TIDA-010232: Riso Calculation

Hello Sky,

Thank for reaching out. There is a mistake in the formulas for calculating RisoN and RisoP.
The correct formulas are:

RisoN = (-(RinAMC + Rst) * (VDC + VN - VP)) / VN 

RisoP = ((RinAMC + Rst) * (VDC + VN - VP)) / VP

This is a known problem and the design guide update will be released very soon.

Best regards,

Andreas

Hi Sky,

Yes you are right, I swapped the the names.

With this approach it is not possible to be very accurate over the complete resistance range, since the ratio between the switched in branch Rst + RinAMC compared to the insulation resistance defines how big the voltage drop at the AMC3330 input is. See the equivalent circuit for SP closed below.

If RisoN in this case get very high, VP will be very low and offset errors of AMC3330 and TLV6001 will have a big impact.
Increasing RstP and RinAMC helps in a limited way only since RisoP is parallel to them and limits the resulting equivalent resistance ( RisoP|| (RinAMC+Rst)) which defines VP.

The design was optimized for insulation resistance values between 40k and 200k, since these are typically the critical values in a 400V system. If insulation resistances are high, typically the accuracy is not that important, since low resistance values are the fault scenario.

Best regards,

Andreas