MOSFET Switching, Cgs and Cgd

Part 5d · Voltage Regulation

MOSFET switching

Key point: switching loss is \(V_{\mathrm{DS}}\) overlapping \(I_D\) while \(I_G\) charges \(C_{\mathrm{GS}}\), then \(C_{\mathrm{GD}}\). The flat spot on \(V_{\mathrm{GS}}\) is the \(C_{\mathrm{GD}}\) interval.

Driver is a 10 V step behind \(R_{\mathrm{drive}} = 2~\Omega\). \(V_{\mathrm{th}} = 2~\mathrm{V}\). The plateau is 3.4 V. \(I_G = (10 - V_{\mathrm{GS}}) / (2 + R_g)\).

1
Delay. The channel is still off.
Wide die, \(V_{\mathrm{DD}} = 12~\mathrm{V}\), \(I_D = 15~\mathrm{A}\), \(C_{\mathrm{GS}} = 2.6~\mathrm{nF}\), \(C_{\mathrm{GD}} = 0.28~\mathrm{nF}\), \(R_g = 1~\Omega\). Scene is Delay. \(Q_g\) is \(2.75~\mathrm{nC} / 28.0~\mathrm{nC}\). \(I_G\) is \(2.98~\mathrm{A}\).
2
Current rise. The drain is still at the rail.
Same die. Scene is Current rise. \(Q_g\) is \(7.10~\mathrm{nC} / 28.0~\mathrm{nC}\). \(I_G\) is \(2.42~\mathrm{A}\), still into \(C_{\mathrm{GS}}\). \(t_r\) is \(1.5~\mathrm{ns}\). \(E_{\mathrm{sw}}\) for the edge is \(0.27~\mu\mathrm{J}\).
3
Miller. \(V_{\mathrm{GS}}\) sits on the shelf.
Same die. Scene is Miller. \(Q_g\) is \(10.5~\mathrm{nC} / 28.0~\mathrm{nC}\). \(Q_{\mathrm{GD}}\) is \(3.34~\mathrm{nC}\). \(t_f\) is \(1.5~\mathrm{ns}\), the same as \(t_r\). \(I_G\) on the shelf is \(2.20~\mathrm{A}\). At 200 kHz that edge is 54 mW.
4
Finish. \(V_{\mathrm{DS}}\) is \(I_D R_{\mathrm{DS(on)}}\).
Same die. Scene is Finish. \(V_{\mathrm{DS,on}}\) is 60.0 mV (4.0 mΩ). \(Q_g\) is \(24.6~\mathrm{nC} / 28.0~\mathrm{nC}\). \(I_G\) is \(605~\mathrm{mA}\).
5
\(R_g = 5~\Omega\). Same \(Q_g\), longer edge.
Same die, \(R_g = 5~\Omega\), scene back on Miller. \(Q_g\) total is \(28.0~\mathrm{nC}\). \(Q_{\mathrm{GD}}\) is \(3.34~\mathrm{nC}\). \(I_G\) on the shelf is \(943~\mathrm{mA}\). \(t_r + t_f\) is \(7.0~\mathrm{ns}\). \(E_{\mathrm{sw}}\) is \(0.64~\mu\mathrm{J}\), 127 mW at 200 kHz. Gate-drive heat is 56 mW.
6
48 V. \(Q_{\mathrm{GD}}\) is the long shelf.
\(V_{\mathrm{DD}} = 48~\mathrm{V}\), \(I_D = 4~\mathrm{A}\), \(C_{\mathrm{GS}} = 1.6~\mathrm{nF}\), \(C_{\mathrm{GD}} = 0.18~\mathrm{nF}\), \(R_g = 1~\Omega\). Scene is Miller. \(Q_{\mathrm{GD}}\) is \(8.60~\mathrm{nC}\). \(t_f\) is \(3.9~\mathrm{ns}\) and \(t_r\) is \(0.9~\mathrm{ns}\). \(E_{\mathrm{sw}}\) is \(0.47~\mu\mathrm{J}\), 93 mW at 200 kHz. \(V_{\mathrm{DS,on}}\) is 220 mV (55.0 mΩ).

Turn-on only, constant capacitors. \(P_{\mathrm{sw}} = E_{\mathrm{sw}} f_{\mathrm{sw}}\). The Low \(Q_g\) preset is a small die: \(3.24~\mathrm{nC}\) total, \(V_{\mathrm{DS,on}} = 320~\mathrm{mV}\), \(E_{\mathrm{sw}} = 0.01~\mu\mathrm{J}\). A full loss stack is in the MOSFET Efficiency Tool.

Scrub the turn-on

Gold line is this instant

Delay —

\(V_{\mathrm{DD}}\) (V) \(I_D\) (A) \(C_{\mathrm{GS}}\) (nF) \(C_{\mathrm{GD}}\) (nF) \(f_{\mathrm{sw}}\) (kHz)

Through the event —

Device · which capacitor \(I_G\) is filling

Rg Ω

Turn-on · \(V_{\mathrm{GS}}\), \(I_D\), \(V_{\mathrm{DS}}\)

\(V_{\mathrm{GS}}\) vs \(Q_g\)

Scene—

\(Q_g\) so far—

\(Q_{\mathrm{GD}}\)—

\(t_r+t_f\)—

\(E_{\mathrm{sw}}\)—

\(P_{\mathrm{sw}}\)—

\(P_{\mathrm{gate}}\)—

\(V_{\mathrm{DS,on}}\)—

\(t_f = Q_{\mathrm{GD}} / I_G\)

→ —

\(E_{\mathrm{sw}} \approx \tfrac12 V_{\mathrm{DD}} I_D t_r + \tfrac12 I_D (V_{\mathrm{DD}} + V_{\mathrm{on}}) t_f\)

→ —