Author Topic: Strange interaction between multiple buck converters  (Read 469 times)

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Offline dcarr

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Strange interaction between multiple buck converters
« on: June 28, 2018, 08:01:25 pm »

I recently designed a 28V->12V 15A buck converter that lives on a 28V bus inside a vehicle.  I designed it myself because it needs to meet strict EMI requirements and the off the shelf units I purchased failed miserably.  Anyway...

The converter seems to work great except for one thing---there are two other pieces of equipment on the bus which contain ~30W buck converters and when those buck converters first start up, it causes my converter to dip down <10V for approximately 500ms.  Interestingly, the main 28V bus doesn't sag much at all during this time so there must be some interaction between the other converters and mine.  Perhaps when the other converters are charging their own bulk capacitance they present a load to the bus that causes my converter to become unstable?

Has anyone run into something like this before?  It's a bit difficult to get a scope out to check the signals further, but that's on my to do list.

Thanks for any ideas.
 

Offline jmelson

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Re: Strange interaction between multiple buck converters
« Reply #1 on: June 28, 2018, 08:06:46 pm »
Are these other converters fed off the same "branch", ie. on the same fuse or breaker as yours?  They may be causing a sudden dip in the 28 V seen at your converter input.

I think you may just have to lug a digital scope out there so you can observe the transient.  My GUESS, and it is just  a guess, is that there is a long run of wire from some main panel to your (and the other) devices, and when they are turned on, the resistance (and possibly inductance) are causing a big sag in the voltage.

Jon
 

Offline T3sl4co1l

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Re: Strange interaction between multiple buck converters
« Reply #2 on: June 29, 2018, 07:51:56 pm »
You were so preoccupied with emissions that you didn't check susceptibility? :P

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 
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Offline David Hess

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Re: Strange interaction between multiple buck converters
« Reply #3 on: June 30, 2018, 02:49:00 pm »
You might check how much input capacitance the various switching regulators have.  Adding some bulk input capacitance might be enough.

If these are non-isolated buck converters, are their outputs grounded creating a ground loop?

Measuring the input voltages with an oscilloscope will rule out some possibilities.
 
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