Original question:
Tool/software:
I am using TPSI2140-Q1 ic to measure isolation between battery positive to chassis ground and battery negative to chassis ground, but unable to calculate resistor value and I am using reference design from datasheet (Figure 9-3. Battery V- Reference Architecture)
our isolation voltage is 300Vdc
Hello Tilden,
Thanks for your support,
In excel calculation we found some negative o/p response please have a look of attached files. (schematic & excel calculation)
Hello Manish,
When would you expect to measure 1.73-V and 4.017-V for the voltage measurements? It's impossible to measure more than 4-V with the entered values, so of course the RisoP and RisoN measurements will be off. If VPACK = 300-V, worst case is hard short across RisoP. VADC = RDIV1/(RDIV1+RDIV2)*VPACK = 4-V.
You can use this calculator to find ideal VADCx (VCHx) values.
For the schematic, can you please show the passive component values? I'm assuming LOD+ is 300V referenced to PGND. Are those resistors connected between LOD+/PGND to Earth meant to model insulation resistance?
Best regards,
Tilden Chen
Solid State Relays | Applications Engineer
There is a doubt that I am not able to clarify, please help, these RisoP and RisoN are just measured value that's are calculated behalf of getting voltages OR we have to physically placed some value of resistor in between positive & earth AND between negative & earth.
please also help with calculating the divider value for 28v battery system in which isolation measured at 100Mohm @250v
Hello Manish,
Thanks for your reply. Typically, RisoP and RisoN are unknown resistance values that must be calculated from voltage measurements to ensure a minimum insulation and protect the end user. We obtain the voltage measurements from closing the TPSI2140s and measuring the leakage. This leakage will change depending on the absolute values of RisoP and RisoN.
I am interpreting your system as drawn below:
28-V seems pretty low, I haven't seen a system with this low of voltage require insulation before. What are your insulation requirements? We typically might see something like warning at 500 ohm/V and fault at 100 ohm/V. In this case, warning once RisoP or RisoN reach 15k ohm and fault once RisoP or RisoN reach 3k ohm.
Using the calculator attached in my previous response, if we assume RisoP and RisoN = 30 kohm, then we would measure 167 mV at the ADC, which is pretty low. Would want to change the size of the resistors (R1-R4 in below schematic) for better accuracy on the voltage measurements. Usually want to keep them similar to the RisoP and RisoN values you expect or want to detect.
Best regards,
Tilden Chen
Solid State Relays | Applications Engineer
