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

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MOSFET Selection

MOSFET requirements:

• MOSFET(s) resistance (Rdson) < 2mOhm (in order to have less power loss than just using the EV200HAANA relays)

• MOSFET must be normally closed open (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

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This fet meets max voltage and current requirements. Put two of these fets in parallel for 1.55mOhm of resistance for $23.96.

Optocoupler and Zener Diode 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 MOSFETs on the Motor Switch (HV).

Voltage to gate will be provided by HV (up which can range from 90V 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. To ensure Rdson of the motor switch will be lower than that of a relay, it is ideal to maintain a relatively consistent Vgs in order to maintain a consistent Rdson. This constant voltage should be maintained at ~5V, given that the max Vgs of the MOSFETs is 6V.

Spec’ing Optocoupler

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.

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• Transistor max voltage (Vceo) > 160VTransistor max cont. current (Ic) > 2.5mA

• Minimize cost

Optocoupler Selected:

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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

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Condition 2: Pre-charge finished

MCI lights LED inside the optocoupler, the transistor closes and the MOSFETs allow current to pass through

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Note: MOSFET resistance is negligible, effectively shorting the 680Ohm resistor.Spec’ing Zener Diode

The minimum voltage 90V from the battery pack will be decreased to ~5V using Voltage Divider Rule with a 4.7MOhm and 274kOhm resistor. In the maximum voltage possible of 160V, this will provide too much voltage to the gate and fry the fet, so a zener diode spec’d for ~5V will be placed connected to gate and source to prevent Vgs from exceeding ~5V.

Zener Diode Selected:

Digikey: https://www.digikey.ca/en/products/detail/on-semiconductor/MM3Z5V1ST1G/661671

Datasheet: https://datasheet.ciiva.com/10630/mm3z2v4st1-d-10630224.pdf?src-supplier=Avnet