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Other Testing
Playing with the shutdown pin does allow us to start perfectly when in the restart loop, however, trying to use the valid outputs did not work (possibly due to the resistors values on the gate and the pullup on valid).
Adding the extra capacitance for valid time delay on the AUX does work.
Inrush Current Testing
So I’ve been doing a little more testing with the inrush current required to charge the capacitors. The aux battery is being used as the voltage source, and using the Keysight clamp meter, wrapping the wires around multiple times to get more resolution, and yes, the results below are the actual values (not the value shown on the screen) taking into account the number of turns.
On the DC-DC Input:
Turning On Properly: 1.7A
Stuck in Reset Loop: 0.65A
On the AUX Input:
Note that I have replaced the UVLO time delay cap with a 1.1uF capacitor (2x 2.2uF in series since I didn’t have any 1uF lying around) to increase the delay time to approximately 120mS according to equation 6 in the datasheet.
Turning on Properly: 7.5A
with the capacitor change, the AUX no longer goes into any reset loop.
Holding Artificial UV condition
In this test, I was attempting to see where the inrush current is mainly going - is it to charge the decoupling cap on the input or the bulk capacitors on the output?
DC-DC Input
The top side of R44 (UV threshold) was held LOW and the inrush current was re-rested using the same setup.
We measured about 1.8A.
Once removing the artificial UV condition, the chip started up immediately (the first time). Then if recreating the artificial UV and removing it with the board still powered, then we enter the reset loop again.
Inrush current during restart loop: 0.7A
Remember, however that this is the inrush current measured by the current clamp, and must be sustained for approx. 1ms. Looking at it scope, we can see that the spike happens much quicker, on the order of 10uS.
Aux Input
Holding the UV on Aux, we measure about 1.8A inrush current.
When releasing the forced UV, we measure another 7.5A of inrush current.
Other Testing
Playing with the shutdown pin does allow us to start perfectly when in the restart loop, however, trying to use the valid outputs did not work (possibly due to the resistors values on the gate and the pullup on valid).
Adding the extra capacitance for valid time delay on the AUX does work.
So we are still getting too much of a voltage dip on the DC-DC input. So I tried a few other things.
Adding Extra capacitance to the input, we still get a large voltage dip:
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Changing the location of the DC-DC voltage measurement so that its not directly under the FET and is out of the way of the high current inrush current path from the DC-DC capacitor to the Vout capacitors:
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Removed one of the capacitors from the Vout, to get a total of 100uF instyaed of 200uF:
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And the DC-DC UV measurement pin in the same conditions to show the undervoltage fault:
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DC-DC supply Cable resistance: 50mOhm
Measuring the current with an oscilloscope:
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We get a value of 0.9 V / 0.05Ohm = 18A
This current is not out of the range of possibility. If we look at all the resistances in the current path, we get a surprisingly high value:
battery IR + GND cable R + PWR cable IR + fuse cold resistance= 0.18+0.04+0.05+0.05 = 0.32 Ohms
The cables and fuse cause 0.32Ohm * 18A = 5.76V of voltage droop on the input - not too good.
The voltage drop from 13.8V down to 8V has a magnitude of 5.8V. 5.76V of which is accounted for by the cable’s resistance. The rest could come from parasitic inductance, other resistances, but most likely inarccurate measurements of the resistances. So we have a few options that will all help:
Extend the UV fault time so that the inrush current passes without a fault
Add a capacitor to the UV fault pin like the AUX inpt has
Reduce the inrush current from the battery
Increase the capacitance for V1, V2, V3 on the Power Selection board
Decrease the Vout capacitance
Reduce the slew rate on the Gate of the FETs to achieve a slower turn-on (like the AUX)
Force a soft start condition by pulling Vout low or manipulating shutdown
Add slew rate limiting circuitry (diode, cap, resistor)
Reduce the resistance between the power source and the power selection measurement
Relocate the measurement point
Create the better cables (instead of 50cm 18AWG with 2 connectors)
I tried this, and it did not help much. The fuse causes the bulk of the resistance.
Find a lower resistance fuse (might be tricky)
Looking at the transition on the AUX, we see that it takes a lot longer (100uS) and thus reduces the inrush current.
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So I decided to continue with implementing the gate slew rate limiting on the DC-DC input as well. I put a 5nF capacitor from the gate to VBAT and a 2.2k resistor in series with VG2, values chosen according to what I had on hand and what was similar to the AUX values.
Below is the result
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As you can see, we successfully have our lower voltage droop and thus less inrush current.
I have still been able to get it stuck in the reset loop when the DC-DC input is at the low end of its voltage range, with the little bit of inrush current there is causing it to dip below the threshold. The DC-DC is regulated, so it will not be in this range, and the AUX will be monitored before we hit it with high currents. But overall, we just had too much current and too much resistance. So was all this testing worth it for a problem that we might not have actually encountered in the car? From a time perspective, maybe not, but from a learning perspective, definitely!