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full bridge current sensing method

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ljy4468

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Hi all,

I have a curiosity about sensing current on full bridge.

Please see attached, and
would you tell me difference during those two circuit?

it seems that there'll be no difference, but anyone can tell me about those circuit.

Thanks in advance.

asd.png
 

- The first one senses the motor current, the second one the H-bridge input current. That's an important difference referring to the voltage and current conversion capability of the H-bridge. During some states, motor current will be e.g. circulated through the upper or lower switches and doesnt't contribute to the current seen in circuit 2.

- As a practical problem, the first circuit requires a common mode range equal to the bus voltage.
 

- The first one senses the motor current, the second one the H-bridge input current. That's an important difference referring to the voltage and current conversion capability of the H-bridge. During some states, motor current will be e.g. circulated through the upper or lower switches and doesnt't contribute to the current seen in circuit 2.

- As a practical problem, the first circuit requires a common mode range equal to the bus voltage.


Thank you for your answer FVM,

I understand your comment, but if current are not circulating e.g. not motor application,
does it have differences b/w that when only measure h-bridge input current?

Thanks/
 

it seems that there'll be no difference, but anyone can tell me about those circuit.

View attachment 70790

in the first schematic the opamp input voltage will be changing polarity and will result in positive and negative voltage output while in the second schematic the opamp input voltage polarity doesn't change so you get only positive output.
 

Suppose the upper-right and lower-left transistors are on.
Now you switch off the upper-right.
The current will keep going through the motor from the lower-right diode to lower-left transistor:
the first circuit will measure this current, the second will not.
 

You should choose the current measurement method according to it's intended function for your system, applied PWM scheme and available current measurement options. As long as we don't know details about any of it, we can hardly suggest a preferred method.
 

The key difference is that the first one measures current magnitude and direction, while the second one measures only current magnitude.
 

Using the second method you don't actually need a differential amplifier, only optional voltage divider + zener diode to protect the ADC. This is because differential voltage equals voltage on the upper side of the resistor minus voltage on the lower side of it, and the lower side is connected to GND. Hence potential of the upper side or resistor referenced to common gnd equals to differential voltage drop across this resistor.
 

Using the second method you don't actually need a differential amplifier, only optional voltage divider + zener diode to protect the ADC. This is because differential voltage equals voltage on the upper side of the resistor minus voltage on the lower side of it, and the lower side is connected to GND. Hence potential of the upper side or resistor referenced to common gnd equals to differential voltage drop across this resistor.

You assume a shunt resistor with a value high enough to give an output in the proper range for the ADC but I would expect resistors like 0.1 ohm or 20m ohm to be used and the an opamp would be required.

Why do you suggest a voltage divider and zener, how large voltage do you expect to get from the shunt resistor?
 

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