Overview

Objective: Replace previous arbitrarily timed pre-charge with one based on actual state of hardware.

Active Precharge: Primary purpose is to safely charge the high-voltage components of the EV such as the inverter, motor, and battery, before full power is applied. 

• Initial High Current Surge Prevention

  • EVs gradually energize its high-voltage components instead of instantly connecting the full battery power to them because the sudden application of full battery power to uncharged components could cause a huge surge of current, potentially damaging the components and reducing the lifespan of the battery. 

• Capacitor Charging

  • Many high-voltage components in an EV have large capacitors that need to be charged when the vehicle is turned on. Charging these capacitors too quickly can cause damage due to the high current. The active precharge circuit slowly charges these capacitors to the required voltage before the main contactors (which fully connect the battery to the system) close.

• Improved Safety

  • By preventing high current surges, the active precharge circuit enhances the safety of the EV's electrical system. It reduces the risk of electrical arcing, overheating, and potential fires.

• How It Works

  • The active precharge circuit typically includes a resistor and a relay (or contactor). When the vehicle is turned on, the resistor is first connected in series with the high-voltage components. This resistor limits the current flow, allowing the capacitors to charge slowly. Once the capacitors are charged to a safe level, the relay bypasses the resistor, allowing full power to flow to the components.

• Active Precharge Design:

  • When the EV is powered on, the active precharge circuit gradually increases the voltage applied to the components.

  • This is typically done using a combination of a precharge resistor (a temporary resistor) and additional active components like transistors or MOSFETs, which can finely control the current and voltage during the precharge phase.

  • The active circuit continuously monitors the voltage across the components and adjusts the current flow to ensure a smooth and controlled precharge.

  • Once the precharge is complete, the circuit bypasses the precharge resistor, allowing full power to flow.

Old Design

In the previous design, a simple contactor board would drive a single contactor off a signal from controls. Controls would send the motor contactor enable signal, wait a set period of time, and then send the motor precharge enable signal. BPS would do the same with the array contactor.

Problem

This both was inefficient as the software wait was much longer than the actual charge time for safety. Additionally, there was no actual check for if the voltage had been equalized or if the contactor had been closed or not. This lead to a situation where current was being drawn before the precharge contactor had been closed, leading to a high current draw through the precharge resistor, resulting to it getting burnt out.

Research

• Wisconsin Racing’s Documentation: https://www.wisconsinracing.org/wp-content/uploads/2024/05/Release-of-223E-Electrical-Designs.pdf 

  • Description: “The Precharge circuit board is a completely non-programmable circuit board that houses our Precharge circuitry and HV Indicator LED circuitry. The Precharge makes use of a dual channel comparator to monitor the HV bus voltage output to control both circuits. The Precharge board is sectioned into two sides, the left side holds all the Low Voltage circuitry, and the right side holds all the high voltage circuitry. The two sides are separated by a visible air gap, and only cross with an optocoupler and the precharge relay.”

  • Circuitry

    • The comparator uses COMP OUT ( 0.99% of the output bus voltage) and COMP IN (90% of 1% of the input voltage) to decide when to stop the precharge.

    • The output of the comparator is placed on the low side of the optocoupler, so that current can flow through the optocoupler to the comparator when COMP IN = COMP OUT, signaling the end of precharge.

    • By the time the precharge is completed, the component being charged will reach approximately 90% of the maximum battery pack voltage.

  • HV Indicator LED Circuit: 

    • Designed to give a visual signal to a person monitoring the circuit. When the high voltage bus reaches a specific threshold, the LED lights up.

• TI’s active precharge: https://www.ti.com/lit/pdf/tiduf21 

  Open

  

  • Summary of design:

    • The comparator measures the voltage across the shunt resistor and compares it to the high (VH) and low (VL) thresholds. If the voltage is below VL, the comparator turns on the FET, allowing current to flow through with minimal resistance and gradually charge the capacitor. When the voltage exceeds VH, the comparator turns off the FET, stopping the current flow to prevent overcharging and ensure safe operational conditions.

  • Isolation and Power Management:

    • TPSI3052-Q1: Acts as the primary switch driver, managing power transfer and supplying necessary voltages to other components.

    • VDDP, VDDH, VDDM: Various power supply rails derived from TPSI3052-Q1 to power different sections of the circuit.

  • Control Logic:

    • Comparators (TLV7011): Monitor current through the inductor via a shunt resistor (RSHUNT) and provide hysteresis to control the FET (M1) switching. Push-Pull output to not have an intermediate state.

    • UCC27517A-Q1: Serves as a high-speed gate driver for the FET. FETs require a significant amount of current to charge and discharge the gate capacitance rapidly.

  • Inductor and Capacitor

    • L (Inductor): Stores energy and smooths current fluctuations during the precharge cycle.

    • C (Capacitor): Represents the load capacitance that needs to be precharged.

  • Switching Elements

    • M1 (FET): Acts as the main switching element controlled by the comparator outputs.

    • D1 (Free-Wheeling Diode): Provides a path for inductor current when M1 is turned off, preventing voltage spikes.

Solution

UT Austin’s Precharge Circuit:

1. Battery Contactor closes

2. Compare Battery Voltage at Node B (Nb) and voltage at Node M (Nm)

3. Once Nm = 0.9 Nb, close Motor Precharge Contactor.

   1. (When Nm is determined to be 90% of Nb, they are supposedly close enough in their voltages to safely close the contractor according to UWisconsin)

   2. Determine later if 90% is something we agree with (need more research)

Open

1. Voltage divider gets 1/111 of the battery voltage, and another voltage divider gets 1/101 of the motor voltage. 

2. Voltage values get sent to a comparator op-amp where the battery voltage is sent to the positive input and the motor voltage is sent to the negative output. This op amp is provided with a 3.3v voltage source. If the motor voltage is greater than the battery voltage (aka the motor voltage is greater than 91% of the battery voltage), then the comparator will output a signal voltage of 3.3v.

3. [unsure if the optocoupler is really necessary] That signal is sent through the optocoupler to the low voltage part of the circuit board. 

4. That signal will get sent to a nmos. The nmos is connected on the drain side to the contactor connector which has a flyback diode and the connected to a 12v source on the other side. The 12v source is provided by the supplemental battery. The nmos is connected to ground on the source side. 

5. This will provide the contactor with 12v and up to 4A if the signal is high. Once the contactor closes, the voltage over the precharge resistor becomes zero and the motor system will be connected to the battery.

Problem: Edge Cases

Just using a simple comparator will work theoretically when there is just one contactor that will connect the battery to the motor. However, in the actual HV diagram, there are several other contactors and factors that have to be considered. For example, prior to closing the battery contactor, the voltages on both sides of the precharge resistor are the same. This may cause the precharge board to close the contactor in certain cases. Then, if the battery is connected, the inrush current will bypass the precharge resistor directly through the contactor.

To solve these issues, we decided to add a few failsafes including an nmos that takes HVref as the gate signal. This prevents the case where voltage on both sides is zero. Additionally, hysteresis is used to prevent a rapid switching on and off when the reference voltage crosses the threshold on the comparator. We compiled the following table on the situations for every single contactor and the possible states they could be in. 0 indicates open, 1 indicates closed, and MP+ and AP+ are the indicated future state of motor precharge and array precharge.

The highlighted sections are not ideal. Therefore additional design considerations have been made. Unifying all the contactors onto one contactor driver board allows for precharge decisions to include the contactor states. Additionally, software using a nucleo can communicate with controls and BPS to make decisions.

Board Connectors:

• 3 pin - motor voltage sense connector

  • Motor Hin, Motor HRef, HVGND

• 3 pin - Array voltage sense connector

  • Array Hin, Array HRef, HVGND

• 2 pin - LV Power

  • LV+, LVGND

• 4 pin - Motor Precharge Contactor

  • LV+ (drive contactor), LV+ (sense?), Precharge sense, precharge con-

• 4 pin - Array Precharge Contactor

  • LV+ (drive contactor), LV+ (sense?), Precharge sense, precharge con-

• 4 pin - Motor contactor

  • LV+, LV+, contactor sense, con-