SOM Power Architecture

Background

With the new zonal architecture requirement, we will be running many PeripheralSOMs across the car, so we decided to run the Peripherals at 12V to avoid voltage drops across the car. The old design had the peripherals run at 5V
Our current CAN chip (ISO1044BD) is an isolated chip that does not produce its own isolated power. This means that we need to provide two powers on both side of the CAN chip that are isolated from each other.
You can provide either 3.3V or 5V for the Vcc of the CAN chip
The decision was made to keep the CarCAN bus at the 5V since the load of the CAN chip on the receiver side is a few mA unlike the peripheral load which will be much more

No big load will be running off the CarCAN 5V, so not much power needs to be provided
Due to the nature of the CAN chip needing two isolated powers, we decided to have a singular shared 5V and GND for the entire CANbus to power the isolated side
The other side of the CAN chip will be powered by any bucks or LDOs from the 12V power input on the board
With this architecture the node at the start of the bus needs to provide that shared isolated 5V and GND for the entire bus
- There are two options for whether how we do this:
  - Have the isolation circuitry on the LeaderSOM
  - Have the isolation circuitry on a separate PCB (specifically the power distribution board

	Option 1	Option 2

Option 1

Option 2

Overview

Put the circuitry to isolate the 5V and provide power on the LeaderSOM

Have the circuitry to provide the isolated power on a separate board

This board would then be on the

Pros and Cons

Standalone testing will be easier

Circuit will be unused on any LeaderSOM who is not the first node in the CANbus

Allows the LeaderSOM to be simpler and more of an intersection of common functionality

Testing requires a board to provide power to the CANBus

The decision was made to have the isolation circuitry on a separate board

Every PeripheralSOM will be powered by their relevant LeaderSOM
- For example the BPS Leader board (with a LeaderSOM) will provide power and CAN for three other PeripheralSOMs
As previously mentioned the Peripherals will be powered by a 12V line, but they will not break out that 12V onto the daughterboard to avoid the need to isolate that 12V power
The need for isolation on the PeripheralSOM stems from the different situations that the SOM daughterboards may be in
- For example, the Voltage daughter board for BPS will be directly connected to the Battery Pack, so we want to avoid return paths from the Battery to the rest of our system
To avoid losses, instead of isolating the 12V line on every PeripheralSOM, we will instead isolate the 3.3V line on every PeripheralSOM since not much of a load is being drawn on the 3.3V line and pass that to the daughterboard
Since our CAN chip can take 3.3V or 5V we also decided to buck directly from 12V to 3.3V