Hello one and all!

I am currently in the process of implementing a boost PFC converter and I seem to be running into issues with regards to the voltage sensing portion of my system. I am trying to sense a voltage of 300V. Initially I decided to design a voltage divider circuit that would be connected to a ACPL-7900-000e(isolated amplifier) and then to a differential amplifier, as shown in the picture below. The voltage divider resistor values were chosen to be 1Mohm and 560 ohms for the high-side and low-side respectfully.


However my results are plagued with these strange spikes (as can be seen in yellow) whereas the signal im actual tracking is quite clean.



As i carried on with my debugging process I decided to actually measure the signals coming from my physical board by means of changing voltage divider from the previously stated value to 100kohm high side and 1kohm on the low side. And it seems that the spikes are still pretty consistent. Could anyone kindly provide any guidance in this regard? Your advice will be highly appreciated.

Thank you

 

However my results are plagued with these strange spikes (as can be seen in yellow) whereas the signal im actual tracking is quite clean.

hi arch,
I would not say your 300Vac is clean at those output spike points, in fact, the output spikes occur at the same instant as the spikes on the 300V.?
E

 

Why are you using an isolation amplifier?
I’ve never seen one used in a pfc boost circuit before.

 

The main reason for that is to isolate my micro controller from any dangerous spikes

 

hi arch,
I would not say your 300Vac is clean at those output spike points, in fact, the output spikes occur at the same instant as the spikes on the 300V.?
E

Hi Eric

Thank you so much for pointing that out, it seems that I may have not noticed that. Do you perhaps have any suggestions as to how I could amend this? My main concern as it stands is that I would like to plug the output of voltage sensor to my micro-controller

 

Hi arch,
Which input of the MCU, I assume it is an ADC input.
What voltage is the MCU supply?
E

 

Hi arch,
Which input of the MCU, I assume it is an ADC input.
What voltage is the MCU supply?
E

Sorry by input I mean the ADCs limit of 3.3V

 

Hi arch,
One method would be a series resistor and two Schottky diodes, used to clip the spikes to within safe limits.
Do you know how to to do that?
E

 

Hi arch,
One method would be a series resistor and two Schottky diodes, used to clip the spikes to within safe limits.
Do you know how to to do that?
E

I do not know to do that, would you kindly assist me in this regard?

 

Hi arch,
One method would be a series resistor and two Schottky diodes, used to clip the spikes to within safe limits.
Do you know how to to do that?
E

And the diodes are built in on many micros.

What capacitor is C2? A 10 or 100nF ceramic should suppress them pretty well.

 

And the diodes are built in on many micros.

What capacitor is C2? A 10 or 100nF ceramic should suppress them pretty well.

I used a 6.7pF ceramic capacitor for C2

 

Hi arch,
Check this for one option, you can vary the value of the capacitor to give you the best result
The input is an over voltage test sine wave.

Post your scope images.
E


Updated:
Added an AC analysis plot for a modified circuit.

 

Hello Eric
I went ahead and connected my voltage sensor to my micro-controller, and turns out it was noise emitted from the physical voltage probes. Thank you so much for all the time you took out to answer my question. Really appreciate it!