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 parallel N-channel MOSFETs 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
Circuit Architecture¶
Block 1: Boost Converter (5V → 12V)¶
Purpose: Generate clean 12V supply for gate driver logic from 5V input.
IC: U21 - MT3608 Step-Up Converter - Input: 5V via C63 (4.7µF, 20V input filter) - Output: ~12V (adjustable via feedback network) - Enable: C42 (1µF) provides soft-start/enable functionality
Key Components: - L4: SRP6540-220M (220µH inductor) - Saturation current: ~1-3A (check datasheet) - Critical for energy storage during switching - D2: BAT54J Schottky diode - Forward voltage: ~0.25V - Current limit: ~200mA continuous - likely system bottleneck - Consider upgrading to 1N5819 or similar for higher current applications - Output filtering: C43 (1µF) || C64 (4.7µF, 20V)
Feedback Network: - R94: 10kΩ (upper divider resistor, SW pin to FB pin) - R95: 576Ω (lower divider resistor, FB pin to GND) - Current configuration: Vout ≈ 10.8V - For 12V output: 511Ω
Feedback Calculation:
V_FB = 0.6V (MT3608 internal reference)
V_SW = V_FB × (R94 + R95) / R95
V_out = V_SW - V_f(D2) = V_SW - 0.25V
Current: V_out = 0.6 × (10000 + 576) / 576 - 0.25 = 10.8V
Target: V_out = 0.6 × (10000 + 511) / 511 - 0.25 = 12.0V
Current Limitations: 1. BAT54J diode: 200mA continuous 2. MT3608 internal MOSFET: ~4A peak 3. Inductor saturation: 1-3A typical 4. Thermal: PCB layout dependent
Block 2: High-Side MOSFET Driver¶
Purpose: Provide isolated high-side gate drive for field current switching.
IC: U22 - LM5109AMA High-Side/Low-Side Driver
- VDD: 12V from boost converter
- PWM Input: OUT_PWM via R54 (100kΩ pull-down)
- Bootstrap supply for high-side drive
Bootstrap Circuit: - D7: Diode (part number: TOSHIBA CRH01(TE85L,Q)) - C4: 1µF, 100V bootstrap capacitor - R96: 2.2Ω gate drive resistor
Bootstrap Operation: 1. When low-side conducts, D7 forward biases 2. C4 charges to VDD (~12V) minus diode drop 3. When high-side switches, C4 provides floating gate drive 4. Gate drive voltage = C4 voltage + switching node voltage
Frequency Limitations: - Bootstrap refresh requires low-side conduction time - Higher frequencies reduce refresh time - 1µF C4 adequate for >20kHz operation - Consider reducing R96 to 1Ω for >50kHz operation
Block 3: Power MOSFETs¶
MOSFETs: Q3, Q7 - BSC072N08NS5ATMA1 (Parallel Configuration) - Voltage Rating: 80V - Rds(on): 7.2mΩ @ 25°C, Vgs=10V - Gate Charge: ~50nC total - Current Rating: >15A each (thermally limited)
Single MOSFET Analysis: - Conduction losses: I²R = 15² × 0.0072 = 1.62W - Thermal resistance: Check datasheet for θjc, θja - Parallel MOSFETs likely unnecessary - single device should handle 15A with proper thermal management
Protection: - D5: FSV20100V flyback diode - Critical for inductive load protection - Provides path for field current when MOSFETs switch off - Prevents inductive voltage spikes that would destroy MOSFETs - 100V rating, fast recovery for high-frequency PWM
System Operation¶
Normal Operation¶
- 5V input powers boost converter
- MT3608 generates 12V for gate driver
- PWM signal controls LM5109A driver
- High-side MOSFETs switch field current on/off
- Flyback diode handles inductive energy during switch-off
Field Current Path¶
- ON state: VIN_2-60 → MOSFETs → ALTERNATORFIELD → GND
- OFF state: ALTERNATORFIELD → D5 → VIN_2-60 (freewheeling)
PWM Control¶
- Frequency range: 100Hz - 20kHz+ (limited by thermal dissipation)
- Duty cycle: 0-100% (limited by bootstrap refresh at high frequency)
- Current control: Average field current = Duty Cycle × (VIN_2-60 / R_field)
Design Considerations & Limitations¶
Current Design Bottlenecks¶
- BAT54J diode (D2): 200mA limit in boost converter
- Thermal management: MOSFET heating at high frequency/current
- Bootstrap refresh: High duty cycle + high frequency combinations
Recommended Modifications¶
For Higher Current Boost: - Replace D2 (BAT54J) with 1N5819 or similar 1A+ Schottky - Verify inductor saturation current rating
For Higher Frequency Operation (>20kHz): - Reduce R96 from 2.2Ω to 1Ω - Add gate drive bypass capacitance - Improve thermal management for MOSFETs
Single MOSFET Operation: - DNP either Q3 or Q7 after thermal verification - Simplifies gate drive, reduces parasitic inductance
Component Selection Rationale¶
Critical Components¶
- MT3608: Proven boost controller, adequate current handling
- LM5109A: Purpose-built high-side driver with bootstrap
- BSC072N08NS5: Low Rds(on), fast switching, automotive qualified
- FSV20100V: Fast recovery, high current flyback diode
Standard Components¶
- Capacitor values: Standard practice for respective functions
- 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 |
|---|---|---|---|---|---|
| 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 |
| Q7 | BSC072N08NS5ATMA1 | N-Channel MOSFET | SuperSO8 | 80V, 7.2mΩ | Parallel power switching MOSFET |
Passive Components¶
| Designator | Value | Rating | Package | Description | Function |
|---|---|---|---|---|---|
| L4 | 220µH | - | SRP6540 | Inductor (SRP6540-220M) | Boost converter energy storage |
| C63 | 4.7µF | 20V | 0805 | Ceramic capacitor | Boost input filter |
| C42 | 1µF | - | 0603 | Ceramic capacitor | Enable/soft-start |
| C43 | 1µF | - | 0603 | Ceramic capacitor | Boost output filter |
| C64 | 4.7µF | 20V | 0805 | Ceramic capacitor | Boost output filter |
| C4 | 1µF | 100V | 0805 | Ceramic capacitor | Bootstrap capacitor |
| R94 | 10kΩ | 1% | 0603 | Precision resistor | Feedback divider upper |
| R95 | 576Ω | - | 0603 | Resistor | Feedback divider lower |
| R96 | 2.2Ω | - | 0603 | Resistor | Gate drive resistor |
| R54 | 100kΩ | - | 0603 | Resistor | PWM input pull-down |
Diodes¶
| Designator | Part Number | Package | Rating | Description | Function |
|---|---|---|---|---|---|
| D2 | BAT54J | SOD-323 | 30V, 200mA | Schottky diode | Boost converter output rectifier |
| D7 | CRH01(TE85L,Q) | - | - | Diode | Bootstrap charging diode |
| D5 | FSV20100V | - | 100V, 20A | Fast recovery diode | Flyback/freewheeling diode |
Net Connections¶
Power Rails¶
5V Input Rail: - Connected to: U21 pin 1 (VIN), C63 positive terminal, C42 positive terminal, U21 pin 4 (EN via C42)
11V/12V Boost Output Rail: - Connected to: D2 cathode, C43 positive terminal, C64 positive terminal, U22 pin 1 (VDD), U22 pin 8 (HB via bootstrap circuit)
VIN_2-60 Rail (High Voltage): - Connected to: Q3 drain, Q7 drain, D5 cathode
GND (Ground): - Connected to: U21 pin 3 (GND), U21 pin 2 (FB via R95), C63 negative terminal, C42 negative terminal, C43 negative terminal, C64 negative terminal, U22 pin 5 (VSS), R54 terminal, D5 anode
Signal Connections¶
OUT_PWM (PWM Input): - Connected to: R54 terminal, U22 pin 2 (HI)
ALTERNATORFIELD (Load Output): - Connected to: Q3 source, Q7 source, D5 anode
Internal IC Connections¶
MT3608 (U21) Pin Connections: - Pin 1 (SW): Connected to L4, D2 anode, R94 - Pin 2 (GND): Connected to ground rail - Pin 3 (FB): Connected to R94-R95 junction - Pin 4 (EN): Connected to C42, 5V rail - Pin 5 (VIN): Connected to 5V input rail - Pin 6 (NC): No connection
LM5109AMA (U22) Pin Connections: - Pin 1 (VDD): Connected to 12V boost rail - Pin 2 (HI): Connected to OUT_PWM via R54 - Pin 3 (LI): Connected to ground (not used in this application) - Pin 4 (VSS): Connected to ground - Pin 5 (LO): No connection (not used) - Pin 6 (HS): Connected to Q3 gate, Q7 gate via R96 - Pin 7 (HO): Connected to bootstrap high side - Pin 8 (HB): Connected to bootstrap circuit (C4, D7)
Bootstrap Circuit Connections¶
Bootstrap Network: - D7 anode: Connected to 12V rail (U22 VDD) - D7 cathode: Connected to C4 positive terminal, U22 pin 8 (HB) - C4 negative terminal: Connected to switching node (Q3/Q7 sources, ALTERNATORFIELD) - U22 pin 7 (HO): High-side gate drive output - U22 pin 6 (HS): High-side return (connected to switching node)
Feedback Network Connections¶
Voltage Divider: - R94 top terminal: Connected to U21 pin 1 (SW switching node) - R94 bottom terminal: Connected to R95 top terminal, U21 pin 3 (FB) - R95 bottom terminal: Connected to ground
Power MOSFET Connections¶
Q3 (Primary MOSFET): - Drain: Connected to VIN_2-60 rail - Gate: Connected to U22 pin 6 (HS) via R96 - Source: Connected to ALTERNATORFIELD, Q7 source, D5 anode
Q7 (Parallel MOSFET): - Drain: Connected to VIN_2-60 rail, Q3 drain - Gate: Connected to U22 pin 6 (HS) via R96, Q3 gate - Source: Connected to ALTERNATORFIELD, Q3 source, D5 anode
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, R94
D2 (Boost Rectifier): - Anode: Connected to L4, U21 pin 1 (SW), R94 - Cathode: Connected to 12V rail, C43, C64, U22 VDD
D5 (Flyback Diode): - Anode: Connected to ALTERNATORFIELD, Q3 source, Q7 source - Cathode: Connected to VIN_2-60 rail, Q3 drain, Q7 drain
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
- 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