Purpose
To facilitate pre-charge during car power-up sequence to prevent a large in-rush current, which can damage boards.
Location
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Located inside battery box. Note that resistor parallel to switch is not on the motor switch board but is rather on MCI to allow for back compatibility with a relay.
Schematic
MOSFET Selection
MOSFET requirements:
MOSFET(s) resistance (Rdson) < 2mOhm
MOSFET must be normally closed (enhancement mode)
MOSFET must withstand 60A max current from source to drain
MOSFET must withstand 160V max voltage across source to drain
Minimize cost
Minimize # of fets required in parallel
MOSFET selected: EPC2304ENGRT
Digikey Link: https://www.digikey.ca/en/products/detail/epc/EPC2304ENGRT/16785484
Datasheet: https://epc-co.com/epc/Portals/0/epc/documents/datasheets/EPC2304_datasheet.pdf
Spec | Value |
|---|---|
Max Drain to Source Voltage, Vds (Continuous) | 200V |
Max Continuous Current, Id (@ 25C) | 102A |
Max Gate to Source Voltage, Vgs | 6V |
Typical Threshold Voltage, Vgs(th) | 1.1V |
Drain-Source On Resistance, Rdson | 3.1mOhm |
Cost/Unit | $11.98 |
This fet meets max voltage and current requirements. Put two of these fets in parallel for 1.55mOhm of resistance for $23.96.
Optocoupler Selection
ASC regs requires electrical isolation between low voltage (LV) and high voltage (HV) systems. As we need the MCI (LV) to send the the signal to close the switch, we can use an optocoupler to isolate between it and the Motor Switch (HV).
Voltage to gate will be provided by HV (up to 160V). This will be decreased from 160V to 5V to not fry the MOSFET using Voltage Divider Rule with two resistors, 62kOhm and 2kOhm. Using Ohm’s Law, 160V/64kOhm = 2.5mA.
In an open state, the voltage across the transistor portion of the optocoupler can go up to 160V, so it must be able to withstand that.
Optocoupler Requirements:
Transistor max voltage (Vceo) > 160V
Transistor max cont. current (Ic) > 2.5mA
Minimize cost
Optocoupler Selected:
Digikey: https://www.digikey.ca/en/products/detail/toshiba-semiconductor-and-storage/TLP388-GB-TPR-E/7644160
Datasheet: https://toshiba.semicon-storage.com/info/TLP388_datasheet_en_20220518.pdf?did=35921&prodName=TLP388
Spec | Value |
|---|---|
Max Collector-Emitter Voltage (Vceo) | 350V |
Collector Current | 50mA |
Cost/Unit | $1.18 |
This optocoupler meets max voltage and current requirements and is relatively cheap.
Conditions
Condition 1: Pre-charge not ready
MCI does not light the LED inside the optocoupler, the transistor remains open
Condition 2: Pre-charge finished
MCI lights LED inside the optocoupler, the transistor closes and the MOSFETs allow current to pass through
Note: MOSFET resistance is negligible, effectively shorting the 680Ohm resistor.