EV charger power stage

An EV charger — whether a 7 kW residential wallbox or a 350 kW DC fast charger — is fundamentally three things stacked on top of each other: a power-electronic conversion stage that turns grid AC into a controlled DC current at the battery's voltage, a digital control system that regulates the conversion (running on a Cortex-M4 or M7), and a communication system that negotiates with the vehicle to agree on safe charging parameters. Get any one wrong and the system either fails to deliver power, delivers it inefficiently, or — in the worst case — damages the battery or starts a fire.

This page covers the power-electronics architecture (PFC, LLC, isolation), the control-loop algorithms running on the embedded MCU, and the communication stack (Control Pilot signalling for AC charging, ISO 15118 PLC for DC fast charging).

AC charger architecture

An AC charger ("Level 2" in US terminology, "Mode 3" in IEC) does not actually convert AC to DC — the vehicle's onboard charger (OBC) does that. The wallbox is a smart contactor: it negotiates a current limit with the vehicle, switches on the AC supply, monitors the connection for safety, and switches off if anything goes wrong. The "charging" happens inside the car.

The wallbox's tasks:

1. Authenticate / authorise the user (RFID, app, plug-and-charge).
2. Verify connection — the vehicle plug is fully seated; the Proximity Pilot pin tells the cable's current rating.
3. Negotiate current — drive the Control Pilot signal at the right duty cycle.
4. Close the main contactor — energise the AC pins.
5. Monitor — current per phase, leakage current via RCD/GFCI, plug temperature, vehicle state changes.
6. Disengage safely — open contactor and signal vehicle if a fault occurs.

The hardware is a Cortex-M0+ or M4 (no DSP performance needed), a high-voltage AC contactor relay, current transformers per phase, a Type-A or Type-B RCD for leakage detection, the CP/PP signal conditioning, and a communication interface (Ethernet, Wi-Fi, cellular for backhaul to the operator's network).

Control Pilot signalling

The CP pin carries a 1 kHz square wave, 12 V peak-to-peak (+12 V to -12 V), with the duty cycle encoding the current the wallbox can supply:

For example, 16% duty = 9.6 A available; 50% = 30 A available; the formula is current_A = duty% × 0.6 valid in the 10-85% range. The vehicle measures the duty cycle and limits its onboard charger to that current.

The CP voltage also encodes vehicle state via the amplitude on the negative half-cycle:

The wallbox MCU samples both the positive and negative CP excursions every cycle, detects state transitions, and reacts within ~100 ms.

DC fast charger architecture

A DC fast charger does the AC-to-DC conversion inside the charger and delivers regulated DC directly to the battery — bypassing the vehicle's onboard charger. This lets you push 50–350+ kW through the cable, far more than any onboard charger.

The power-electronic chain:

Grid 3-phase 400 V AC
  → EMI filter
  → 3-phase PFC (Vienna or active rectifier) → 800 V DC bus, PF ≈ 1
  → DC-DC isolation stage (LLC or PSFB) → 200-1000 V DC output
  → Output filter + contactors
  → CCS / NACS connector → vehicle battery

Three-phase PFC

For powers above ~7 kW, single-phase boost PFC isn't viable — the grid would see large 100 Hz current pulsation. Three-phase PFC is mandatory.

The Vienna rectifier is a popular topology: three switches plus six diodes, three-level output, achieves PF > 0.99 and THD < 5%. Each switch operates at 40-100 kHz; the switching is staggered between the three phases to reduce DC-bus ripple. The control loop is:

• Inner: per-phase current loop running at the switching frequency. Inputs: phase currents (Hall or shunt), reference from outer loop. Output: switch duty cycle. Bandwidth ~5 kHz.
• Outer: DC-bus voltage loop, ~100 Hz bandwidth, slow enough not to interact with grid 50/60 Hz harmonics. Outputs current reference scaled by sinusoidal phase reference (computed from phase-locked loop on grid voltage).

The PLL on grid voltage is a critical sub-block. It must lock to the fundamental, reject harmonics, and respond to phase jumps (e.g. from grid faults) within a half-cycle. The standard implementation is an SRF-PLL (Synchronous Reference Frame PLL) with a quadrature signal generator.

Isolated DC-DC

Galvanic isolation is mandatory between mains and the vehicle — both for safety (no DC fault current path between vehicle chassis and grid earth) and for regulatory compliance (IEC 61851-23, UL 2202). The isolated DC-DC stage uses a high-frequency transformer for the isolation barrier.

Two dominant topologies:

LLC resonant converter. A half-bridge or full-bridge primary drives a resonant tank (L_r, L_m, C_r) feeding the transformer. The resonance lets the switches turn on at zero voltage (ZVS) and zero current (ZCS), eliminating switching losses and reaching ~95-97% peak efficiency. The control variable is switching frequency — operating above resonance gives buck-mode (vehicle voltage < bus / N), below resonance gives boost-mode. Output regulation is via PFM (Pulse Frequency Modulation).

LLC is the dominant choice for new fast-charger designs because of the efficiency at partial load. Limitation: it has limited output-voltage range without burst-mode operation, and at very light load the resonance becomes hard to control.

Phase-Shifted Full Bridge (PSFB). Four switches in a full bridge primary; the regulation is by the phase shift between the two half-bridges. Wider output voltage range, slightly lower peak efficiency than LLC. More tolerant of wide-input wide-output operation, used in some fast chargers that need to support both 400 V and 800 V battery packs.

Multi-stage modular architecture

Real 350 kW chargers don't have one 350 kW LLC stage. They have N × 50 kW modules in parallel, each module being an LLC connected to a shared DC bus and a shared output. A module-level MCU runs the LLC control loop; a charger-level MCU coordinates load-sharing between modules, manages thermal, runs the user interface, and talks to the vehicle.

Module modularity buys:

• Efficiency at partial power. At 50 kW out of 350 kW, only one module is active running near peak efficiency. Without modularity, the single big converter would run at 14% load — very inefficient.
• Redundancy. One module failure drops capacity to 6/7 of full, no service outage.
• Easier service. Field-replaceable modules.
• Common stack. Same module across charger sizes.

A 350 kW charger has perhaps 7-8 power modules plus a control panel, each running its own Cortex-M.

Cortex-M's role in the control loops

The PFC and LLC control loops require:

• Synchronised ADC sampling. Sample current and voltage at the PWM cycle boundary, ideally at the midpoint of the switching interval (lowest noise). Cortex-M timers with ADC trigger output handle this; ST's HRTIM on STM32G4 is particularly well-suited.
• Tight ISR latency. The current-loop ISR runs at the switching frequency. At 100 kHz, that's a 10 µs window — the ISR must complete in <2 µs to leave room for the rest of the firmware. Cortex-M4F at 170 MHz handles it comfortably; M0+ at 48 MHz does not.
• Floating-point math. The PLL, the inverse Park, and the PI controllers all benefit from single-precision FPU. Without FPU, fixed-point implementations are tractable but tedious.
• DSP instructions. Park / Clarke transforms use SIN/COS; the ARM DSP extension's MAC and saturating arithmetic accelerate them. The CORDIC accelerator on STM32G4 computes sin/cos in ~50 cycles.
• Hardware fault response. Over-current must shut down PWM within microseconds. The "break input" on advanced timers does this in hardware without CPU involvement — a comparator output drives the timer's BRK pin, the timer immediately outputs the configured safe state.

ISO 15118 and Plug & Charge

For DC fast charging, the vehicle and charger must agree on parameters orders of magnitude more sophisticated than the CP duty cycle can carry: target voltage, current ramp profile, SoC, charging stop conditions, payment authentication. The standard is ISO 15118 — a Layer 7 protocol stack carried over PLC (Power Line Communication) on the CP wire.

PLC carries OFDM-modulated data at ~10 Mbit/s on top of the existing 1 kHz CP signal. The vehicle and charger each have a PLC modem (typically Qualcomm QCA7000-family). On the Cortex-M side, the modem appears as a SPI-attached Ethernet bridge; the firmware runs an IPv6 stack (link-local addresses, SLAAC), a TLS 1.3 client, and an EXI-encoded XML protocol parser.

ISO 15118-20 (2022 edition) adds Plug & Charge: the vehicle's contract certificate is provisioned during initial registration with the operator; at each subsequent plug-in, the charger validates the certificate and starts the session without user intervention. The cryptographic stack (X.509 chain validation, ECC P-256 signatures, TLS) lives on the Cortex-M and demands substantial Flash (~500 KB) and CPU.

Safety supervision

A charger is a multi-hundred-amp DC supply running into something that could be a person, an animal, or a slightly damaged battery. Safety supervision is independent of the main control loop, typically running on a separate Cortex-M0+ supervisor that monitors:

• Insulation resistance of the DC output relative to chassis — must be > 100 Ω/V; failure latches contactors open.
• Pre-charge of the output capacitance before closing the main contactor — limits in-rush.
• Over-current, hardware-latched.
• Over-temperature at every power component and at the plug.
• CP signal continuity — if the CP duty drops, the vehicle wants to stop; charger must comply within 100 ms.
• Watchdog of the main controller — supervisor will trip the contactors if main control fails to refresh.

These run on a separate MCU because the main controller is allowed to fail; the safety supervisor isn't. In some designs, the safety supervisor uses a TÜV-certified safety-MCU like the Infineon AURIX, STMicro SPC58 (Power Architecture), or Renesas RH850 — chips with native ASIL-D capability and certified safety manuals.

Cross-references

• Motor FOC — closely related power-electronics control problem.
• STM32 mainstream — G4 with HRTIM and CORDIC is the dominant power-stage MCU.
• AUTOSAR — the safety-MCU side runs AUTOSAR Classic in some implementations.
• TI Sitara — alternative for the charger-level supervisor with HMI.
• IEC 61851-1, -23, -24 — electric vehicle conductive charging standards.
• ISO 15118-2, -20 — V2G communication protocol.
• "Vienna Rectifier" reference designs from Infineon, Wolfspeed, Texas Instruments.