Tool/software:
Hi everyone,
I’m using the LM5050 as an ideal diode to protect a 54.5V / 30A power supply from the back EMF generated by an ESC when braking a motor.
Context
In normal operation, the power supply feeds the ESC with 54.5V at up to 30A. When the motor is suddenly stopped, it generates a back EMF. In this case, the LM5050 drives the MOSFET to turn off and block the reverse current, protecting the PSU.
However, I encountered the following issues and behaviors:
1. Without R14
If I don’t include resistor R14 (connected from GATE to SOURCE of the MOSFET), the MOSFET does not turn off fast enough, and the reverse voltage from the ESC reaches the PSU.
My assumption is that this is caused by the parasitic gate capacitance of the MOSFET, which forms an RC time constant and delays the gate discharge. R14 helps by discharging this capacitance faster.
2. Motor stop – Back EMF
When I fully stop the motor, a large back EMF is generated. In this case, the MOSFET turns off correctly and the PSU is protected.
3. Braking the motor – small voltage appears first
When I brake the motor abruptly, I see a small voltage spike before the large back EMF. During this small initial voltage increase, the MOSFET does not turn off, and the reverse current goes through to the PSU.
I’m not sure why the LM5050 does not react fast enough or why the MOSFET stays on during this initial event.
4. Light braking (~ -10% throttle)
During light braking, for example a small decrease in motor speed, I observe a 1.75V spike before the MOSFET finally turns off.
What I’ve Tried
- I experimented with different values for R14, but the issue persists.
- I also tried adding a diode in parallel with R14, still without success.
- Tried different mosfet, still no difference.
My Questions
- Why does the MOSFET not turn off immediately during these smaller back EMF events?
- Is there something wrong or missing in my schematic?
- Is there a better way to discharge the gate capacitance faster without affecting LM5050 behavior?
Thanks in advance for your help!
Let me know if more details or measurements are needed.
Best regards,
