voltage feedback from full bridge dc-ac converter by instrumentation amplifier

[zizi110]

Hi all,
i am designing closed loop system that consist of a dc-ac converter(inverter) and a controller,
i want to control output voltage of inverter by means of feedback voltage that goes to microcontroller ADC. in micro i have a C code that is my controller algorithm.
my supervisor said that i should use instrumentation amplifier for voltage feedbacking.
but i dont know ... i guess may be there is problem with isolation between power stage and microcontroller,
there are instrumentation amplifier like op184/284/484, but my question is the isolation issue between inverter and micro.
is it necessary to use optocoupler for isolation or not?

thanks for your time.

 

[biswaIITH]

yes u can use optocouplers(TLP250 or 350 or any other) for providing isolation between the load and the micro....

 

[red_alert]

He needs a high linearity optocoupler like this:

 

[zizi110]

Hi friends,
thanks for your reply.
i have HCPL7520 optocoupler from avago, is it ok?
i saw that it is used for volatge and current sensing!

 

[red_alert]

That's perfect!

 

[zizi110]

Hi ,thanks for your help
ok, so first of all we feedback the output voltage of full bridge inverter by an instrumentation amplifier (just like AD8227 which has wide input range), and then for isolation between power stage and microcontroller we use optoisolator (like HCLP7520) and give its output to ADC of micro.
is it right?

 

[Surendhar M]

It is also better to add the opto isolator at the drive pins from microcontroller to the full bridge inverter. So that you are completly isolating the power stage and the microcontroller

 

[red_alert]

You could narrow down the output voltage range by using a resistive voltage divider across the load.

 

[zizi110]

Yea surendhar M , i do it.
my VDC is 24v, so instrumentation amplifier could have input voltage range in this limit,
how can i use resistor divider?! i have to measure output voltage in differential manner.

 

[Surendhar M]

For narrowing down the voltage adjust the gain of the instrumentation amplifier by placing a gain resistor across Pin 2 and 3 instead of using a voltage divider to do it.

Sorry You can't reduce the gain of the circuit. Because it has been set with default 5. May be if you are using 3 opamps to make instrumentation then narrowing down the voltage is possible.

 

[red_alert]

Put three series resistors across the load and measure the voltage across the middle one.

- - - Updated - - -

I've just seen your VDC is 24V so you might not need a voltage divider at all. I thought you could have a higher voltage VDC (350 V).

 

[zizi110]

thanks guys,
so i write the whole of scenario, but i'm not sure my understanding is right or not. please help me if i'm in wrong way.
if assume that instrumentation amplifier is AD8220 , based on its datasheet, its gain can varies from 1 through 1000 . we can use it as single supply rail to rail output,. the supply voltage range of this IA in single supply mode can be select up to 36 v. so i connect VS- to ground and VS+ to 24 v. so input voltage range of IA (instrumentation amplifier) can support full bridge output voltage variations. but becuase of negative cycle of ac output voltage of full bridge i must select the vref of instrumentatiom amplifier to +12v?
in my case ( VDC is 24v) i guess gain=10 is fair, so max output of this instrumentation amplifier is 2.4v ( if i understand right) and it is ok for ADC input voltage range ( i'm using ARM controller and its MAX ADC voltage is 3.3v)
i worry about just one thing that is isolation between power stage and micro, and after discussion on this topic i found that must have an optoisolator after IA.

thanks for your time
any suggestion appreciated

- - - Updated - - -

it is datasheet of AD8220:
http://www.analog.com/media/en/technical-documentation/data-sheets/ad8220.pdf

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one more question ,
is there any instrumentation amplifier that has all of features of AD8220 + isolation b/w input and output in it?

 

[red_alert]

Take a look at avagotech website, you'll find a lot of app notes and product suggestions. Then search for availability at your local suppliers.

 

[zizi110]

isolation and gain reduction in feedback path between power stage and microcontroller

Hi all,
i have a dc-ac full bridge converter that its output voltage must be controlled, so i used an instrumentation amplifier for voltage feedback , VDC of converter is 24v and instrumentation amplifier is AD627 and its supply voltage is 24v( single suuply mode)
i must connect the output of this instrumentation amplifier to ADC of micro, but how can i isolate the power stage and microcontroller , and also i want to reduce the output volatge of IA to 3.3 ( that is max volatge ADC can tolerate).
please help me

thnks