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Rev1 Testing

During testing, multiple issues were found with this design. Most notably the P-Ch MOSFET Q1 had the drain and source wired in reverse, and the latch latched high in the beginning. This is an issue as initially when the HV battery is off and power supplied to the board, pre-charge will close the main relay as it is currently configured. To fix this, it is proposed that an AND gate be added to the comparator output and the negative input of the comparator. This way, the output will only be high if the comparator is high AND the battery relay is closed.  

Hysteresis

By observing the opto-isolator output, it was observed that during the transition from high to low to close the main relay, it bounced between the two states multiple times for more than a second. This was remedied by adding a positive feedback between the comparator output and Pins 5/6 as recommended in Figure 1. As seen in the plot below, bouncing was significantly reduced with 240mV below 12V of positive feedback. 

From this, we can also see that after adding hysteresis, switching takes almost double the time. This can be remedied by either unbalancing the resistor divider and moving the switching point earlier, or experimenting with different values of hysteresis for different voltage thresholds. Because the unique way in which it is implemented as recommended by TI, adding trim potentiometers and slightly unbalancing the resistor divider is the easier and more reliable solution. 

NAND gate

Because the positive input of the comparator is set to take in the motor controller side of the pre-charge resistor, this means the comparator goes from a logical low to logical high when pre-charge is complete. However, if the comparator is powered but the inputs to it aren't, such as when the battery relay is open, then it goes into a logical high state as well. This results in the output going from high → low → high. To solve this, an AND gate was added during testing with the inputs being the battery side of the comparator input, and the output of the comparator. This way, the output will only be high if the battery relay is closed, pre-charge relay is closed, and comparator output is a logical high. 

We can see that in Fig. 1, the comparator goes from high → low → high as described earlier, but with the AND gate it remains logical low. Also, currently the AND gate goes to a logic high level after ~3.6 seconds, which is when the capacitor is at 29.6V, 97% of the 30.4V applied to it. This can be further adjusted by the trim potentiometers installed, it may be better if different values of trim pots or the possibility of adding resistors in series/parallel to it was implemented. For example, a 0 Ohm resistor can be placed in series normally, but if we deemed it necessary to have the pot at say 5.25k, then we can put a 5k resistor in place of the 0 Ohm, and use a small valued trim pot for fine adjustments. This can be helpful as every extra 1-percent pre-charged means 1.5V less differential between the battery and the motor controller at 150V, however it should be noted that since capacitors follow a exponential function there will be diminishing returns after a certain amount. 

Now since the opto-isolator we are using inverts the logic, it makes sense to use a NAND gate instead. This way, output from the opto-isolator will be the same as Fig. 3. above, and can then be latched with the SR Latch. 

Changes for Rev2

  • Fix diode silkscreen on the bottom of the board
  • Remove R1, replace R2 with a smaller value for more current into comparator
  • Q1 should have 1/2/4/6 and 4 swapped 
  • Ultra-fit and Dura-Clik connectors instead of Clik-Mates
  • Add passthrough for relay sense line (shorted when relay is closed, open circuit when relay is open)
  • Fix voltage drop with MOSFET stages
  • Add hysteresis to avoid bouncing 
  • Add an NAND gate between negative input and comparator output
  • Try to increase voltage going into opto-isolator. Around 2V right now due to voltage division
  • Consider using non-tented vias or through hole test points. Some TPs were ripped off
    • Also don't put TPs directly on a trace as if it falls off it takes the entire trace with it 
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