes an enable function to adjust the startup time, and a PGOOD output indicator to optimize system functional safety. This broad product line supports output currents from 0.6 to 4.0 A (Figure 2).

Figure 2: ROHM Semiconductor BD9S series typical application circuit [Source: ROHM Semiconductor]

Cypress's S6BP20x series (S6BP201A, S6BP202A, and S6BP203A) are single-channel buck-boost DC/DC converters for automotive and industrial applications. They are ideal for automotive applications such as body control modules, instrument clusters, and ADAS. These PMICs feature low standby current and provide stable power delivery over a wide input voltage range.

Cypress's unique buck-boost technology enables you to design smaller PMIC systems by eliminating bulky and expensive electrolytic capacitors used to filter the input. This smaller design allows you to create a small, cost-effective, and energy-efficient solution (Figure 3).

Figure 3: Block diagram of the S6BP201A

Bidirectional DC-DC

The automotive industry, especially after the recent innovation push of hybrid and electric vehicles, requires the use of power systems consisting of multiple batteries in addition to the traditional 12 V primary battery. In addition, the use of power regeneration systems requires bidirectional power transmission (from battery to user and vice versa).

The introduction of fuel consumption reduction devices such as the start and stop system requires the use of two battery power supply systems. The traditional and familiar 12 V lead-acid battery is flanked by 48 V lithium batteries to start the engine. Therefore, power needs to be transferred bidirectionally between the two batteries according to the specific needs of the entire system.

A single Renesas ISL78226 device can deliver up to 3.75 kW of maximum power with over 95% conversion efficiency and can also be integrated into a modular master/slave architecture to deliver higher power. This innovative design enables designers to support the rapid adoption of 48 V hybrid powertrains that reduce emissions and fuel consumption used in mild hybrid vehicles, where an electric motor is connected to an internal combustion engine (Figure 4).

Figure 4: Typical application diagram of ISL78226 [Source: Renesas Electronics]

in conclusion

The increasing popularity of hybrid and electric vehicles requires the use of dual battery power supply systems. Moreover, even in conventional vehicles equipped with internal combustion engines, the demand for electric power has been increasing in recent years, which is necessary to power the numerous electronic devices and accessories installed in the vehicle.