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)

Block Diagram

Ports

Front

Board	Nom. Current (A)	Max. Current (A)	HSD
Peripheral Displays & Cameras	0.278	0.329	Light
Front Lights Board	1.889	4.163	Heavy
Steering Board	0.017	0.071	Light
Pedal Board	0.013	0.067	Light
Center Console	0.952	2.146	Light
Horn	0	8	Heavy

Rear

Board	Nom. Current (A)	Max. Current (A)	HSD
Motor Interface	0.179	4.958	Light
BMS Carrier (Master & Slave)	0.814	19.800	Heavy
Solar Master	0.150	0.482	Light
Rear Lights Board	0.022	2.879	Heavy
Rear View Camera	0.5	0.5	Light
Charging Interface	0.026	0.133	Light
Front Power Distribution	3.250	15.97	Heavy

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