Hello everyone!,
First time posting here, so I don't know what to expect. I'm currently trying to design a very precise current loop through a solenoid. I have found a fully integrated H-bridge that I am happy with, but I am having some trouble with measuring current.
I want to use low-side current measurement through a 0.08 Ohm resistor. Apparently when the H-bridge switches states, there will be a voltage spike present across the sense resistor. This is not due to the expected current flow, but more due to the intrinsic capacitance of the FETs. Regardless of the reasoning, I am not sure what to expect. I am using a very nice precision low-offset / offset drift opamp that cannot tolerate more than 5.3V or less than 0.3V at its input.
The maximum RMS current that I plan to measure is 4A, yielding 320mV across the resistor. I cannot measure these spikes, so I wanted to plan for the worst. Is there a way to block these from causing potential damage?
In the past, I have seen capacitors placed across the resistor to cut off the spike, but I don't want to effect the dynamic response of my controller by introduction of an unnecessary pole. I tried to find analog switches that I could actively turn off and on with my controller, but all fault-tolerant switches are out of my supply range. I have heard of the classic series resistor and two schottky diodes to both VCC and GND, but I think that may effect my offset stability over temperature.
I was thinking of using a transistor as a switch to pull down a small resistance in series with the sense resistor output. I'm really worried about any unnecessary noise issues and was hoping for an elegant solution. Can anyone offer advice?
My application is really critical and I have spared no expense in its implementation. The project is basically a sensorless active magnetic bearing controller, as part of my degree requirements.
Thanks a lot !
Stephen
First time posting here, so I don't know what to expect. I'm currently trying to design a very precise current loop through a solenoid. I have found a fully integrated H-bridge that I am happy with, but I am having some trouble with measuring current.
I want to use low-side current measurement through a 0.08 Ohm resistor. Apparently when the H-bridge switches states, there will be a voltage spike present across the sense resistor. This is not due to the expected current flow, but more due to the intrinsic capacitance of the FETs. Regardless of the reasoning, I am not sure what to expect. I am using a very nice precision low-offset / offset drift opamp that cannot tolerate more than 5.3V or less than 0.3V at its input.
The maximum RMS current that I plan to measure is 4A, yielding 320mV across the resistor. I cannot measure these spikes, so I wanted to plan for the worst. Is there a way to block these from causing potential damage?
In the past, I have seen capacitors placed across the resistor to cut off the spike, but I don't want to effect the dynamic response of my controller by introduction of an unnecessary pole. I tried to find analog switches that I could actively turn off and on with my controller, but all fault-tolerant switches are out of my supply range. I have heard of the classic series resistor and two schottky diodes to both VCC and GND, but I think that may effect my offset stability over temperature.
I was thinking of using a transistor as a switch to pull down a small resistance in series with the sense resistor output. I'm really worried about any unnecessary noise issues and was hoping for an elegant solution. Can anyone offer advice?
My application is really critical and I have spared no expense in its implementation. The project is basically a sensorless active magnetic bearing controller, as part of my degree requirements.
Thanks a lot !
Stephen