How to Sense Floating Output Voltage of Inverting Buck-Boost Converter With MCU Control

I have designed an inverting Buck-Boost converter whose output is floating. I would like the microcontroller to change the duty cycle of the converter to keep a steady voltage regardless of the impedance of the load. I want a maximum voltage of about 60v and absolutely not lower than 50v. The negative rail dips to about -32v when the power supply is pushed to this point.
Essentially, I cannot reference the outputs to the ground of my microcontroller without, at best, getting nonsense results, or at worst, destroying the IC.
My thought was to use a differential amplifier to sense the output of the converter and give a signal to the microcontroller. This has not worked as planned, for a few reasons. Primarily, the negative output goes below my ground voltage, and every op-amp I have been able to find does not allow a voltage lower than -0.3v below the negative rail. I need to be able to sense the difference in voltage between the positive and negative outputs regardless of whether one of the outputs goes below "ground" as seen by the op-amp. I do not want to have to give my op-amp a negative rail voltage of -32v unless this is the only way.
My circuit is essentially the same as the prototypical buck-boost converter (attached), except I have chosen to low-side switch for efficiency purposes.

Bit out of my area this, but are you after a true/false indication of the output voltage 'in-spec' or an actual representation of the output voltage?Many many years ago, I saw an arrangement using a bulb and an LDR. Quite elegant in its day I suspect. Higher the output voltage, brighter the bulb so lower the LDR resistance. An opto isolator with the LED being driven by a Zener and resistor can give you a true/false indication of output voltage in-spec. You could also use an opto isolator driven by a voltage to frequency converter, and then get the microcontroller to monitor the frequency on the opto output.Not a problem I've ever had to solve... but an interesting one.  
Regards,Joe

Hi Justin-This is often accomplished by using a TL431 shunt regulator/comparator driving a linear optocoupler such as a PC367N2J000F. HERE is an example of how it's done.  Look on page 2.Another possibility is to use an isolation amplifier, such as the HCPL7520.

-Rick

All good ideas; I went with the isolation amplifier. This seems to work! Thank you for the help! I wonder if anyone can offer any advice on improving the accuracy of the circuit? The circuit regulates to a 45-52v range while set to 48v. The allowable range is 43-60v, 48v nominal. The average is right on target, but sometimes the circuit will drift in one direction or another for a very short time before recovering.  I have done a little improvement in code by averaging a small number of feedback samples (25) before making a decision on how to adjust the duty cycle. Introducing a small delay seems to help as well, but there is surely a balance to be struck between quick response and overshoot.

Hi Justin-Glad to hear that it's working.  Post a schematic and I'll try to help make improvements.  You might also try the other circuit using the TL431.  That is a VERY popular approach for regulating isolated supplies.  The TL431 has a very accurate voltage reference and very high gain, so it's well suited for this application.-Rick 

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Hi 
Glad to hear that it's working. Post a schematic and I'll try to help make improvements. You might also try the other circuit using the TL431. 
That is a VERY popular approach for regulating isolated supplies. 
The TL431 has a very accurate voltage reference and very high gain, so it's well suited for this application.
Thanks for the great feedback.

Glad to hear that it's working. Post a schematic and I'll try to help make improvements. You might also try the other circuit using the TL431. 
That is a VERY popular approach for regulating isolated supplies. 
The TL431 has a very accurate voltage reference and very high gain, so it's well suited for this application.

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