Alternator Field Drive¶
Purpose¶
This circuit provides a high-side PWM drive to the gate of the N-channel MOSFET controlling the alternator field coil. The goal is to achieve efficient, reliable operation up to 100kHz PWM while minimizing quiescent current when the circuit is inactive.
Source Signal¶
- Origin: ESP32 PWM-capable GPIO
- Voltage: 3.3V logic level
Main Components¶
| Component | Function |
|---|---|
| LM5109A | High-speed high-side gate driver (10-12V VDD) |
| MT3608 | Boost converter from 5V to 12V for gate driver |
| BSC072N08NS5 | N-channel MOSFET to switch field coil |
| 74LVC1G04 | Inverter to enable/disable the 5V rail via logic |
| BSS84 | P-MOSFET to gate 74LVC1G04 enable logic |
| Bootstrap Diode (BAS16LT1G) | Charges bootstrap cap during low-side on |
| Bootstrap Cap (100nF–1uF, 100V) | Supplies transient gate drive voltage |
| Gate Resistor (~10Ω) | Limits gate inrush and dampens ringing |
Operating Voltages¶
- Logic Source: 3.3V from ESP32
- Driver Supply (VDD): 10–12V from MT3608
- PWM Frequency Target: Up to 100kHz
Power Management¶
- 5V supply is software-controlled via Field Enable + ON/OFF + Alert! logic
- If 5V is off, MT3608 also ceases operation (since input is cut), minimizing idle current
- No TVS or gate clamp added (assumes clean layout and short traces)
Performance Goals¶
- Reliable switching with >10V Vgs
- Minimal idle current draw when disabled
- Efficient field coil control with minimal MOSFET dissipation
Notes¶
- Bootstrap capacitor sizing must support fast switching without voltage sag
MOSFET Thermal and Current Limit Analysis¶
Key Parameters¶
- Gate drive voltage (VGS): 12 V
- Estimated RDS(on) at 12 V: ≈ 5.5 mΩ (conservative extrapolation from datasheet graph)
- PCB configuration: 4-layer board with good copper and via stitching
- No external heatsink
- Max junction temperature (TJ,max): 100 °C
- Ambient temperature (TA): 25 °C
- Estimated thermal resistance (RθJA): ~25 °C/W (based on good PCB design for SON-8 package)
Max Power Dissipation¶
[ \Delta T = T_{J,max} - T_A = 100 - 25 = 75^\circ C ]
[ P_{max} = \frac{75}{25} = 3\,W ]
Max Continuous Current¶
[ P = I^2 \cdot R_{DS(on)} \Rightarrow I_{max} = \sqrt{\frac{3}{0.0055}} \approx 23.3\,A ]
Practical Limits¶
| Limit Type | Estimate |
|---|---|
| Power-limited current | ~23 A |
| Safe continuous current | 15–20 A (margin) |
| Likely failure point | > 23 A (thermal) |
Note: Above 20 A, trace heating, solder joint fatigue, and junction rise make failure more likely without forced cooling or heatsinking. Ambient temperature assumption of 25C is not worst case scenario.
Revision Info¶
- Original driver (MAX15054) replaced due to low Vgs drive at 5V rail
- New system uses external 12V boost to properly saturate the MOSFET gate