BQ79731-Q1: UIR Sensor - Isolation monitoring Detection feature

1. The datasheet indicates that SW1 is used to measure isolation resistance. Could you elaborate on the purpose of SW3 in this context? Additionally, could you explain how this method ensures compliance with ASIL requirements? Also, how to decide to select Automotive Isolated Switch (TPSI2140) or MOSFET for SW1, SW2 and SW3?

SW3 is used in this context when not monitoring insulation resistance, and allows the setup to be in power conservation mode. This requires SW1, 2, and 3 to be open. This is for the sake of power conservation, and ASIL requirements are going to need to be addressed through the compliance of the whole system, not just an individual component. Selecting the Automotive Isolated Switch is going to be dependent on the system as well, specifically with the voltage requirement of the system, however, the recommended TPSI2140 will be appropriate. 

2. The equations are not provided in the datasheet, so I have referred design document (Application Note: Insulation Resistance Detection for HEV Applications) for the equations above.
The provided equations assumes that the current through parallel branch (R1 and Risop) and the parallel branch ((R3 + R2) and Rison) will be same, with VF1 considered as the voltage drop across the parallel combination of (R3 + R2) and Rison. However, it appears in the design document that VF1 is the voltage drop across R3 alone. Could you clarify whether the assumption that VF1 applies to the parallel combination is accurate?

Thank you for referring to the Application Note for the additional formulas, this was appropriate. You are correct to say that the current between the two parallel branches are the same, and VF1 is the voltage drop of the parallel combination of (R3 + R2) and Riso,n. The design document does seem as such, but it must be considered with SW2 being closed as well, so VF1 is not just the voltage drop across R3 alone. 

3. Chassis ground and the HV pack negative are at different potentials and all nodes in the circuit are at distinct potentials. 

Ideally, the chassis and device ground are at the same potential, and the nodes in the circuit are at distinct potentials. 

4. Could you provide a detailed derivation of the equations used for the isolation resistance measurement to help us better understand? I have provided a table in the trailing mail which have the measurements of Rison and Risop and in some cases there is deviations in the measurement values and assumed values, which I have highlighted red. We want to understand the reason for the deviation.

I don't have access to the detailed derivation of the equations used, however, as we can see with the majority of tests ran, the equation holds true when comparing the assumed and calculated values. I can connect you with someone who will better be able to provide that information to you. 

5. I have performed simulations and calculations for the resistance values under three conditions for same 500V reference system. For the second condition, the calculated values do not align with the assumed values. Could you provide the guidance to understand this difference?

I see that for the most part, the assumed vs. calculated values are generally the same, but just not for condition 2. Unfortunately, I need you to clarify what the different conditions you are referring to in these tests before I can continue with the debugging process. I'm assuming this has to do with the combination of switches that are being enabled. 

6.. The reference calculations and design in the documents are based on a 500V system. Since we are designing for a 96V and 800V BMS, could you provide guidance on how to select appropriate values for R1, R2, and R3 to ensure compatibility and optimal performance for our 96V and 800V voltage system?

These resistor choices will be dependent on the system that you are creating, in this case the 500V system. Examples of things to account for are insulation requirements, or overshoot requirements. 

7. Is there any other document we can refer to?

I believe you've found all the necessary documentation for this question.