Alternator Field Control¶
Overview¶
This circuit provides PWM control of automotive alternator field current up to 15A. The design consists of a 5V to 12V boost converter driving a high-side MOSFET driver with a single N-channel MOSFET for field current switching.
Key Specifications: - Input Voltage: 5V - Field Supply: VIN_2-60 (2-60V automotive rail) - Maximum Field Current: 15A - PWM Frequency Range: 100Hz - 20kHz+ (thermally limited) - Output: Controls alternator field winding via high-side switching - Control Input: 3.3V PWM from ESP32 microcontroller
Test Results (single MOSFET configuration): - Bank Nominal Voltage: 12V - Date: 9/10/25 - Enclosure: None - Ambient Temp: 63F - PWM Frequency: 1200hz - PWM Duty Cycle: 100% - Battery Voltage: 12.93V - Field Voltage: 12.83V - Field Current (engine off): 4.2A - Maximum PCB Temp: ~30C and most of that heat is left over from soldering - Conclusion: Not expecting any thermal issues in this design, V3 issues were related to high MOSFET Rds(on) due to low Vgs
Circuit Architecture¶
Block 1: Level Shifter and System Enable Logic (3.3V → 5V Logic)¶
Purpose: Translate ESP32 3.3V FIELD_ENABLE signal to 5V logic for boost converter control while providing multiple system disable conditions.
IC: U15 - SN74LVC1T45DBV Single-Bit Dual-Supply Bus Transceiver
- VCCA (Pin 1): 5V supply for A-side
- VCCB (Pin 6): 3V3 supply for B-side
- B (Pin 4): Combined input from three control signals
- A (Pin 3): Output to MT3608 Enable pin (5V logic)
- DIR (Pin 5): Direction control - tied to B pin (Pin 4)
- GND (Pin 2): Ground
System Enable/Disable Logic:
The system uses a wired-AND configuration at U15 Pin 4 that combines three control signals:
- FIELD_ENABLE (ESP32 GPIO): Active-HIGH enable signal
- 3.3V output from ESP32 through 10kΩ series resistor
- Provides pull-up voltage when system should be enabled
-
When LOW or ESP32 powered down: disables entire system
-
ALERT! (INA228 Alert Pin): Active-LOW disable signal
- Open-drain output from current monitor IC
- When INA228 detects overcurrent/fault: pulls pin to ground
-
Overrides FIELD_ENABLE pull-up, disabling system immediately
-
ON/OFF (Control Panel Toggle Switch): Active-LOW disable signal
- Manual toggle switch that can short pin to ground
- Operator control for manual system disable
- When closed (OFF position): overrides FIELD_ENABLE, disables system
Logic Function at U15 Pin 4: - System ENABLED when: FIELD_ENABLE = HIGH AND ALERT! = open (no fault) AND ON/OFF = open (ON position) - System DISABLED when: FIELD_ENABLE = LOW OR ALERT! = active (pulls low) OR ON/OFF = closed (OFF position)
Operation: - U15 level-shifts the combined 3.3V logic signal to 5V for MT3608 enable - Any single disable condition (FIELD_ENABLE low, ALERT active, or ON/OFF switch) will disable the boost converter - This immediately removes 12V power from the LM5109A gate driver, turning off field current - Provides multiple layers of protection: software control, hardware fault detection, and manual override
Enable Signal Path: FIELD_ENABLE/ALERT!/ON/OFF → U15 Pin 4 → U15 Pin 3 (5V) → U21 Pin 4 (EN) → MT3608 boost converter → 12V supply for LM5109A
Block 2: Boost Converter (5V → 12V)¶
Purpose: Generate clean 12V supply for gate driver logic from 5V input.
IC: U21 - MT3608 (SOT-23-6, fixed-freq current-mode boost) - VIN (Pin 5): 5V via C42 (22µF ceramic) - VOUT (target): 12.07V (set by feedback divider from VOUT → FB → GND). FB regulates to 0.6V - EN (Pin 4): From U15 (5V logic). EN high ≥1.5V - fSW: ~1.2MHz internal switching frequency
Key Components: - L4 (Inductor): Shun Xiang Nuo SMNR4020-22UH = 22µH - Use low DCR; choose Isat high enough to avoid saturation near switch current limit - D2 (Boost Schottky): Guangdong Hottech 1N5819WS (JLC C191023) - Anode = SW (Pin 1), Cathode = VOUT - 40V, 1A Schottky, SOD-323 - Appropriate class of diode for MT3608 boost rectification - Even with a light average load, the MT3608 output diode must be treated as a boost rectifier, not a signal diode — it carries pulsed inductor current, not just average DC load current - Low forward drop improves efficiency and output headroom - Output caps (C43, C63): 22µF each (44µF total), ≥25V rating, close to D2 cathode-GND
Feedback Network (VOUT → FB → GND): - R94: 10kΩ (upper divider resistor, VOUT to FB) - R95: 523Ω (lower divider resistor, FB to GND) - Setpoint formula: V_OUT = 0.6V × (1 + R94/R95) - Current configuration: V_OUT = 0.6V × (1 + 10000/523) = 12.07V
Feedback Network Calculation:
V_FB = 0.6V (MT3608 internal reference)
V_OUT = 0.6V × (1 + 10000/523) = 12.07V
Note: Feedback divider senses VOUT (after diode), not the SW switching node. This ensures regulation of the actual output voltage.
Current Limitations: 1. D2 = 1N5819WS is a proper 1A Schottky boost rectifier; no BAT54-class signal diode limitation applies 2. MT3608 switch current limit: controller-limited; must be verified against the actual datasheet and operating conditions — the ~4A peak figure is approximate and not a guaranteed design current 3. Inductor saturation: Check L4 (SMNR4020-22UH) Isat specification 4. Input capacitor C42 (22µF) meets the datasheet ≥22µF recommendation; output caps C43+C63 (44µF total) exceed it
Block 3: High-Side MOSFET Driver¶
Purpose: Provide floating high-side gate drive for field current switching.
IC: U22 - LM5109AMA High-Side/Low-Side Driver
- VDD: 12.07V from boost converter
- PWM Input: OUT_PWM via input conditioning network (series R + pull-down)
- Bootstrap supply for high-side drive
PWM Input Network (input conditioning): - R89: 100Ω (series resistor) - R54: 10kΩ (pull-down resistor) - Input voltage: HI sees essentially the full 3.3V logic level (~3.27V nominal due to 100Ω / 10kΩ loading) - Signal path: ESP32 → R89 → Pin 2 (HI) - Pull-down: Pin 2 → R54 → GND
Pin Configuration: - Pin 1 (VDD): 12.07V from boost converter - Pin 2 (HI): PWM input (3.27V from ESP32) - Pin 3 (LI): Tied to ground (VSS) - unused input per datasheet requirement - Pin 4 (VSS): Ground - Pin 5 (LO): Not connected (unused output) - Pin 6 (HS): High-side return (connected to switching node) - Pin 7 (HO): High-side gate drive output - Pin 8 (HB): Bootstrap supply input
Bootstrap Circuit: - D7: US1M (JLC C412437) bootstrap charging diode - 1A ultrafast diode, SMA package - Not the ideal bootstrap diode — chosen because it is a JLC Basic part - Higher forward voltage drop than CRH01 is the main electrical penalty; it reduces the effective bootstrap recharge voltage - Reverse recovery time is not a meaningful concern at 100Hz–20kHz PWM - C4: 0.1µF, 100V bootstrap capacitor - 100V rating is conservative; the actual HB−HS differential is only ~12V. Rating was chosen for availability and margin, not because the voltage stress requires it. - R96: 10Ω gate drive series resistor (reduces switching ringing)
Bootstrap diode selection note: US1M is acceptable in this role but is not the best technical choice. Forward voltage drop is the dominant limitation: US1M charges C4 to a lower voltage than CRH01 would, reducing HB−HS headroom by roughly several tenths of a volt. This is tolerable for the present design using a single BSC072N08NS5ATMA1 with a 12V gate-drive supply, but Vf is the first thing to revisit if gate drive margin becomes a problem. Reverse recovery time difference between US1M and CRH01 is negligible at these PWM frequencies and is not a design concern.
Bootstrap Operation: 1. When MOSFETs are off, HS node at ground potential 2. D7 forward biases, C4 charges to VDD (~12.07V) minus diode drop 3. When HI goes high, C4 provides floating gate drive 4. Gate drive voltage = C4 voltage + switching node voltage 5. Adequate for frequencies up to 20kHz with proper duty cycle
Bootstrap Capacitance and Gate-Charge Check
Only one BSC072N08NS5ATMA1 MOSFET is used. Gate charge is approximately 29nC max @ 10V. With C4 = 0.1µF, estimated bootstrap droop per turn-on event is:
ΔV ≈ Q / C = 29nC / 0.1µF ≈ 0.29V
This is acceptable. Bootstrap capacitor size is not a limiting factor in this design. The more important limitation is recharge headroom through D7, especially the diode forward voltage drop.
Frequency Limitations: - Bootstrap refresh still requires off-time with HS near ground - With one BSC072N08NS5ATMA1 and 0.1µF bootstrap capacitance, charge storage is ample for the intended PWM range - At 100Hz to 20kHz, the practical diode disadvantage of US1M is mainly higher forward voltage drop, not reverse-recovery time - The difference between approximately 35ns and 75ns trr is negligible at these PWM frequencies - High duty cycle operation still reduces refresh margin because HB must be recharged through D7 during low-side intervals
Block 4: Power MOSFET¶
MOSFET: Q3 - BSC072N08NS5ATMA1 - Voltage Rating: 80V - Rds(on): 7.2mΩ max @ 37A, Vgs = 10V - Gate Charge (Qg): 29nC max @ 10V - Continuous Drain Current: 74A (Tc) - Vgs max: ±20V - Ciss: 2100pF max @ 40V
Single-MOSFET Operation: - Design is based on one BSC072N08NS5ATMA1; parallel configuration has been removed - This simplifies gate drive, reduces gate charge demand, and reduces layout parasitics - At 15A and 7.2mΩ max Rds(on), conduction loss is approximately: P = I²R = 15² × 0.0072 ≈ 1.62W - This is acceptable with proper PCB thermal spreading and confirms that the parallel-MOSFET configuration is unnecessary - Prior bench testing indicates one MOSFET is adequate for the target field current in this design
Protection: - D5: FSV20100V flyback diode - Critical for inductive load protection - Provides path for field current when MOSFET switches off - Prevents inductive voltage spikes that would destroy the MOSFET - 100V rating, fast recovery for high-frequency PWM - R_GS (designator TBD): 10kΩ gate-to-source pull-down resistor - Holds gate at source potential when driver output is high-impedance - Prevents unintended turn-on during startup, power-down, or undefined transient states - Connected directly between Q3 gate and source (ALTERNATORFIELD node)
System Operation¶
Normal Operation¶
- ESP32 FIELD_ENABLE signal enables boost converter via U15 (requires ALERT! inactive and ON/OFF switch in ON position)
- 5V input powers boost converter
- MT3608 generates 12.07V for gate driver
- ESP32 PWM signal drives LM5109A HI pin via input conditioning network
- High-side MOSFET PWM-switches field current
- Flyback diode handles inductive energy during switch-off
System Enable/Disable Conditions¶
System ENABLED when ALL conditions met:
- FIELD_ENABLE = HIGH (ESP32 GPIO active)
- ALERT! = inactive/open (no current monitor fault)
- ON/OFF = open (manual switch in ON position)
System DISABLED when ANY condition occurs: - FIELD_ENABLE = LOW (ESP32 disables or powers down) - ALERT! = active (INA228 detects overcurrent/fault) - ON/OFF = closed (manual switch in OFF position)
Disable Response: 1. Any disable condition pulls U15 input low 2. U15 output goes low, disabling MT3608 3. 12V supply to LM5109A is removed 4. Gate driver stops switching MOSFETs 5. Field current ceases immediately
Field Current Path¶
- ON state: VIN_2-60 → MOSFET → ALTERNATORFIELD → Load → GND
- OFF state: ALTERNATORFIELD → D5 → VIN_2-60 (freewheeling)
PWM Control¶
- Input: 3.3V PWM from ESP32
- Processing: Voltage divider reduces to 3.27V at LM5109A
- Frequency range: 100Hz - 20kHz+ (limited by thermal dissipation and bootstrap refresh)
- Duty cycle: nominally 0–100%, but sustained high duty cycle is constrained by bootstrap refresh requirements
- Current control: Average field current = Duty Cycle × (VIN_2-60 / R_field)
Design Considerations & Limitations¶
Current Design Characteristics¶
- D2 = 1N5819WS: proper 1A Schottky boost rectifier for MT3608
- D7 = US1M: acceptable bootstrap diode, chosen as a Basic part, with the tradeoff of higher Vf and larger package versus CRH01
- Capacitor values: C42 (22µF input) and C43+C63 (44µF output total) meet or exceed MT3608 datasheet recommendations
- Losses: main losses are in MOSFET conduction and flyback/current-loop behavior; bootstrap losses are minor
- Bootstrap refresh: still depends on off-time, but 0.1µF bootstrap capacitance provides ample charge storage for one MOSFET
Recommended Modifications / Current Status¶
- D2 has already been upgraded to 1N5819WS, which is the correct diode class for the MT3608 boost rectifier
- Single MOSFET operation is now the intended configuration
- Output setpoint remains approximately 12.07V
- MT3608 input (C42, 22µF) and output (C43+C63, 44µF total) capacitor values now meet the datasheet recommendations
- Gate drive series resistor R96 = 10Ω; if ringing is still observed, value can be adjusted
- If future testing shows weak high-side gate drive at extreme duty cycle or frequency, D7 would be the first component to revisit because its forward voltage directly affects HB recharge headroom
Component Selection Rationale¶
Critical Components¶
- SN74LVC1T45DBV: Reliable 3.3V to 5V level translation with bidirectional capability
- MT3608: Proven boost controller, adequate current handling
- LM5109A: Purpose-built high-side driver with bootstrap
- BSC072N08NS5ATMA1: Low Rds(on), moderate gate charge, and sufficient voltage/current rating for single-MOSFET alternator field switching
- 1N5819WS: Correct class of Schottky diode for MT3608 boost rectification
- US1M: Acceptable JLC Basic bootstrap diode; chosen as a practical compromise between availability and performance
- FSV20100V: Fast recovery, high current flyback diode
Standard Components¶
- Capacitor values: Functional for application but smaller than datasheet optimal values
- Resistor values: Calculated for specific voltage/current requirements
Alternator Field Controller - BOM and Net Connections¶
Bill of Materials (BOM)¶
Active Components¶
| Designator | Part Number | Description | Package | Value/Rating | Notes |
|---|---|---|---|---|---|
| U15 | SN74LVC1T45DBV | Single-Bit Dual-Supply Bus Transceiver | SOT-23-6 | 5.5V, 32mA | Level shifter 3.3V→5V with enable logic |
| U21 | MT3608 | Step-Up Boost Converter | SOT-23-6 | 2A, 24V | Main boost controller IC |
| U22 | LM5109AMA | High-Side/Low-Side MOSFET Driver | SOIC-8 | 100V, 2A | Gate driver with bootstrap |
| Q3 | BSC072N08NS5ATMA1 | N-Channel MOSFET | SuperSO8 | 80V, 7.2mΩ | Power switching MOSFET |
Passive Components¶
| Designator | Value | Rating | Package | Description | Function |
|---|---|---|---|---|---|
| L4 | 22µH | - | SMNR4020 | Inductor (Shun Xiang Nuo SMNR4020-22UH) | Boost converter energy storage |
| C42 | 22µF | 25V | 0805 | Ceramic capacitor | Boost input filter |
| C43 | 22µF | 25V | 0805 | Ceramic capacitor | Boost output filter |
| C63 | 22µF | 25V | 0805 | Ceramic capacitor | Boost output filter |
| C4 | 0.1µF | 100V | 0805 | Ceramic capacitor | Bootstrap capacitor |
| C11 | 1µF | - | 0603 | Ceramic capacitor | Level shifter supply filter |
| R94 | 10kΩ | 1% | 0603 | Precision resistor | Feedback divider upper |
| R95 | 523Ω | - | 0603 | Resistor | Feedback divider lower |
| R96 | 10Ω | - | 0603 | Resistor | Gate drive series resistor (ringing suppression) |
| R89 | 100Ω | - | 0603 | Resistor | PWM input series resistor |
| R54 | 10kΩ | - | 0603 | Resistor | PWM input pull-down |
| R_GS | 10kΩ | - | 0603 | Resistor | Gate-source pull-down (designator TBD) |
Diodes¶
| Designator | Part Number | Package | Rating | Description | Function |
|---|---|---|---|---|---|
| D2 | 1N5819WS (C191023) | SOD-323 | 40V, 1A | Schottky diode | Boost converter output rectifier |
| D7 | US1M (C412437) | SMA | 1000V, 1A | Fast recovery diode | Bootstrap charging diode |
| D5 | FSV20100V | - | 100V, 20A | Fast recovery diode | Flyback/freewheeling diode |
Net Connections¶
Power Rails¶
3V3 Rail: - Connected to: U15 pin 6 (VCCB), C11 positive terminal
5V Input Rail: - Connected to: U15 pin 1 (VCCA), U21 pin 5 (VIN), C42 positive terminal
12.07V Boost Output Rail: - Connected to: D2 cathode, C43 positive terminal, C63 positive terminal, U22 pin 1 (VDD), D7 anode, R94 (feedback upper resistor)
VIN_2-60 Rail (High Voltage): - Connected to: Q3 drain, D5 cathode
GND (Ground): - Connected to: U15 pin 2, U21 pin 2, U22 pin 4 (VSS), U22 pin 3 (LI), C42 negative, C43 negative, C63 negative, C11 negative, R95, R54, D5 anode, R_GS lower terminal
Signal Connections¶
FIELD_ENABLE (System Enable): - Connected to: U15 pin 4 (B input) through 10kΩ series resistor from ESP32 GPIO
ALERT! (INA228 Alert Pin): - Connected to: U15 pin 4 (B input) - open drain output, pulls low on fault
ON/OFF (Manual Toggle Switch): - Connected to: U15 pin 4 (B input) - shorts to ground when in OFF position
OUT_PWM (PWM Input from ESP32): - Connected to: R89 → U22 pin 2 (HI), R54 pull-down to GND
ALTERNATORFIELD (Load Output): - Connected to: Q3 source, D5 anode, C4 negative terminal, U22 pin 6 (HS)
Internal IC Connections¶
SN74LVC1T45DBV (U15) Pin Connections: - Pin 1 (VCCA): 5V rail - Pin 2 (GND): Ground - Pin 3 (A): Output to U21 pin 4 (EN) - Pin 4 (B): Combined input from FIELD_ENABLE (via 10kΩ resistor), ALERT! (open drain), and ON/OFF (toggle switch) - Pin 5 (DIR): Direction control - tied to Pin 4 (B input) - Pin 6 (VCCB): 3V3 rail
MT3608 (U21) Pin Connections: - Pin 1 (SW): Connected to L4 terminal 2, D2 anode - Pin 2 (GND): Connected to ground rail - Pin 3 (FB): Connected to R94-R95 junction (feedback divider midpoint) - Pin 4 (EN): Connected to U15 pin 3 output - Pin 5 (VIN): Connected to 5V input rail, L4 terminal 1, C42 - Pin 6 (NC): No connection
LM5109AMA (U22) Pin Connections: - Pin 1 (VDD): Connected to 12.07V boost rail - Pin 2 (HI): Connected to OUT_PWM via R89, pulled down by R54 - Pin 3 (LI): Connected to ground (VSS) per datasheet requirement - Pin 4 (VSS): Connected to ground - Pin 5 (LO): No connection (unused output) - Pin 6 (HS): Connected to switching node (Q3 source, ALTERNATORFIELD) - Pin 7 (HO): Connected to Q3 gate via R96 - Pin 8 (HB): Connected to bootstrap circuit (C4 positive, D7 cathode)
Bootstrap Circuit Connections¶
Bootstrap Network: - D7 anode: Connected to 12.07V rail (U22 VDD) - D7 cathode: Connected to C4 positive terminal, U22 pin 8 (HB) - C4 negative terminal: Connected to switching node (Q3 source, ALTERNATORFIELD, U22 pin 6) - U22 pin 7 (HO): High-side gate drive output to MOSFET gates - U22 pin 6 (HS): High-side return (connected to switching node)
Feedback Network Connections¶
Voltage Divider (senses VOUT after D2): - R94 top terminal: Connected to boost output rail (D2 cathode, 12.07V) - R94 bottom terminal: Connected to R95 top terminal, U21 pin 3 (FB) - R95 bottom terminal: Connected to ground
Power MOSFET Connections¶
Q3 (Power MOSFET): - Drain: Connected to VIN_2-60 rail - Gate: Connected to U22 pin 7 (HO) via R96; R_GS upper terminal also connects here - Source: Connected to ALTERNATORFIELD, D5 anode, U22 pin 6 (HS), C4 negative, R_GS lower terminal
R_GS (Gate-Source Pull-Down): - Upper terminal: Connected to Q3 gate, R96 output, U22 pin 7 (HO) - Lower terminal: Connected to Q3 source (ALTERNATORFIELD node), GND-referenced switching node
Inductor and Diode Connections¶
L4 (Boost Inductor): - Terminal 1: Connected to 5V rail, U21 pin 5 (VIN) - Terminal 2: Connected to U21 pin 1 (SW), D2 anode
D2 (Boost Rectifier): - Anode: Connected to L4 terminal 2, U21 pin 1 (SW) - Cathode: Connected to 12.07V rail, C43, C63, U22 VDD, R94
D5 (Flyback Diode): - Anode: Connected to ALTERNATORFIELD, Q3 source, U22 pin 6 (HS), C4 negative - Cathode: Connected to VIN_2-60 rail, Q3 drain
Enable Logic Connections (Wired-AND Configuration)¶
U15 Pin 4 (Combined Enable Input): - FIELD_ENABLE: ESP32 GPIO → 10kΩ resistor → U15 Pin 4 (provides pull-up when active) - ALERT!: INA228 alert pin → U15 Pin 4 (open-drain, pulls low on fault) - ON/OFF: Manual toggle switch → U15 Pin 4 (shorts to ground when in OFF position) - Pull-down: R_PD (100kΩ) → U15 Pin 4 to GND (ensures defined LOW state when all signals inactive)
Logic Function: - Pin 4 voltage HIGH (3.0V) when: FIELD_ENABLE high AND ALERT! inactive (open) AND ON/OFF open (ON position) - Pin 4 voltage LOW (0V) when: FIELD_ENABLE low OR ALERT! active (low) OR ON/OFF closed (OFF position) - ESP32 current draw: 30µA through 10kΩ + 100kΩ voltage divider
Physical Layout Considerations¶
High Current Paths¶
- VIN_2-60 to MOSFET drains: Wide traces, heavy copper
- MOSFET sources to ALTERNATORFIELD: Wide traces, heavy copper
- Ground connections: Solid ground plane preferred
Switching Node (High dV/dt)¶
- U21 pin 1 to L4 to D2: Minimize loop area
- Keep switching node traces short and wide
- Minimize parasitic inductance and capacitance
Gate Drive Connections¶
- U22 to MOSFET gates: Controlled impedance, minimize length
- Bootstrap circuit: Keep C4 and D7 close to U22
- D7 is an SMA-package diode (US1M); ensure local placement still keeps the VDD→D7→HB→C4→HS bootstrap loop reasonably compact despite the larger footprint
- Ground returns: Low impedance path to common ground
Thermal Considerations¶
- MOSFET thermal pads: Connect to ground plane via thermal vias
- Power dissipating components: Adequate copper pour for heat spreading
- Component spacing: Allow for airflow around power components
Signal Integrity¶
- PWM input traces: Keep away from switching nodes
- Bootstrap circuit: Minimize loop area for HB-HS
- Level shifter: Adequate supply filtering near IC
- Feedback traces: Route away from SW node to minimize noise pickup
- Enable logic traces: Keep FIELD_ENABLE, ALERT!, and ON/OFF traces separated from high-frequency switching nodes
MT3608 Specific Layout Requirements¶
- Input capacitor C63: Place close to VIN-GND pins for tight current loop
- Output capacitors C43/C63: Place close to D2 cathode-GND for tight current loop
- SW node: Minimize copper area and trace length to reduce EMI
- Feedback routing: Keep FB traces away from SW node switching area
Enable Logic Layout¶
- U15 placement: Position close to enable signal sources to minimize trace lengths
- Control signal routing: Keep ALERT! and ON/OFF traces away from switching nodes
- Pull-up resistor: Place 10kΩ FIELD_ENABLE series resistor close to ESP32 output
- Ground connections: Ensure solid ground reference for control logic