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feedback circuit question

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Full Member level 3
May 10, 2010
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Hi, i've to design a feedback for my isolated push-pull converter.
The feedback is an OR between voltage and current sense.
In that way my DC/DC should work as a laboratory power supply.
I've drawn the schematic in figure below


Apart from compensation network and from the voltage and currents involved I have some doubts about how i expected the power supply of Operational Amplifier. I'm not sure that the differential stage for current sensing could work in such configuration.

Could you help me?
Could you suggest me the most suitable configuration?

Your circuit will work. You will have all the variations in the 12V interfering with the feedback signal.
If you can put the current sense resistor on the ground of the load it will be simpler circuit.

What do you think about opa-2140 used in this configuration? how the circuit will be simpler putting the sense resistor on ground?

Why 0.1V reference for current comparator? My purpose is to amplify current signal by 50 times.

Change the 0.1V to what you need. I don't know what current flows through your load so I just put a number. My circuit is only a guide and not a final circuit.
There could be a confusion about this feedback issue. Feedback in most cases is sampling of the output and feeding it back to the regulator for the purpose of controlling the output.
In other cases a certain state of the output is fed back for the purpose of protecting the circuit.
I assumed that your circuit is for the purpose of protecting your power supply and not for controlling the output voltage.
If you need to control your output voltage you cannot use an optocoupler because an optocoupler has only on or off state.

BTW, I read your previous thread about RL in an old inverter. It is common for engineers to use a resistor as a bobbin for winding a coil on it. The engineer uses a resistor which is the right physical size for the coil, the value of the resistor isn't important. 100 Ohm in parallel to less than 0.1 Ohm of the coil wire is insignificant.

The feedback pin of optocoupler is the input called COMP in my IC so the error amplifier now is the op-amp and optocoupler pin goes directly to pwm controller. The pull-up resistor is inside the IC (LM5030). The output current should be 1A.

Sorry franticEB, your complete project is beyond me. I could give you an advice only about sensing the current.

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