3-14. Performance of MOSFETs: Characteristic of Capacitance

Capacitance characteristics of C_iss, C_rss and C_oss are important factors affecting switching characteristics of MOSFET.

C_iss: input capacitance (C_iss  = C_gd + C_gs)

 ⇒Sum of gate-drain and gate-source capacitance: It influences delay time; the bigger the C_iss, the longer the delay time.

C_rss: Reverse transfer capacitance (C_rss  = C_gd)

 ⇒Gate-drain capacitance: The bigger the C_rss, the more the characteristic of drain current rising deteriorates, which is disadvantageous for MOSFETs’ loss. Low capacitance is needed to drive at high speed.

C_oss: Output capacitance (C_oss = C_gd + C_ds)

 ⇒Sum of gate-drain and drain-source capacitance: It influences turn-off characteristic, and loss with light load. In the case of large C_oss, turn-off  dv/dt decreases, which is advantageous for noise. But loss with light load increases.

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