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Goals
- Safely pre-charge the motor controller by applying 12V to the pre-charge relay
- Discharge the motor controller to ground whenever 12V is not applied
Background and strategic fit
The pre-charge and discharge controllers will each be responsible for one motor controller. It's job is to safely charge the motor controller up to the same voltage as the battery before allowing the main relay to connect; and to discharge the motor controller to ground whenever power is not being applied. This allows the main relay to connect without arcing, and de-energize the motor controller when the vehicle is not running.
Based on the requirements, it was determined that the high voltage from the battery would be reduced to 5-12V using two resistors in series, and an instrumentation amplifier would measure the difference in voltage across the pre-charge resistor. Once the differential is minimum, the main relay would then be closed with a MOSFET. The discharge portion of the circuit would simply be a normally closed relay that would open when the vehicle is in it's run state.
Requirements
| # | Title | User Story | Importance | Notes |
|---|---|---|---|---|
| 1 | Voltage Isolation | High voltages must be isolated from the 12V rail of the vehicle, so in case of a failure in the resistors, HV will not go into the 12V rail | Must Have |
Optoisolator | |||
| 2 | Reliable Operation | Must reliably initiate pre-charge and discharge when powered on and powered off | Must Have |
| Discharge is entered whenever power is cut to the board with a SPDT relay |
Questions
Below is a list of questions to be addressed as a result of this requirements document:
| Question | Outcome |
|---|---|
| Minimum differential voltage before the main relay can close | < 0.7V should be enough to prevent any arcing |
| P-ch MOSFET goes to DCDC and Pre-charge relay, with RC circuit going to Pre-charge relay for >5ms. What voltage does DCDC need? Add another N-ch MOSFET for higher current? | 12VDC for DCDC, Use Additional mosfet and RC circuit for time delay |
Proposed Parts
Changes:
- Look at Chaos MOSFETS
- Change to N channel mosfets, standardize
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