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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
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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 | - Looking into Optoisolators or Digital Isolators with a isolated power supply
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| 2 | | | | |
User interaction and design
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Below is a list of questions to be addressed as a result of this requirements document:
| Question | Outcome |
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| 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? |
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Proposed Parts
P620KBYCT 620K 4W 0603search/en?keywords=P620KBYCT-NDP47KBYCT 47K /4W 0603P47KBYCTPB1248CT| Pre-Charge |
| PB1250-ND | Telecom Relay |
SPST-NO A Surface Mount14620413PB1248CT21269541209| 1208-ND | Isolated Module DC DC Converter 1 Output |
12V 21mA 1212S12092256389
http://mustcalculate.com/electronics/capacitorchargeanddischarge.php?vfrom=0&vto=10.8&vs=12&r=120&time=1m