Description

12S pre-charge controller module for controlled start-up / shut-down behavior of entire system power distribution network.

Engineer: @Robert Tang

Supervisor: @Kevin Li

Requirements

• 12S rated

• Pre-charge/discharge circuit for controlled on/off behavior of entire power distribution network

• Battery voltage / current sense ?

  • 12S

  • 100A pulse current / 60A continuous current

• Dedicated MCU

• CAN interface

System Background/Research

Pre-charge circuits

Open

Why it is needed?

Without the pre-charge circuit, when connecting a high voltage source to the load, which can be modelled as a capacitor, there will be a large potential difference, and this will cause inrush currents that can damage the components. When the pre-charge circuit is used, the load capacitance can be safely charged up, and then the switch can be closed.

Reference documents

Why Pre-Charge Circuits Are Necessary in High Voltage Systems

High-Voltage Solid-State Relay Active Precharge Reference Design Reference design for this TPSI3050-Q1

Managing Inrush Current Not the exact same as pre-charge, but gives an general idea of managing in-rush current, and some of the methods in the doc are relevant to the pre-charge circuit (integrated load switches)

Designing a BCM® Pre-Charge Circuit

Pre-Charge Circuits for Lithium-Ion Battery Packs

FSAE/Precharge/Precharge at master · michaelruppe/FSAE Pre-charge circuit for tractive system

Pre-charge Component Selection

Search on DigiKey for: high-side FET driver, not looking for driver with internal FET (can reflect by no R_ds_on in the spec)

IC & Link	Type

IC & Link	Type
TPS4811-Q1 TPS48111LQDGXRQ1 | DigiKey Electronics	High Side NMOS Driver	The similar IC (4810) that was used in 12V->5V @ 5A Buck (without pre-charge, 48111 is with pre-charge) 3.5V to 80V input
TPSI3050-Q1 TPSI3050QDWZRQ1 | DigiKey Electronics	Isolated Switch Driver	Might be overkill for our application Open
LM5069 LM5069	Current Controller	Is a bit different, does not mention pre-charge directly, but have in-rush current protection and have helped decrease dv/dt
BQ76200 BQ76200PWR | DigiKey Electronics	High side NMOS Driver	*With PMOS pre-charge FET driver function

CAN (and MCU)

Resources:

CAN - Controller Area Network

ESC CAN Adapter

CAN - Controller Area Network (EFS team)

The CAN circuit and the MCU will be copied from Meghan’s ESC CAN Adapter, the selected STM32 MCU is STM32L431KCU6.

The STM32 will require external 3.3V and 5V input to power it on, this can be done by placing a Buck (convert 12S voltage to 5V) and then a 5V to 3.3V LDO, this will help reduce noise.

Current sense

Some pre-charge FET driver IC, such as the TPS48111 chip, has integrated current sensing feature, however I need to do calculations to validate the feasibility of the feature.

If the internal current sensor does not work, an external current sensing circuit is needed

System Design

System Block Diagram

Component Selection

STM32 Power Component Selection

BUCK

Requirement: 12S battery to 5V, the required current is determined by adding up all currents; >50V input; simple architecture (does not require additional features) Look for DC to DC switching regulator

All of the load current consumed by the BUCK output, and accounting for ripples is around 1A.

Selection Table:

Buck Component Selection

Output voltage set:

The output voltage is set to 6.1V to account for DCR losses

Inductor selection:

Search Query

12S Pre-charge Inductor Link to Desmos calculation

Since the current is small (less than 1A), the DCR of the inductor will not matter that much, the selected inductor has a DCR of 310mOhm.

Finally selected inductor: https://www.digikey.ca/en/products/detail/bourns-inc/SRP7050TA-470M/5429688

Capacitors:

All needed capacitors are selected from the WARG component library

LDO

The LDO is selected from 6S Servo Module, converting 6V output from the BUCK to 5V and then to 3.3V using two cascading LDOs.

Pre-Charge IC Related Component Selection

Current Sense Resistor

R_SNS from the calculation above is calculated 0.0002Ohms

Search Query

Selected: WSLP3921L2000FEA

R_SET

100Ohm, 1% found from the library

R_IWRN (Overcurrent protection threshold)

The I_OC (Overcurrent threshold) is set to be 120A, (1.2*I_Max)

R_ISCP (Short-circuit protection threshold)

The I_SC (Short-circuit threshold) is set to be 144A, (1.2*I_OC)

C_TMR (Fault timer period)

Main MOSFET (Q1 and Q2)

2 options: First one is to use two large FETs in parallel, second option is to use 3 smaller FETs in parallel. Finally selected to go with 2 in parallel, since the ones that I have found on DigiKey has lower R_DSON and similar price, and better performance than the small FETs

REQUIREMENTS:

1. Max V_GS the TPS chip can drive is 13V, so select FET with V_GS larger than 15V

2. V_DS at least 60V (something with 80V should be optimal)

3. Expected wattage is 2.5W-3.5W (calculated by 150/thermal resistance of junction to ambient)

   1. Choose R_DSON around 1mOhm is best

Search Query

Final Selected: NVBLS0D8N08XTXG

Pre-charge FET (Q3) and Pre-charge Resistor

Requirements: 1. Handle I_INRUSH; 2. Voltage rating is the same for other FETs (V_DS); 3. V_GS is the same as well, since they are all powered by the charge pump.

The pre-charge FET and pre-charge resistor will be connected in parallel with the main FETs

The I_INRUSH is set to be 0.5A (500mA)

R_PRECHARGE=V_IN/I_INRUSH=48V/0.5A=96Ohms

Selected 2 110 Ohm resistors in parallel to account for power dissipation

https://www.digikey.ca/en/products/detail/vishay-dale/CRCW2512220RJNEGHP/2222118 , 2512 sized

Search Query

C_BST Bootstrap Capacitor

Rated 25V+

IMON Resistor

EN/UVLO Set