Brief Description/Purpose:

What does this circuit do? Why do we need it?

This board takes inputs from the DCDC converter and the Supplemental Battery and selects which source to output to the fuse box. DCDC is the power from the high voltage primary battery pack that gets stepped down to 12V through the DCDC converter. The supplemental battery pack is the low voltage 12V NiMH battery pack. When DCDC is on, the board will select DCDC, and when DCDC is off, the board will select the Supplemental Battery. The supplemental should only be selected during the ignition and the fault states. The board selects the source using a 10V zener diode that will break down once DCDC reaches 10V.

This board also interfaces with both the Supplemental Monitoring PCB and Ignition Switch:

For the Supplemental Monitoring PCB, this board sends the voltage of the Supplemental Battery out from J1.
For the Ignition Switch, this board sends Supplemental Power to the Controls Board through J6, which is then routed back to this board through J7 when the Ignition Switch is switched from the off position. This signal is routed to the gate of Q3 which turns on the NMOS and starts sending power to the fuse box.

Pertinent Regulation(s)

Link to Regs: https://www.americansolarchallenge.org/regulations/2024-american-solar-challenge-regulations/

Regulation	Description of the Regulation	How Regulation is Met
8.2.C.1	Supplemental, replaceable batteries carried in the solar car may be used to power: Main Power Switch, driver ventilation fans (if solely used for driver ventilation), telemetry, and the horn. All other systems present on the car must be powered off the main battery.	Under normal operations, all systems in the car are powered off by the main battery as the DCDC voltage should be high.
8.2.C.2	Supplemental battery power may be used to momentarily power the battery protection system (BPS) as defined by Reg. 8.3 to verify safe battery parameters before energizing the Main Power Switch.	Upon startup, the supplemental battery will be selected by the Power Board and be used to power all LV systems until BPS determines the battery is safe.
8.2.C.3	During a battery fault in which the BPS has automatically opened the Main Power Switch, the supplemental battery must be used to power the BPS, BPS Strobe, and BPS Fault Driver Indication. While in such a fault state, the supplemental battery may be used to power any other systems (signal lights, rear view camera, telemetry, etc) that do not provide motive power to the vehicle.	Under fault state, all systems will be powered off of the supplemental battery as the DCDC voltage will be low.

Context

Location of the Board: The power board will be located in the electronics enclosure(HV section if the electronics enclosure is split).

List of I/O and Connections:

#	Name	Type	Ideal Voltage	Notes
J1	SUPP PWR MON	1x02_P3.00mm_Vertical Female	+12V	9.9 mm tall
J2	SUPP PWR 1	1x02_P7.49mm_Vertical Female	+12V	14.76 mm tall
J3	SUPP PWR 2	1x02_P7.49mm_Vertical Female	+12V	14.76 mm tall
J4	DCDC PWR	1x02_P7.49mm_Vertical Female	+12V	14.76 mm tall
J5	FUSE OUT	1x02_P7.49mm_Vertical Female	+12V	14.76 mm tall
J6	SUPP OUT	1x02_P3.00mm_Vertical Female	+12V	9.9 mm tall
J7	LV ENABLE	1x02_P3.00mm_Vertical Female	+12V	9.9 mm tall

Main

Schematic

List of Circuit Components

SUP90P06-09L

Description: Power PMOS
Why is it necessary: This MOSFET is placed on the power lines for each source and functions as a switch that is used to connect and disconnect each source from the output.
Justification for selection of specific part: Able to handle high current. Should be changed in the next iteration for a smaller and less expensive MOSFET.
Datasheet link: https://www.vishay.com/docs/73010/sup90p06-09l.pdf
Associated passives/components:
- Precharge Aux Enable connector
- Main Contactor Enable connector
- 10k resistors
Notes: Two of these MOSFETs are placed in series with each other with their directions switched to prevent current from flowing into one of the sources through the body diode while the MOSFET is deactivated.

UM6K1N

Description: Logic NMOS
Why is it necessary: The first MOSFET in the package is used to flip the logic when the zener diode breaks down. Once the zener diode breaks down, the gate of this MOSFET will pull high and therefore pull the DCDC_Gate connection low, activating the PMOS on the DCDC power line and connecting DCDC to Fuse Out. The second MOSFET in the package is used in the inverter that flips the DCDC_Gate signal for the Supp_Gate signal.
Justification for selection of specific part: Has the proper threshold voltage and comes in a dual package.
Datasheet link: https://fscdn.rohm.com/en/products/databook/datasheet-nrnd/discrete/transistor/mosfet/um6k1n.pdf
Associated passives/components:
- 10k pull-up resistor
- 1M pull-down resistor
Notes:

UM6J1N

Description: Logic PMOS
Why is it necessary: The first MOSFET is used in the inverter that flips the DCDC_Gate signal for the Supp_Gate signal.
Justification for selection of specific part: Has a similar threshold voltage to the Logic NMOS. For the next iteration we can probably buy an inverting IC for more reliability.
Datasheet link: https://fscdn.rohm.com/en/products/databook/datasheet-nrnd/discrete/transistor/mosfet/um6k1n.pdf
Associated passives/components: N/A
Notes: N/A

BZX384-Q

Description: Zener Diode
Why is it necessary: This diode ensures that the gate of the logic NMOS is pulled low only once the DCDC voltage surpasses the breakdown voltage of the diode. This way, the source swap only occurs once the DCDC is ~11V as opposed to when it surpasses the threshold voltage of the logic NMOS.
Justification for selection of specific part: Appropriate breakdown voltage.
Datasheet link: https://www.mouser.com/datasheet/2/916/BZX384_Q_SER-2721638.pdf
Associated passives/components: N/A
Notes: Confirm the exact part number ordered in the mouser order.

Layout

Dimensions: 82mm x 62mm

Requirements/Constraints: N/A

Design Choices:

Heavy use of planes and large traces to sustain the high current flowing along the power lines
Used MOSFETs for logic as opposed to Op-Amps due to fast switching time and to avoid feedback

PCB:

3D Model:

Additional Considerations

Redesign for the next generation to avoid doing logic at 12V.