Regenerative Braking Design Discussion

flux

I renamed the project and am adding the main power-path components plus connectors and support passives now.

flux

The generator inputs, bridge rectifiers, TVS clamp, supercapacitor equivalent, bleeder, and protected bus are wired; I’m wiring the charger, battery, and load rails next.

flux

Created the full schematic for the regenerative braking charger and renamed the project to Regenerative Braking Charger. I added/wired the motor inputs, dual bridge rectifiers, TVS clamp, supercapacitor equivalent, BQ25750 charger stage with external MOSFETs, 2S battery connector/protection, 5 V rail, 3.3 V RC522 rail, LCD/MCU/RFID connectors, and support passives.
Open items:
  • BQ25750 bootstrap/compensation/feedback values still need datasheet-final tuning before layout.
  • Supercapacitor bank is represented as an equivalent C1; final cell count, balancing, fuse, ESR, and discharge timing need sizing.
  • I2C is pulled to 3.3 V for IC safety; add level shifting if your LCD module forces 5 V I2C pull-ups.
Suggested next steps:
  1. Finalize BQ25750 component values from the TI datasheet.
  2. Size the supercapacitor bank from expected braking energy.
  3. Move to PCB layout/placement after those values are locked.
  4. Use the new Board Bring-Up Plan for first-power validation.
Finalize charger and layout
Finalize BQ25750 values
Size supercapacitor bank
Add safety protections
Proceed to placement
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Files

Schematic

Layout

Preview

Regenerative Braking Power Path: Part Selection and Schematic Outline
Assumptions
  • Two brushed DC motors with gearboxes are used as generators on the rear wheels.
  • Rectified motor output can range from 0 V to >24 V during braking spikes.
  • Energy is buffered in supercapacitors, then used by a buck-boost charger for a 2S Li-ion pack.
  • Battery pack maximum charge voltage is 8.4 V.
  • Loads are a microcontroller, 5 V LCD display, and RC522 RFID reader.
  • RC522 power must be a clean, regulated 3.3 V rail and must not be powered from 5 V.

Table

StageFunctionRecommended partFlux part UIDWhy this part
1Bridge rectifier, one per motorTS25P05Gad6dc3ce-0aa9-40d9-b3b1-3310854f350bSingle-phase bridge rectifier, 600 V through-hole TS-6P package, strong voltage margin for motor spikes.
1 alternateBridge rectifierB250C5000-3300A2501c276-ed2d-0b0c-fc9a-6f6b2651a759600 V, ~5.8 A bridge rectifier; suitable if available.
2Input transient clampSMBJ24A7319d82b-aa7f-48ce-8889-d95e8df2d22424 V working TVS, 600 W peak pulse, ~38.9 V clamp; appropriate for protecting a wide-input charger from motor spikes.
3Supercapacitor bankSupercapacitor cells + balancing resistorsUse library supercapacitors sized by energy/current requirementMust be voltage-rated above clamp/operating voltage. Series cells need balancing.
4Buck-boost 2S Li-ion chargerBQ257500ac5aae6-7196-42be-a572-e388c94dc94eWide-input buck-boost battery charge controller, supports Li-ion/Li-polymer charging and wide 4.2–70 V input range.
4 alternateBuck/boost chargerMP2650GV variants1c45b2b8-bfa0-4d68-971a-4da5569faac6, d1566088-c877-4607-9c7d-c05192bb0a112S–4S buck/boost charger family, but input/source constraints need datasheet verification.
52S pack protection/BMS ICBQ29209DRBR5453eef3-b803-441a-94d3-703aa772513e2-cell Li-ion/Li-polymer protection/management IC with OV/UV/OC protection and balancing.
65 V rail from 2S batteryLTC3388 Modulee464524f-fb7a-4dae-8d57-a9aca387e1b9Buck converter module with selectable 3.3 V/5 V output. Best for modest logic/LCD loads.
6 alternate5 V high-current buckXL40164fd1b4a2-8d64-4c18-b90c-0bb99f02074eCommon high-current buck, but not ideal if the 2S pack can fall near 6 V because dropout/headroom may be insufficient.
63.3 V RC522 railMIC5365-3.3YC52b3b96b3-76de-4307-9b90-42a1f57b48a4Fixed 3.3 V, 150 mA low-noise LDO. Use from the 5 V rail, not directly from the 2S battery.
TVS / Clamp Selection
For a downstream charger that accepts at least 30 V input, the clamp network should prevent sustained or transient input excursions above the charger’s safe operating range while not conducting heavily during normal generator operation.
Recommended starting point:
  • Use a unidirectional TVS diode across the protected rectified bus.
  • Choose VRWM ≈ 24 V so it does not conduct during normal 24 V peaks.
  • Expected clamp for SMBJ24A is around 38.9 V at peak pulse current.
  • Use 600 W minimum peak pulse rating for a small graduation-project prototype.
  • For high-energy braking pulses, upgrade to SMCJ/5KP class TVS or a dedicated braking dump load, because a TVS alone is not meant to absorb continuous generator energy.
Important: if the charger input absolute maximum is only 30 V, SMBJ24A may clamp too high. In that case use either a lower-voltage TVS plus series impedance/current limiting, or select a charger with higher input tolerance such as BQ25750.
Supercapacitor Bank Guidance
  • Place the supercapacitor bank after the rectifiers and TVS clamp.
  • Voltage rating must exceed the maximum allowed bus voltage.
  • If using 2.7 V supercapacitor cells, series cells are required. For a 24–30 V bus, this means many cells in series, so active or resistor balancing is required.
  • Add a fuse or current-limiting element between the rectified motor bus and supercapacitor bank to control inrush/fault current.
  • Add a bleed resistor so the capacitor bank discharges safely after power-off.
Schematic Wiring Outline
Stage 1: Motor rectification
  1. Add two motor input connectors: MOTOR1_A, MOTOR1_B, MOTOR2_A, MOTOR2_B.
  2. Add one bridge rectifier per motor.
  3. Connect each motor’s two generator terminals to the ~ AC inputs of its bridge rectifier.
  4. Tie both rectifier positive outputs together into net VRECT_RAW.
  5. Tie both rectifier negative outputs together into net PGND.
Stage 2: Overvoltage protection
  1. Place the TVS diode directly across VRECT_RAW and PGND, physically close to the rectifier outputs.
  2. For a unidirectional TVS, connect cathode to VRECT_RAW and anode to PGND.
  3. Add optional fuse/PTC or low-value series resistor/current limiter upstream of the protected bus.
  4. Rename the protected bus after the protection node as VGEN_PROT if using a series protection element.
Stage 3: Supercapacitor buffer
  1. Connect supercapacitor bank positive to VGEN_PROT.
  2. Connect supercapacitor bank negative to PGND.
  3. Add balancing resistors across each series supercapacitor cell.
  4. Add a bleed/discharge resistor across the full bank.
  5. Add a voltage divider to allow the microcontroller to monitor VGEN_PROT, if telemetry is required.
Stage 4: Buck-boost battery charger
  1. Connect charger input/VBUS/VIN to VGEN_PROT.
  2. Connect charger power ground to PGND.
  3. Configure charge voltage for 8.4 V for a 2S Li-ion pack.
  4. Set charge current conservatively based on motor energy, supercap current limit, battery capacity, and thermal limits.
  5. Add all datasheet-required power inductor, input capacitors, output capacitors, sense resistor, bootstrap parts, compensation parts, and I²C pull-ups if using BQ25750.
Stage 5: Battery storage and BMS
  1. Connect charger BAT output to the protected 2S battery/BMS pack positive path.
  2. Connect charger ground/BAT negative to pack/BMS negative path as required by the BMS topology.
  3. Expose pack connector pins: B+, BMID, B- for 2S balancing if using cell-level protection.
  4. Do not bypass the BMS/protection path for system loads unless the protection IC datasheet explicitly allows it.
Stage 6: Load rails
  1. From protected battery output VBAT_2S, feed the 5 V buck converter input.
  2. Connect buck output to net +5V.
  3. Power LCD and microcontroller from +5V if those modules are 5 V-compatible.
  4. Feed MIC5365-3.3YC5 input from +5V.
  5. Connect MIC5365 output to net +3V3_RFID.
  6. Connect RC522 VCC only to +3V3_RFID.
  7. Add input/output capacitors for both regulators per datasheets.
  8. If microcontroller logic is 5 V and RC522 logic is 3.3 V-only, add level shifting on SPI lines: SCK, MOSI, MISO, NSS/SDA, and RST as needed.
Safety Notes
  • A TVS is a transient clamp, not a braking resistor. If the motors can produce sustained high energy during braking, add a dump load or active braking controller.
  • Supercapacitors can deliver dangerous fault currents. Use fusing and safe discharge paths.
  • Li-ion charging requires correct cell configuration, charge voltage, charge current, temperature monitoring, and protection circuitry.
  • Verify every charger/BMS connection against the exact datasheet before PCB layout.

  • Assumptions

  • Recommended Flux Library Parts

  • TVS / Clamp Selection

  • Supercapacitor Bank Guidance

  • Schematic Wiring Outline

  • Stage 1: Motor rectification

  • Stage 2: Overvoltage protection

  • Stage 3: Supercapacitor buffer

  • Stage 4: Buck-boost battery charger

  • Stage 5: Battery storage and BMS

  • Stage 6: Load rails

  • Safety Notes