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I used the clamp meter in peak mode to get an estimate of the inrush current since I don’t have an appropriate shunt resistor here.
Peak current observed was 2.2A (with a 13.5V supply and a current limit on the power supply of 5A).
This seemed like it was high than Liam’s specs, so I tested with the oscilloscope, using the power supply cable as the shunt resistor (measuring the voltage at both ends of it). The cable has a voltage drop of 28mV on a 2A current draw, and the peak current measured was about 8A. Note that this only lasted about 3uS. I am not confident in the oscilloscope measurements since we are dealing with inrush currents and thus EMC - and my test setup included long leads for the oscilloscope cables which increases the error. I propose that we find a better way to measure inrush current - we need a board or adapter that we can attach to any power supply (or ultra-fit/micro-fit connector) and oscilloscope to measure the inrush current easily, without much setup.
The kill switch operation was also checked at the point, and it works as expected. The relay opens when the kill switch is pressed. The circuit is so simple, so I didn’t expect any issues here.
Inrush Current Revisited on May 14, 2020
Funny how implementing some of the industry ‘best practices' tend to give you more accurate results - they’re not just best practices for a reason - most of them are based on fundamental laws and relationships in electronics. In this case, we were dealing with the rate of current rise, controlled directly by inductance. So the goal with the measurement is to minimize the inductance of your measurement setup. In this case, minimize the loop area of the GND return path to the oscilloscope. To achieve this I did 2 things:
Created a short GND return path for the oscilloscope probe (as opposed to the alligator clips) by using a piece of 22AWG solid core wire and twisting it around the barrel of the scope probe:
In order to make use of this, I had to provide a resistive element which also had a low inductance - which a current sense resistor is great for! In this case, I used one from a cheap 4S BMS that I got off Aliexpress a long time ago (the BMS itself is terrible). I would expect probably a 5% tolerance or so on the resistor given how cheap the module was.
And we got some much better results:
Below we see the inrush current across the 5mOhm shunt resistor. Ignoring the super high frequency transient at the 0us mark (which is likely cause due to wire inductance, and is just a measurement artifact) we see at voltage at cursor A of 4.6mV. Using Ohm’s law, I = V/R = 4.6mV / 5mOhm = 0.92A. Notice that we also see the current dropping in somewhat of an exponential manner, as we would expect for an RC circuit. I am much more confident in this result than with the earlier testing. The second peak, at cursor B, is just over 2mV (the actual peak is hidden behind the cursor line) which gives us 0.4A. This could be where the precharge circuit activates the bypass FET, but it does seem a bit early for it to kick in given the 10mS designed target. This testing was done activating the relay on a 10s period with 5s off time, so the capacitor on the gate of the FET might not have fully discharged before activating again. Either way, the current is within an acceptable range.
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How is this ‘acceptable range’ determined?
The purpose of limiting inrush current is to limit the supply voltage droop at the input to the board during these high current moments. Too high of a voltage droop, and you could risk resetting sensitive measurements or the MCU, which we certainly don’t want to happen on the car. We have had issues in the past relating to inrush current with the Power Distribution Board Rev 1.0 in the power selection section (later moved to its own board) which caused reset loops.
The operating range of our AUX battery is anywhere from 9-15V or at least around there (its 10S 1P NiMH cells). As long as our supply voltage does not dip below 9V, we should be fine. We will later implement a limit in firmware to determine if the AUX has enough charge left to provide the inrush current and not droop below 9V, causing the car to reset (we had issues with this in MSXII as well). Let’s take a look at the graphs.
Powered from a Power Supply, with V = 13V
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Powered from the AUX battery, V = 13.2
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With either supply, the voltage droop measured at the input connector of the BMS Carrier is below 2V. This voltage droop test should be re-run with the contactor connected and measure the voltage droop when switching the contactor (almost a 4A load) as that load is larger than the inrush current observed. So this is all good from the perspective of the capacitor inrush current.
Enable Relays
Pins set according to instructions (PB9, PB3, and PB4 all HIGH)
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Now to reconnect the isoSPI ICs and test them.
The 5V converter seems to be turning ON by default… Turns out that in the code I’m using the default state for initialized pins is high, so no issue here.
My firmware knowledge if very limited at the moment, so I will wait until I learn to generate a clock pulse or just jump straight to the AFE validation and hope it works.
Testing out the AFEs - we got them working, so isoSPI seems to work well!