MOSFET as a Switch — Voltage Regulation

Along the way

1
Different corner of the same map. Part 5’s flat region is MOSFET saturation (amp bias). A converter FET that is ON lives at low \(V_{\mathrm{DS}}\) in triode.
2
Deep triode ≈ a resistor. When \(V_{\mathrm{DS}}\ll V_{\mathrm{GS}}-V_{\mathrm{th}}\), \(I_D\approx V_{\mathrm{DS}}/R_{\mathrm{DS(on)}}\) and \(R_{\mathrm{DS(on)}}\propto 1/(V_{\mathrm{GS}}-V_{\mathrm{th}})\).
3
Force the current, read the drop. The rose line is the load current. The FET finds \(V_{\mathrm{DS}}\) on the blue \(V_{\mathrm{GS}}\) curve. Gold dot is the ON state.
4
Gate voltage is the lever. More \(V_{\mathrm{GS}}\) → more inversion charge → lower \(R_{\mathrm{DS(on)}}\). That is why 6 V vs 8 V (or logic-level vs 10 V drive) matters.
5
Conduction loss heats the die. \(P_{\mathrm{cond}}=I_D^2 R_{\mathrm{DS(on)}}\). Weak gate plus high current is the classic hot FET.
6
Stay deep. Leave and it stops being a switch. If forced \(I_D\) approaches \(I_{D,\mathrm{sat}}\), Q walks toward pinch-off. Raise the gate or lower the current.

Source at 0 V. \(R_{\mathrm{DS0}}\) is the on-resistance at \(V_{\mathrm{GS}}=10\,\mathrm{V}\). This page is ON-state only — not switching loss or the body diode.

Lab · Force \(I_D\) · park in deep triode

ON-state only · gold Q 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)

Rose line = forced current. Blue = your gate. In this corner \(V_{\mathrm{DS}}\approx I_D R_{\mathrm{DS(on)}}\) — you do not set \(V_{\mathrm{DS}}\). \(R_{\mathrm{DS0}}\) is \(R_{\mathrm{DS}}\) at 10 V gate.

Fully ON ≈ —

\(I_D\) vs \(V_{\mathrm{DS}}\) · near origin · rose = forced current

X-axis max (\(V_{\mathrm{DS}}\))

\(R_{\mathrm{DS(on)}}\) vs \(V_{\mathrm{GS}}\) · gate is the lever

\(R_{\mathrm{DS(on)}}\)—

\(V_{\mathrm{DS}}/I_D\)—

\(V_{\mathrm{DS}}\)—

\(V_{\mathrm{ov}}\)—

\(P_{\mathrm{cond}}\)—

Regime—

\(I_{D,\mathrm{sat}}\)—

\(V_{\mathrm{DS}}/V_{\mathrm{ov}}\)—

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

→ —

\[ V_{\mathrm{DS,ON}} \approx I_D R_{\mathrm{DS(on)}} \]

→ —

\[ P_{\mathrm{cond}} = I_D^2 R_{\mathrm{DS(on)}} \]

→ —

\(R_{\mathrm{DS0}}\) is \(R_{\mathrm{DS}}\) at 10 V gate: \(k=1/(R_{\mathrm{DS0}}(10-V_{\mathrm{th}}))\). The gold point solves Level-1 at the forced current; the linearized \(R_{\mathrm{DS(on)}}\) is the deep-triode slope.

Try it: (1) Textbook ON — gold near the origin, tiny drop. (2) Weak gate — same current, more volts and heat. (3) High current — loss scales as \(I_D^2\). (4) Logic-level vs 10 V drive — same die, different gate. (5) Leaving triode — Q walks toward \(V_{\mathrm{DS,sat}}\).

Full conduction + switching loss: MOSFET Efficiency Tool.

Analog Intuition

MOSFET as a switch · deep triode

Lab · Force \(I_D\) · park in deep triode