Brief grid disturbances — voltage sags lasting 100–500ms — can cause a DC fast charger's main controller to reset, interrupting an active charging session and triggering OCPP error handling on the backend. A supercapacitor backup system provides holdup energy during these events, allowing the controller to continue operating through the disturbance without a session interruption.
The LTC3350
Linear Technology's LTC3350 (now Analog Devices) is a monolithic supercapacitor charger and backup controller that handles the complete supercapacitor management function: charging the stack from the main power supply, balancing voltage across cells in a series stack, monitoring stack health, and controlling the power path switchover during a backup event.
The LTC3350 operates in one of two modes:
- Charge mode: Charges the supercapacitor stack from the primary supply via an internal synchronous buck converter (up to 3.5A charge current)
- Backup mode: Powers the load from the supercapacitor stack via an internal boost converter (load up to the stack charge current)
Switchover between modes is automatic and requires no firmware intervention — the LTC3350 detects when the primary supply drops below a configurable threshold and transitions to backup mode within microseconds.
Supercapacitor Stack Sizing
Sizing the supercapacitor stack requires knowing:
- The load power during holdup (controller, communication ICs, display — typically 5–15W for a DCFC controller board)
- The required holdup duration (typically 300–500ms to ride through standard sag events)
- The usable voltage window of the supercapacitor stack
The LTC3350 supports stacks of 1–4 series supercapacitors. For a 12V controller supply with a 5V–12V usable window and 10W load power:
Energy required = 10W × 0.5s = 5J
Capacitance = 2 × Energy / (Vmax² − Vmin²) = 2 × 5 / (144 − 25) ≈ 84mF
Use standard supercapacitor values: four 100F/2.7V capacitors in series give 25F effective capacitance at 10.8V — more than sufficient. The practical stack is four 100F/2.7V cells in a 2S2P configuration if PCB height is constrained, or 4S 10F cells if holdup time requirements are modest.
Cell Balancing
Series supercapacitors require active cell balancing because their leakage currents differ between units — without balancing, the cell with the lowest leakage charges to a higher voltage and can be over-stressed. The LTC3350 implements resistive cell balancing: it shunts excess voltage from over-charged cells through internal shunt resistors.
For 2.7V-rated cells with a 2.5V charge voltage target, the LTC3350's balance threshold is set via the VCAPFB pin resistor divider. Cells are balanced during both charge and backup modes.
Firmware Integration
The LTC3350 communicates over I²C, exposing registers for:
- Stack voltage and individual cell voltages
- Charge current and state
- Stack capacitance (measured via internal coulomb counting during charge)
- Alarm flags (under-voltage, over-temperature, charger fault)
Firmware integration involves:
- Reading cell voltages periodically and logging to OCPP telemetry (useful for predictive maintenance)
- Monitoring the CHRG_STATUS register to confirm the stack is fully charged before allowing a charging session to start
- Implementing an alarm handler for the LTC3350's INT pin — supercapacitor failure should generate an OCPP StatusNotification with an appropriate fault code
Field Performance
On the 240kW DCFC platform where this design was deployed, field data from 80+ units shows that the supercapacitor backup system activates an average of 2–3 times per unit per year. In each logged event, the controller maintained continuous operation through the voltage sag with zero session interruptions attributable to the backup event. Supercapacitor stack capacitance measurements (read via I²C from deployed units) show less than 10% capacitance degradation after 18 months of field operation — within the expected aging curve for the selected cell chemistry.