MOSFET as a switch

Part 5c · Voltage Regulation

Key point: a switch that is on does not sit on the flat saturation plateau. It parks near the origin, in deep triode, as \(R_{\mathrm{DS(on)}}(V_{\mathrm{GS}})\).

You force \(I_D\). The FET finds \(V_{\mathrm{DS}}\). \(R_{\mathrm{DS0}}\) is the on-resistance at a 10 V gate. Deep triode means \(V_{\mathrm{DS}}/V_{\mathrm{ov}} \le 0.25\).

1
Fully on is a 5 mΩ resistor.
10 V gate, 5 A, \(V_{\mathrm{th}} = 2~\mathrm{V}\), \(R_{\mathrm{DS0}} = 5~\mathrm{m}\Omega\). \(R_{\mathrm{DS(on)}} = 5.00~\mathrm{m}\Omega\). \(V_{\mathrm{DS}} = 25.0~\mathrm{mV}\). Heat is 125 mW.
2
A weaker gate raises that resistor.
Same die, same 5 A, gate down to 4.5 V. \(R_{\mathrm{DS(on)}} = 16.00~\mathrm{m}\Omega\). \(V_{\mathrm{DS}} = 81.3~\mathrm{mV}\). Heat is 407 mW. Still deep triode.
3
A logic-level part at 4.5 V.
\(V_{\mathrm{th}} = 1.2~\mathrm{V}\), \(R_{\mathrm{DS0}} = 4~\mathrm{m}\Omega\), 8 A. At 4.5 V, \(R_{\mathrm{DS(on)}} = 10.67~\mathrm{m}\Omega\), \(V_{\mathrm{DS}} = 86.5~\mathrm{mV}\), heat 692 mW.
4
The same die at 10 V.
Still 8 A. \(R_{\mathrm{DS(on)}} = 4.00~\mathrm{m}\Omega\). \(V_{\mathrm{DS}} = 32.1~\mathrm{mV}\). Heat falls to 256 mW. The lever is gate voltage, not saturation current.
5
Leave deep triode and it stops being a switch.
4 V gate, 23 A, \(R_{\mathrm{DS0}} = 10~\mathrm{m}\Omega\). \(I_{D,\mathrm{sat}} = 25.0~\mathrm{A}\). \(V_{\mathrm{DS}} = 1.43~\mathrm{V}\). \(V_{\mathrm{DS}}/I_D = 62.36~\mathrm{m}\Omega\), not the 40.00 mΩ deep-triode number. Heat is 33.0 W.

ON-state only. No switching loss and no body diode. The X-axis max moves with the step so the gold dot stays on the plot. \(P_{\mathrm{cond}} = I_D^2 R\), using the actual \(V_{\mathrm{DS}}/I_D\) once you leave the straight part.

Force current, read the drop

Gold dot is \(V_{\mathrm{DS}}\) at this current and gate

Deep triode —

\(V_{\mathrm{GS}}\) (V) \(I_D\) (A) \(V_{\mathrm{th}}\) (V) \(R_{\mathrm{DS0}}\) (mΩ)

\(I_D\) vs \(V_{\mathrm{DS}}\) · rose = forced current · blue = this gate

X-axis max (V)

\(R_{\mathrm{DS(on)}}\) vs \(V_{\mathrm{GS}}\) · gold = this gate

\(R_{\mathrm{DS(on)}}\)—

\(V_{\mathrm{DS}}/I_D\)—

\(V_{\mathrm{DS}}\)—

\(V_{\mathrm{ov}}\)—

\(P_{\mathrm{cond}}\)—

Regime—

\(I_{D,\mathrm{sat}}\)—

\(V_{\mathrm{DS}}/V_{\mathrm{ov}}\)—

\(R_{\mathrm{DS(on)}} \approx 1/\bigl(k(V_{\mathrm{GS}}-V_{\mathrm{th}})\bigr)\)

→ —

\(V_{\mathrm{DS}} \approx I_D R\)

→ —

\(P_{\mathrm{cond}} = I_D^2 R\)

→ —