500V 750V 850V 22kw On Board Charger Electric Vehicle obc Charger

When you charge your electric vehicle (EV) using an AC charger, the real work is done by a device inside your car called the on-board charger (OBC). The OBC is a power electronics module that converts alternating current (AC) from the grid into direct current (DC) to safely charge the vehicle's battery. It’s important to note that the AC charging station itself—often called EVSE (Electric Vehicle Supply Equipment)—does not perform conversion. It mainly provides a safe connection to the grid. In contrast, a DC fast charger converts AC to DC externally and delivers high-power DC directly to the battery.

Why Does the OBC Matter?

The OBC determines your EV’s AC charging speed. Think of it like a funnel controlling how quickly energy flows into the battery—the larger the funnel, the faster the charging. Most passenger EVs are equipped with OBCs rated at 7 kW or 11 kW, defining the maximum power they can accept from AC chargers.

How Does an OBC Work?

The conversion process inside the OBC involves multiple stages:

• ● EMI Filtering: The incoming AC (e.g., 220V) is filtered to reduce electrical noise.

• ● Rectification: An AC-DC converter rectifies the filtered AC into DC.

• ● Power Factor Correction (PFC): This circuit improves efficiency and ensures compliance with grid standards.

• ● DC-DC Conversion: Through a switching transformer (such as an LLC resonant converter), the voltage is adjusted to the level required by the battery.

• ● Output Filtering: Final filtering ensures clean, stable DC power to charge the battery.

Why Is OBC Efficiency Lower Than DC Charging?

Several factors contribute to the typically lower efficiency of OBCs compared to DC fast chargers:

• ● Size and Power Constraints:

OBCs are installed inside the vehicle where space, weight, and cooling are limited. Most are designed for 7‒22 kW, whereas DC chargers are external systems that can deliver much higher power with larger components.

• ● Heat Dissipation Challenges:

OBCs are housed within the EV with restricted airflow, leading to higher operating temperatures. This affects the performance of power semiconductors (e.g., MOSFETs, IGBTs), reducing efficiency. DC chargers often include advanced cooling systems (liquid or forced air) for better thermal management.

• ● Multi-Stage Conversion Losses:

Each conversion step in the OBC—AC/DC, PFC, and DC/DC—incurs energy losses. Typical OBC efficiency ranges between 90%–93%. Meanwhile, DC charging systems operate at 95%+ efficiency due to optimized dedicated hardware.

A simple analogy:

• ● DC Charging ⇢ Like a full-sized kitchen, preparing meals quickly with professional equipment.

• ● OBC Charging ⇢ Like a mini rice cooker in your car—functional, but limited by size and power.

Now you know why most EVs come with 7 kW or 11 kW OBCs—and what that means when you’re choosing an AC charger for home or public use.