BLDC motor - how to protect PCB circuitry from sudden stop and generator operation

Thread Starter

tomerbr

Joined Oct 16, 2017
32
Hi, I hope this is right forum, if not please let me know and I will post it under General Electronics Chat or Power Electronics

I have a working board that we are unable to stabilize.
By that I mean some boards get burned with no apparent reason.

The system is an outdoor cleaning robot for solar panels.
It comprises of several building block, the main ones are: a solar charger that charge the battery when the robot is in the docking station, the MCU and the BLDC motor driver.

The battery is a LiFePO4 16.4V (5 cells).

I have two issues.

  • The charger stops charging, something gets ruined there.
  • Parts around the BLDC driver get burned.
I think these two problems are connected because we never see a stop in the charging when the robot is docking only after it returns.
From that I deduce that the charger circuit either get burnt during the operation of the robot or during the entry into the docking station when the solar panel is of course "hot".

I investigated how the robot is being operated in the field and found two issues that I think I need to address.

  • Sudden stop.
  • Manually pushing the robot, effectively turning the motor into a generator.
Both these issues will produce voltage and current above the system rating, I even saw a 50V 1000uF electrolytic cap exploded.

I saw something that is called a breaking resistor, but I am not quite sure how to install it.

I have considered adding a freewheeling diode or rectifiers, but I am not sure where to put it on a BLDC motor and if that even help because it will still pass the entire voltage back to the system and affect the charger and other parts.

The motor works with about 16V and get reach a peak 15 amps in normal operation.

Attached is the basic block diagram, the charger and power stages and the motor stage.

System block diagram.JPGSolar_Charger.JPGPowerSupply.JPGMotor.JPG



Any suggestions?
Drawings are also attached in PDF format in better resolution

Thanks,
Tomer
 

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LowQCab

Joined Nov 6, 2012
2,058
Your description of "parts get burned" is really vague.
Can You duplicate the problem on a fresh Board ?
Specifically which components or traces "burn".
Do You have exploding parts, or over-heated Board-Traces or both ?

At ~15-Amps, I would estimate that You are pushing the limits of
a Printed-Circuit-Board even under "normal" operating conditions.
If all these parts and Circuits are miniaturized down to SMD dimensions,
extreme measures must be taken to insure that everything can withstand any unusual conditions,
You are dissipating a lot of heat, even under "normal" operation.

Are all of your Bridge-Transistors mounted on an adequate Heat-Sink, with adequate Air-Flow ?

Are all of your components rated for at least twice the normal operating Voltages ? ( preferably 3X or more ).

Does this unit operate outside in direct Sunlight ?

What is the expected ambient Temperature ?

Have You measured Component Temperatures and Currents/Voltages under
"real-world" operating conditions ?
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