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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

Figure 1 shows the high voltage side of the pre-charge board. HV_IN is where the circuit is connected to the battery. The two MOSFETs connected in parallel act as a switch which connects our HV line to the motor controllers. The MOSFETs are put in parallel because parallel MOSFETs will balance out, distributing the current a bit and overall handling the load better. This switch will also disconnect the pre-charge circuit once the pre-charge is complete. The line contains four power resistors before the motor controller each rated at 100W, limiting the inrush current. After the motor controller is two resistors in parallel which act as discharge resistors. 

Things to research:

  • 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-.

Fig 2 shows an op-amp comparator taking in IN+ and IN- as its non-inverting and inverting inputs respectively. This comparison will output a logic high when IN+ is greater than IN-, effectively signaling that the motor controller has reached 95% of the battery's voltage and pre-charge is complete. The op-amp is set with a positive feedback loop to prevent the comparator output from bouncing between logic high and logic low which may happen when the circuit is transitioning from a logic high to a logic low. The positive feedback latches the output to a logic high and makes it so there must be a significant difference between IN+ and IN- to switch the output. 

Things to research

  • Op-amps and op-amp comparators
  • Negative feedback and positive feedback

             

 Fig 3. Pre-Charge Logic Sch - AND gate          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. 

An SR latch (the IC in figure 4) is placed after the AND gate in order to latch the output of the AND gate. So long as the enable (ENA) pin is powered (which it always is in accordance with ISO_12V), the latch works like a normal SR latch with S4 and R4 corresponding to set and reset for the output Q4. When S4 receives a logic high, Q4 will be connected to Vdd, gaining a logic high as well. A capacitor is placed between the reset pin and its source to keep the reset input low for a short amount of time. 

Things to research:

  • Fundamental gates (AND, OR, NOT, etc.)
  • SR Latches (go to the datasheet for logic diagram and truth table)

            

       Fig 5. Pre-Charge Logic Sch - Optoisolator and MC Contactor                                  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 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. When this happens, the Vo will output a logic high and close the MOSFET to the contactor driver. This signals the relay connecting HV directly to the motor controllers to close. 

Things to research: 

  • Optoisolators (and the things involved)
  • Isolating and interfacing HV and LV circuits

              

Fig 7. Pre-Charge HV Sch - Pre-charge switch off

Figure 7 shows the port for another output of ISO_LATCH_OUT taken from the SR latch. This controls the 2 N-channel MOSFETS in the HV circuit which in turn controls the P-channel MOSFETs connecting the pre-charge resistors to HV. When ISO_LATCH_OUT outputs a logic high, the first N-channel closes, which opens the second N-channel. This closes the paired P-channel MOSFETs disconnecting HV. 

        

                           Fig 8. Pre-Charge Logic Sch - Isolated DC/DC                                                                      Fig 9. Motor Interface Pcb - DC/DC

The isolated side of the board gets power through the isolated DC/DC, which connects the isolated and non-isolated side. The IC can be seen in the PCB layout as U9, spanning to the isolated side. 









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