measuring voltage of dc/dc full bridge converter

Hi All
I have a current controlled dc/dc full bridge converter with complementary output +85V and -85V that feeds audios amplifier. I'm trying to find the best way of measuring the full bridge output voltage (+85V/-85V) to provide the proper below 2.5V level signal for voltage loop of full bridge. The differential output voltage in steady sate would be +85-(-85)=170V. I think the simplest way to sample these two voltages is to use two general purpose op-amp like TL072 and resistive dividers to make a simple subtractor . However, I was told I may need to use difference opamps like AD629 from analog device. I know it is necessary to use these difference opamps in applications when measuring small milivolts voltages due to common mode signal problem. But do I have to use them in my application also? They are kind of expensive and I do not understand if they have enough advantage in my application to justify using them or not. Please advice.

Hi,

you don´t need a "difference OPAMP" but you need a "difference OPAMP circuit".

It uses a typical OPAMP.

It has two input nodes and it is amplifying the difference voltage of both nodes.

Klaus

Another easy option is sense on a low side mosfet when its turned off (while the one in the other branch is turned on). You can use a voltage divider in parallel with the low side switch in order to adpat the voltage and measure it directly from MCU ADC. You can sense to Vdc. This method avoid using differential op amps because all measures have the same reference.