Hi,
I am designing 115-120VAC switch with mosfets(back-to-back configuration) and I have some questions to ask you. The isolation is provided by isolators and isolated DC/DC converter and to drive mosfets I'm using non-isolated gate driver with respect to iso_gnd. I have also current measurment INA IC to measure load current, it is about 20-25A. I have 5V_ISO, 15V_ISO and Vref_ISO to supply INA and driver IC and they are referenced to iso_gnd which is coming from isolated dc/dc converter. I have also ADC which is using 5V_ISO and iso_gnd as supply.

the first question is can I use this driver circuits to drive mosfets ?
Can I use INA IC in high voltage line to measure current as shown in figure ? Ina inputs will see 120VAC with respect to neutral, but It is referenced to iso_gnd, so It will see the voltage between pins and iso_gnd. am I right ?
If I can, what is the drawbacks of this kind of circuits ?
I attached the sample figure.
thank you,
Regards

 

The principles of the circuit generally look healthy.

And the problem of the isolated ground and the current measurement I suggest to move the 1 mΩ shunt up in the top Power MOS's Source.
There is a small Gate capacitive current as a fault source, but with 20 amps of Load current it should not be a problem.

 

yes thats right, did you check INA302 common mode voltage rating ? I ask this question in e2e and TI experts says something different, please check https://e2e.ti.com/support/amplifiers/f/14/p/814937/3018123#3018123

@Kjeldgaard

 

I don't know the INA302 well enough that I dare answer the question of Common Mode.

But there are indications that the only positive voltage inputs.

 

I don't know the INA302 well enough that I dare answer the question of Common Mode.

But there are indications that the only positive voltage inputs.

Datasheet says It supports 36V cm voltage.
He says when mosfet ON, INA302 inputs directly see 120VAC, thats true but with respect to the neutral line. In my application, I think the pins will see just the voltage between pins and iso_gnd, Do you agree with that ?

In addition, If I put the shunt resistor between top mosfet and iso_gnd, in short circuit condition ( I am detecting ssc with dual comparator on shunt resistor and gate driver opening the mosfets), can gate driver affected by voltage drop of shunt ?

 

Any comments?

 

When the HV Top is -ve w.r.t. to the bottom, ISO Gnd is at diode drop above AC negative.
If the bottom MOSFET is not turned on, then inputs of INA302 will be close to the AC Positive Voltage. That would be exceeding 36V easily!

 

You could tie their gates together and use one gate resistor and one gate source resistor for both MOSFETs.

 

When the HV Top is -ve w.r.t. to the bottom, ISO Gnd is at diode drop above AC negative.
If the bottom MOSFET is not turned on, then inputs of INA302 will be close to the AC Positive Voltage. That would be exceeding 36V easily!

What if I put the shunt resistor between top mosfet source and iso_gnd ?

 

There does not seem to be anything to violate the CM max of the INA302. Very Low inductance shunt needs to be used, and slower turn on to reduce spikes. There would be some delay between the MOSFETs switching on/off.

 

There does not seem to be anything to violate the CM max of the INA302. Very Low inductance shunt needs to be used, and slower turn on to reduce spikes. There would be some delay between the MOSFETs switching on/off.

thank you for your answer, yes it should have a very low inductance and zero voltage turn-on zero current turn off will be done