Corporate666
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So wait, are you saying that you think the spike is killing the control IC itself (through its supply voltage) or just the FET? If it's through the supply voltage, then solution is easy; just supply the chip itself through maybe a 10 ohm resistor and then a good bypass cap. That will damp any spikes without creating losses in the actual power path.Thanks for all the replies. First off, obviously I am not an engineer by education, so I am learning as I go, but very happy to learn, even (and especially) from my mistakes
As for how I know - well, I was having some issues with another boards last year with Alpha Omega switcher regulator failures, so I contacted them and they did a failure analysis on a few of the regulators, and said the internal FETs had failed. My boards worked fine for their engineer, so we focused on what was different and zoomed in on the power supply. Putting a scope on the input showed a voltage spike. Adding a TVS to the board solved the problem.
This time around, a TVS was already on the board. I put a scope on the input to the switcher and I see a 22 volt spike that lasts around 20us when the power supply is connected. The regulator has a Vin Max of 18V, so I assumed this was what was causing the problem. What I am unsure about is that I have a TVS with a 17.5V breakdown voltage that is connected between Vin and ground right where power enters the board. So I am not sure why I would be seeing a 22V spike in the first place. It's a bidirectional TVS. I checked the obvious, it starts to conduct at 17.45V in one direction, 17.6V in the other.
I realize that a TVS alone is not sufficient protection for connecting this circuit up for automotive use, but I am stuck as to why the regulators are failing in testing, and if it's the voltage spike, I am not sure why I am even seeing that spike with the TVS there.
I haven't put a resistor in line with the power input and checked the inrush current, I will do that next. The engineer at AO Semi said this particular switcher has internal soft start, and that inrush current should not be an issue. I have also adjusted the code in my microcontroller to delay when the LED's are turned on, and the failure seems to happen whether there is any load on the switcher or not.
10 ohms is a bit large given your power requirement
Regards, Orson Cart.
Oh wow your right, I wasn't aware the the chip supply and the FET input were shared by one pin. I've never seen that in a buck IC before. If that's the case, I would ditch the chip entirely; that's just really, really poor design.Surely the chip you mention - AOZ1021 - has only one Vcc connection? not a seperate one for power and another for the control part, or are you considering another IC entirely?
Regards, Orson Cart.
it appears they have a zener on the base setting the clamp voltage... and another on the output of the protection circuit. I am unsure what this second diode is for, other than simple reverse hookup protection, but I am unsure why it would be on the emitter side of that NPN. Maybe I am missing something obvious?
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