Analysis on the development trend of vehicle-mounted OBC

At present, there are two ways to charge electric vehicles: one is to use it outside the car, the ground charging pile, and the other is to use it inside the car, the on-board charger OBC.

The on-board charger realizes the energy conversion between electric vehicles and the power grid. It can also be divided into unidirectional and bidirectional. The unidirectional charger directly charges the car battery, while the bidirectional charger allows the battery to feed energy back to the power grid.

At present, the on-board charger OBC is developing rapidly, and the KW level has achieved mass production in various forms.

One-way OBC: 3.3KW, 6.6KW, 11KW, 22KW;

Bidirectional OBC: 3.3KW, 6.6KW, 11KW;

In terms of materials, SIC/GaN mos has been widely used;

Charging piles have achieved rapid leapfrog development in terms of high power, high voltage, bidirectional, super fast charging, and wide range.

OEM vehicle manufacturers have also begun large-scale self-research and development, such as BYD and other car manufacturers.

OBC forward charging process:

The grid outputs the electric energy through AC/DC, and then sends it to the battery after DC/DC transformation;

OBC reverse energy feedback process:

The power battery connects the excess energy to the DC/AC link through the DC/DC reverse, releasing the energy back into the power grid;

Among them, the choice of DC/DC topology has a great influence on the transfer and consumption of energy;

Different topologies have different effects on efficiency. Compared with DAB and LLC, LLC can achieve higher efficiency and lower EMI.

The typical bidirectional OBC circuit topology is shown below:

High efficiency dual inductor design to reduce EMI and reverse recovery time

In terms of component selection, fast main switches and fast anti-parallel diodes should be selected for switching.

Conventional IGBTs cannot meet some of the requirements for high efficiency, so it is necessary to select components that can release quickly;

Fast IGBT can respond quickly at the front end, and automotive-grade devices are recommended first, which can meet the needs of circuit delay, signal noise reduction and control algorithm. Infineon's IKW4N65F5A has a withstand voltage of 650V and a loss of 0.1mJ. This type of component can basically meet conventional requirements.

LLC DC/DC network gain calculation:

After calculating the parameters of the network gain, the selection of the internal capacitance of the circuit can be designed to ensure the internal absorption loop;

In order to achieve adjustable voltage and frequency, it is necessary to select appropriate capacitors.

In the process of bidirectional LLC design, technical difficulties need to be considered, such as excessive current during the opening or closing of the resonant circuit, pulse design, etc.

The efficiency of the circuit needs to be considered, both in the forward and reverse directions, which requires precise control and the ability to switch quickly between the forward and reverse directions.

Adopting design solutions

Why choose SIC devices?

It can improve the performance of the circuit, enhance the working efficiency of the LLC circuit, enhance the reliability, reduce the turn-on and turn-off time, and enhance the circuit's anti-interference performance;

When using the household registration, OBC has been applied and can be carried directly, as shown below

Among them, the reference scheme adopted

Current detection can also use programmable linear Hall plus magnetic ring, such as CHA611 linear Hall sensor

Domestic automotive-grade programmable linear Hall sensor CHA611 can replace Allegro's A1363 series products and solve the shortage of automotive-grade chips

You can also use the integrated Hall current sensor CH704

The chip integrates a precise programmable linear Hall chip, a small magnetic ring and a copper bus with an on-resistance of 0.1mΩ, which can realize current detection of +/-50A, +/-100A, +/-150A, +/-200A.

Application Cases of Domestic Automotive-Grade Hall Current Sensors