Electronics > Projects, Designs, and Technical Stuff

High power current sense resistor resistance/voltage drop during inrush current

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Tomorokoshi:
Do you have a way of determining the frequency response of your current sense amplifier?

Can you also measure the Vo / Vi transfer function of it?

matbob:
Are the gate drive currents for the low side MOSFETs flowing through the sense resistors?

aiq25:

--- Quote from: Tomorokoshi on February 18, 2019, 01:41:47 am ---Do you have a way of determining the frequency response of your current sense amplifier?

Can you also measure the Vo / Vi transfer function of it?

--- End quote ---
I really don't have a way of determining the frequency response because this is part of a much larger circuit. I will try to do it in the future though.

I measured the voltage right across the sense resistor. The amplifier feeds into a micro. Under normal operation the amplifier is measuring correctly and the gain is correct. I have not looked at the output during an inrush because I was just measuring across the sense resistor.


--- Quote from: matbob on February 18, 2019, 03:55:08 am ---Are the gate drive currents for the low side MOSFETs flowing through the sense resistors?

--- End quote ---
Yes. The H-Bridge is driven using an IC and the ground of the low side FET is tied to the ground of the sense resistors.

T3sl4co1l:
What dI/dt is this inrush, and what's the layout look like?

Related question: can this inrush be much more than the circuit is designed for?  Are you not limiting it with a current mode controller?  You've got a whole H-bridge there, with current sense -- it would be a shame to throw away such an excellent opportunity. :)

Tim

ajb:
We're all just guessing right now, we need a lot more information to provide any useful advice. 

One thing that jumps out is that if the current sense resistors total 200-300mOhm (not 0.2-0.3mOhm as originally stated), that means that at 15A the source of the low side switches will be at 3-4.5V, which, depending on the gate drive situation, could mean you get "free" current limiting, or with enough d/dt and the right parasitics, something that rings like a bell and blows up. 

Can you show the layout, some scope traces, and ideally a better schematic?  Are these "inrush" events (really, repetitive pulses from what you've described) caused by the load itself, or a result of the supply fluctuating, or are they induced by the way you're driving the load?

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