Precharge Validation - MCs instead of Capacitor Pack

TL;DR: The precharge sense resistors need to be 2kohms or less total for it to work with our motor controllers. If both motor controllers are in parallel, and we only use one motor controller interface (which is the plan) then I would even suggest that we target less than 1kohm of precharge resistance instead. Note: James said we plan to use 300 ohms for the next revision anyways, so we should be good.

Note that I am defining EPR as Equivalent Parallel Resistance even though this is not a standard term but is the short way of me saying the impedance that results in leakage current.

Precharge had been verified to work with a film capacitor pack with equivalent capacitance of about 520uF (4 paralleled 130uF film caps), but had run into issues executing when connected to the motor controllers. What would happen is that the precharge circuit would not conclude with the latch out closing the relay and ending the precharge sequence. It however, worked flawlessly with the capacitor pack, so some debug was done to see what was going on.

First, the at home setup was done such that a power supply set to 50V was connected to the relay input and the HV in of MCI. Then the HV ground was shared with the motor controller, MCI and the power supply. The relay coil was also connected to MCI, and 12V in was going to the board. The MCU was programmed to enable PA9 because this was a version of MCI that uses only one load switch for the isolated 12V supply.

I tested again to make sure the problem was replicated, which it was with the motor controllers, and it continued to work with the cap pack. I tried connecting the cap pack in parallel to the motor controller to see if there would be a difference with the precharge actually working - there was no change, it still would not reach completion. One problem I was facing was that the surge of current caused by connecting the probes of the multimeter on the probe points of MCI would often cause the latching to happen and end precharge abruptly, also not allowing me to properly measure in+ or in-.

To deal with this problem I opened the motor controller and used the large screw terminals as probe points to measure the voltage of the caps as precharge started. I noticed that with the 50V input, the capacitors would charge to 45V then stop. The schematics state how precharge ends at 95% completion, which means that 45V is out of bound. It seems that the motor controller capacitor charge profile reached that number as an upper limit, so the question is why and how is it different from the cap pack that charges to almost full 50V?

I theorized it had to do with the max charge that could be output from the series resistors, so I shorted two of the 1k series resistors to only have 1kohm series resistance on precharge to the motor controllers. Testing it with this modification made it latch almost immediately (and finish precharge)! So it seems that the series resistance of 3kohm was too high for latching to ever conclude. I changed it to 2kohm series resistance (by shorting only 1 resistor) and found it latched in a reasonable amount of time which was a bit under 2 seconds. The next step was to analyze why this might be happening which is what I believe is the reason and is explained next.

Basically, the biggest difference between our cap pack and the motor controller caps is that there is extremely low impedance on those caps in the motor controllers. At first it didn’t really make sense how this might impact the precharge state, but I then outlined it in a few diagrams. Since the motor controllers use about 48 paralleled 4.7uF (after further discussion, this is actually 48 paralleled 10uF caps in series with another 48 parlleled 10uF caps) ceramic caps (see Diagram 2), they have super low ESR(EPR) and a capacitance of about 230uF, compared to the film capacitor pack I was testing with. The connection of the precharge circuit and the bulk capacitance is shown in Diagram 1 part A. I then found that if we have the ESR(EPR) of the cap at a certain value (20kohm for example) we would get a certain voltage that the caps could charge to (see Part B). If we decrease that ESR(EPR) of the bulk caps because of their extremely low impedance, we find that using 10kohm for example decreases the max voltage that the caps can charge to (y variable in the diagram) - See part C. So to counteract this low ESR(EPR), we can decrease the value of the precharge resistors - such as make them to 1kohm instead of 3kohm as done in Part D - and find that the max voltage the capacitors can charge to is higher to compensate. The reason this is important is because the precharge circuit will measure the in+ node (the 50V node in the diagram) with the in- node (the y node in the diagram) to make sure that precharge is within a range of being over. This resistor divider being formed with the bulk capacitance ESR(EPR) to ground is probably the reason for failures thus far, and therefore we must decrease precharge resistance to 2kohms when testing with one motor controller. When we parallel two motor controllers on the output of precharge, I would suggest limiting precharge resistance to 1kohm to compensate (due to the paralleled ESR(EPR) which should be half the resistance total).

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