Electronics > Projects, Designs, and Technical Stuff
High power current sense resistor resistance/voltage drop during inrush current
aiq25:
Hello. I'm using two 150mOhm current sense resistors in series to measure low side current of an H-Bridge. During normal operation my current is below 1A (voltage on the high side > 150V). During inrush events currents can be as high as 15A. I'm seeing something strange: during an inrush the voltage across both of these resistors is about 1V lower than it should be, according to a current probe placed on the output of the H-Bridge. I'm measuring the voltage across the two ends of the current sense, not individual resistors.
In short, I have a current probe on the output of the H-Bridge which measures the current to be 2-3 times greater than what the current sense resistor is measuring. I measured the voltage drop by directly connecting leads to the resistors. I have verified this is the correct voltage reading by using different methods and oscilloscopes. I have two theory: there is some going on in the PCB that is causing this OR the resistance of the current sense resistors is dropping during these high current peaks.
The strange thing about this is that by reading lower current through the sense resistors, it implies that there is another current path. i have verified the schematic and PCB design and they look correct. I'm confused as to what is causing this. Any feedback would be appreciated.
Due to the lower voltage drop across the sense resistors, the HW protection circuit is kicking in at much higher output currents. The inrush current events are 10-50 us in length and are pulsed currents. The pulsing can last 10-40 ms with about 10-50 us of dead time in between the pulses.
If anyone is curious I'm using 2 of these series parts: Susumu KRL1632E-M
Disclaimer: This is for a work project, so I can't share too many details.
matbob:
Hello aiq25,
Do you have any decoupling capacitors connected near the H-Bridge? Where are the current sense resistors placed? Does the current through the decoupling capacitors flow through the sense resistors?
aiq25:
--- Quote from: matbob on February 17, 2019, 03:20:04 am ---Hello aiq25,
Do you have any decoupling capacitors connected near the H-Bridge? Where are the current sense resistors placed? Does the current through the decoupling capacitors flow through the sense resistors?
--- End quote ---
There is one capacitor that is placed across the two lines of the H-Bridge going out to the load. The current through that capacitor should flow through the resistor.
ajb:
If you have a capacitor directly across the output of the h-bridge, current flowing out of this capacitor will NOT be passing through the current sense resistor.
It would be helpful to see a schematic just so we're all on the same page, and an h-bridge shouldn't be anything proprietary unless you're doing something really unusual.
It's possible that your measurement method is introducing some error. Can you show is that, if only in diagram form?
aiq25:
Attached is the block diagram. The capacitor is directly across the AC lines, so the current from this would have to flow through the current resistor, at least that's my understanding. Please let me know if this is incorrect.
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