Inside an EV On-Board Charger: From AC Outlet to Battery

An OBC is sized by what the AC supply can deliver, not by the battery. The common ratings follow directly from voltage times current:

Single phase, 230 V, 16 A

≈ 3.7 kW

Single phase, 230 V, 32 A

≈ 7.4 kW

Three phase, 230 V per phase, 16 A

3 · 230 · 16 ≈ 11 kW

Three phase, 230 V per phase, 32 A

3 · 230 · 32 ≈ 22 kW

Split phase (North America), 240 V, 32 / 48 A

≈ 7.7 / 11.5 kW

The charging station tells the car how much current it may draw. In the IEC 61851 and SAE J1772 schemes this is done with a 1 kHz PWM signal on the control pilot line; between 10 % and 85 % duty cycle the allowed current is the duty cycle in percent times 0.6 A, so 53 % means 32 A. The OBC firmware must respect that limit.

The first stage rectifies the AC input and shapes the input current into a sine wave in phase with the voltage, which is required by grid harmonic standards and makes full use of the outlet's current rating. Classic designs use a diode bridge followed by a boost converter. Many newer OBCs use a bridgeless totem-pole PFC with SiC or GaN switches, which removes the diode bridge losses. Three-phase OBCs use a three-phase PFC or three single-phase modules, one per phase.

The PFC output is a regulated DC link, typically a few hundred volts for chargers serving 400 V batteries and higher for 800 V batteries. As discussed below, some designs let this voltage move with the battery voltage.

In a single-phase system the input power is not constant: it pulses at twice the line frequency between zero and twice the average. The DC-link capacitor absorbs the difference, so its voltage ripples at 100 Hz (or 120 Hz). The required capacitance follows from an energy balance:

Single-phase DC-link ripple

ΔV_pp ≈ P / (2π · f_line · C · V_dc)

For 7.4 kW at 50 Hz with a 400 V link and 20 V peak-to-peak ripple, C ≈ 7 400 / (2π · 50 · 400 · 20) ≈ 2.9 mF. That is a large, heavy capacitor bank, which is why OBC designers accept more ripple and let the following DC-DC stage reject it, or add active ripple-cancelling circuits. A balanced three-phase input has constant instantaneous power, so this ripple largely disappears in three-phase operation.

This capacitor also needs precharge when the charger connects, for the same reasons as any DC link; the precharge guide on this site covers the sizing.

The second stage provides galvanic isolation between the grid and the battery and controls the charging current. Two topologies dominate:

• LLC resonant converters, valued for zero-voltage switching and high efficiency near resonance. They are naturally unidirectional; the bidirectional variant with a resonant tank on both sides is often called CLLC.
• Dual active bridge converters, which are inherently bidirectional and handle a wide voltage range with phase-shift control, at the cost of more circulating current when the voltage ratio is far from one.

Bidirectional operation matters because vehicle-to-load and vehicle-to-grid features reuse the same hardware to send power back out of the battery.

A nominal 400 V battery may sit somewhere around 250 to 450 V depending on its chemistry, cell count and state of charge, and an 800 V pack spans a similarly wide range in proportion. The exact window depends on the pack design. For an LLC with a fixed DC link, a ratio of almost two to one between the lowest and highest output voltage means a wide gain range, pushing the design toward low m values, high magnetizing current and operation far from resonance at the extremes.

• Variable DC link: the PFC stage adjusts its output voltage so the DC-DC stage always runs close to its ideal gain. The LLC can then stay near resonance and the PFC absorbs the range.
• Topology changes: some designs reconfigure the secondary rectifier, for example between full-bridge and voltage-doubler modes, to cover different battery voltages.
• Phase-shift control: a DAB or CLLC can cover the range by adjusting phase shift, accepting higher circulating current at the extremes.

Lithium-ion batteries are usually charged with constant current until the cell voltage reaches its limit, then with constant voltage while the current tapers. The battery management system decides the current and voltage limits; the OBC follows them while also respecting the grid current limit from the charging station and its own temperature limits. In a typical DC-DC stage that means an outer voltage loop, an inner current loop, and a limiter that picks the lowest of all requested currents.

The same loop-design issues covered in the LLC voltage loop guide apply here: the plant gain changes strongly across the battery voltage range, so a single fixed set of controller gains is rarely optimal.

Each block of an OBC can be explored with the calculators on this site: the LLC resonant tank design tool for the isolated stage, the DAB calculator for a bidirectional alternative, and the inrush resistor calculator for DC-link precharge. Numbers in this guide are illustrative; actual products vary widely in their DC-link voltage, battery window and topology.