Jibin M. Varghese If you think about what happens to a MOSFET when VDS increases it may make sense. Think about the depletion region near the drain when VDS increases. As VDS increases the depletion width increases and therefore the capacitance will decrease (C = V/d). The drain voltage has implication on the underlying semiconductor and will change capacitance.

Maybe this will help:

(https://www.microsemi.com/document-portal/doc_view/14692-mosfet-tutorial#:~:text=The%20reverse%20transfer%20capacitance%20is%20equal%20to%20the%20gate%20to%20drain%20capacitance.&text=The%20reverse%20transfer%20capacitance%2C%20often,the%20turn%2Doff%20delay%20time.)

http://www.aosmd.com/res/application_notes/mosfets/power_mosfet_basics.pdf

-Adam

Dear Jibin M. Varghese

Dr. Adam's answer is good. I just do some additions.

Yes, Ciss = Cgs+Cgd, and Cgd (namely Crss) decreases with the increase of Vds. However, Cgd is usually much smaller than Cgs when Vds is relatively high (e.g. >10V). Thus, Ciss will be dominated by Cgs. Since Vds has little influence on the value of Cgs, the value of Ciss remains almost constant with the increase of Vds when Vds is relatively high. For more details and how to measure these voltage-dependent capacitances, please see our previous publications:

[1] Z. Zhao, K. Y. See, W. Wang, E. K. Chua, A. Weerasinghe, Z. Yang, and W. Chen, "Voltage-dependent capacitance extraction of SiC power MOSFETs using inductively coupled in-circuit impedance measurement technique," IEEE Trans. Electromagn. Compat., vol. 61, no. 4, pp. 1322-1328, Aug. 2019.

[2] Z. Zhao, K. Y. See, E. K. Chua, A. S. Narayanan, A. Weerasinghe, and W. Chen, "Extraction of voltage-dependent capacitances of SiC device through inductive coupling method," in Proc. IEEE Int. Symp. Electromagn. Compat. IEEE Asia-Pacific Symp. Electromagn. Compat. (EMC&APEMC), 2018, pp. 1301-1304.

[3] Y. Liu, Z. Zhao, W. Wang, and J.-S. Lai, "Characterization and extraction of power loop stray inductance with SiC half-bridge power module," IEEE Trans. Electron Dev., vol. 67, no. 10, pp. 4040-4045, Oct. 2020.

[4] Y. Liu, K. Y. See, R. Simanjorang, Z. Lim, and Z. Zhao, "Modeling and simulation of switching characteristics of half-bridge SiC power module in single leg T-type converter for EMI prediction," in Proc. IEEE Int. Symp. Electromagn. Compat. IEEE Asia-Pacific Symp. Electromagn. Compat. (EMC&APEMC), 2018, pp. 1314-1318.