The main purpose of the power distribution board is distributing low-voltage (~12 V) power from the DC-DC converters and the supplemental battery to other low-voltage boards in the electrical system. The Rev 4 board is intended to be tested in 2019 FSGP and distribute power for MS14's electrical system in 2020 ASC.
The main goals for Rev 4 are followings:
- Separate the power distribution board into front and rear power distribution boards
- Charge the supplemental battery with energy from the DC-DC converters
- Collect data on load voltage and current consumption
- Protection against load or harness being shorted to ground
- Add current limiting
Low-Voltage Systems and Current Requirements
Assume all systems are powered by around 12 V. Ideally, the front and the rear power distribution boards should be very similar.
High-Side Driver
Low-Current HSD
Current Limit: < 1A @ 12-13.5V
| Front | Rear |
|---|---|
Peripheral Displays & Cameras (Use a simple circuit to distribute the current) | Solar Master (Front) |
| Steering Board | Solar Master (Rear) |
| Pedal Board | Rear View Camera |
| Fan | Fan |
| Spare | Telemetry |
| Spare | Charger Interface |
Potential Parts
| Part Number | Load Current | Rds On | Number of Outputs | Analog Current Sense |
|---|---|---|---|---|
| TPS2H160BQPWPRQ1 | 2.5A | 155mOhm | 2 | |
| BTS71202EPAXUMA1 | 2A | 61mOhm | 2 | +/-30% @ 50mA, +/-11% @ 500mA |
| AUIPS7091GTR | 1.5A | 80mOhm | 1 | +/-8% @ 50mA, +/-3% @ 500mA |
| AUIPS7081RTRL | 2A | 55mOhm | 1 |
Medium-Current HSD
Current Limit: < 6-8A @ 12-13.5V
| Front | Rear |
|---|---|
| Front Lights Board | Motor Interface |
| Center Console | Rear Lights Board |
| Horn? (8A) | Spare |
Potential Parts
| Part Number | Load Current | Rds On |
|---|---|---|
| BTS70401EPAXUMA1 | 4.5A | 19mOhm |
| BTS70081EPPXUMA1 | 11A | 8.8mOhm |
| BTS70061EPPXUMA1 | 12A | 6.6mOhm |
High-Current HSD
Current Limit: < 20A @ 12-13.5V
| Front | Rear |
|---|---|
Spare | Front Power Distribution (around 16A at the maximum) |
| Spare | BMS Carrier (Master & Slave) (due to high inrush current to the relays) |
Potential Parts
| Part Number | Load Current | Rds On | Current Mirror Error |
|---|---|---|---|
| BTS70041EPPXUMA1 | 15A | 4.4mOhm | ± 8% @ 10 - 15A |
| AUIR3313STRL | 23A (Adjustable) | 5.5mOhm | ± 5% |
| BTS70021EPPXUMA1 | 21A (Fixed) | 2.6mOhm | ± 5% |
Medium-High-Current HSD
Current Limit: 6 - 20A @ 12-13.5V
Potential Parts
| Part Number | Load Current | Rds On | Analog Current Sense | Note |
|---|---|---|---|---|
| TPS2HB08BQPWPRQ1 | 6.4A - 70A (Adjustable) | 8mOhm | ±15% @ > 0.5A | Found on Mouser |
Supplemental Battery
MS12 use a 12 V 10-cell? NiMH battery as the supplemental battery. It is used to power the minimal necessary systems to check the battery system and the reset of the car before closing the main battery relays.
The Rev 4 should be able to:
- Safely charge the NiMH battery
- Monitor the voltage and the temperature of the battery
- Measure the current draw from the battery
- Limit the current draw
Charging NiMH Battery
Discharge Curve
Found many different NiMH discharge curve on the internet. The average voltage for a pack of NiMH battery seems to slightly above 12 V. It is not too certain if we should charge the NiMH battery at 12 V or boost it to slightly higher voltage.
We should compare the discharge curves between charging at constant 12 V and charging by an off-the-shelf NiMH charger.
Charging Methods
- Constant Voltage at 12 V
- Simply make the supplemental battery parallel to the DC-DC converters
- Safe
- The battery might not be fully charged (need measurements to prove)
- Constant Voltage > 12 V
- Require a boost converter
- Ensure the battery is fully charged
- Require temperature and current monitors to ensure the safety of the battery
- Constant Current then Constant Voltage
- Safer and more efficient
- Require a more complicated circuit
- Off-the-shelf Changer
- Safe and efficient
- Extra weight
- Expensive
Solution?
- Charge the supplemental battery with 12 V while the car is racing
- Charge the battery to the maximum capacity with an external charger during the nights?
- Requires a circuit to switch from a 13 V battery to a 12 V DC-DC converter
Load Voltage, Current and Power Sensing
Voltage Sensing
Since the loads are parallel, they should operate at the same voltage. The PWR board only needs to measure the output voltage of the supplemental battery and the DC-DC converters
Current Sensing
Accuracy
The main purpose of the current sensing is understanding how much power is being used by each system while the car is driving/racing. The data will be used to identify systems with high power usage for future optimization. It is more meaningful to track the power consumption of each system relative to the total power rather than the absolute value.
The target power consumption of MSXIV's LV system is 20 W. A reasonable accuracy will be the +/- 0.1% of the total power. Given the supply voltage will be 12-13.5 V, the current sensing circuit should be more accurate than +/- 1.5 mA.
Locations
Given the HSD will be responsible for disconnecting the load when it is shorted to ground, there is no safety advantage for high side current sensing. There are three locations for the current sense resistor:
- Low-Side
- Easier to implement
- Requires a 100 mOhms current sense resistor which consumes some power
- High-Side
- High immunity to ground disturbance
- The common-mode voltage tolerance of the differential op-amp must be greater than 12V
- HSD Current Mirror
- A HSD built-in current mirror with high gain plus a current sense resistor should use less power than the low-side sensing (needs proof)
- The accuracy is around +/- 30% for low current (< 100 mA)