The precharge circuit runs entirely based on hardware (i.e. no firmware is required).
Fig 1. Pre-Charge HV Sch - HV line for pre-charging
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- MOSFETs
- N-channel vs P-channel
- Parallel MOSFETs
- Power resistors vs normal resistors
Fig 2. Pre-Charge HV Sch- power resistors Fig 3. Pre-Charge Logic Sch - comparator
Fig 1 shows the power resistors used to limit the current when pre-charging with two voltage dividers connected to either side. The voltage dividers connected to the left and right sides of the power resistors are used to compare the voltage of the battery and motor controllers and set to reduce the 150V from the HV line to 9.39V on the battery side and 9.84V on the motor controller side. Basically, if the pre-charge circuit is on, IN- will be 9.34V, and IN+ will rise from 0V to 9.84V. Aside from lowering the voltage, the resistances used in the voltage dividers are set such that when the motor controller reaches 95% of the voltage on the battery side, IN+ will be greater than IN-.
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- Op-amps and op-amp comparators
- Negative feedback and positive feedback
Fig 3. Pre-Charge Logic Sch - AND gate Fig Fig 4. Pre-Charge Logic Sch - SR Latch
Using only the comparator shown above presents another problem whereby the comparator may output a logic high when the op-amp is powered but its inputs aren't, such as when the battery relay to the pre-charge circuit is open. The AND gate solves this by taking in the comparator's input as well as a reading from IN+, or the motor controller side of the pre-charge circuit. This way, the AND gate will only output a logic high if the comparator signals pre-charging is finished, and the motor controller is indeed charged. The MOSFETs act as a resistive switch converting the IN+ reading to something that matches the comparator's logic high.
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- Fundamental gates (AND, OR, NOT, etc.)
- SR Latches (go to the datasheet for logic diagram and truth table)
Fig Fig 5. Pre-Charge Logic Sch - Optoisolator and MC Contact Driver Fig 6. Motor Interface Pcb - Optoisolator (U5)
The first IC on the left of figure 5 is an optoisolator (or optocoupler) used to interface between the HV and LV sides of the board. You can see this in figure 6, the optoisolator spans across a gap where the HV side (top) is isolated from the LV side (bottom). Following the latch previously mentioned, when ISO_LATCH_OUT outputs a logic high, the MOSFET (basically a switch) is closed connecting the cathode (CAT) pin to ground. This connects the circuit from the anode (AN) completing the circuit and powering the GaAsP LED inside the IC.