MOSFET — voltage-controlled switch
A MOSFET is a voltage-controlled switch that opens/closes the drain-source channel via gate-source voltage — the core part (×6) of a 3-phase inverter. Its gate is insulated, so it controls via voltage (a field), not current. Unlike a BJT, when ON it shows an on-resistance Rds(on) instead of a fixed drop (VCE(sat)), so I²R heat is the key selection criterion; fabrication also leaves parasitic capacitances (Ciss/Coss/Crss) and a body diode. Switching happens in the low-loss linear (ohmic) region — and since the gate needs ~12V while an STM32 outputs only 3.3V, a gate-driver IC is mandatory.
What a MOSFET is + why the symbol looks like that
- A voltage-controlled device that forms a conducting drain-source channel via gate-source voltage (VGS) to switch current.
- Read just three parts — the gate drawn separate (insulated → voltage control), the broken channel in the middle (OFF until VGS>Vth), and the inner arrow (the body diode, also marking N-/P-channel).
| Symbol part | Meaning |
|---|---|
| Gate drawn apart | Insulated → no current, voltage control |
| Broken channel | OFF normally · ON when VGS>Vth |
| Inner arrow | Body (parasitic) diode |
BJT vs MOSFET — different ON behavior
- Both control a large output with a small input, but a BJT uses base current while a MOSFET uses gate voltage (insulated).
- Trap: the 'fully-on switch' region is called saturation for a BJT but linear (ohmic) for a MOSFET — opposite names. A MOSFET's saturation is actually the lossy transition.
| BJT | MOSFET | |
|---|---|---|
| Control | Base current | Gate voltage (insulated) |
| When ON | VCE(sat) ≈ 0.2–0.3V | Rds(on) (I²R heat) |
| Switching region | Saturation | Linear (ohmic) ← opposite! |
Three regions — switch in the linear (ohmic) region
- Cut-off (VGS<Vth, OFF), linear/ohmic (fully ON, minimum Rds(on), low loss), and saturation (large VDS, lossy transition).
- Switching (90%+ of use) happens in linear; pass quickly through the lossy saturation region for efficiency — hence drive the gate fast and hard.
Electrical characteristics — SQJQ466E datasheet
- Key ratings of the inverter N-channel SQJQ466E — VDS 60V (never exceed), VGS ±20V, ID 200A (25°C)/118A (125°C derated), threshold VGS(th) 3.5V, Rds(on) 1.9mΩ.
- Rds(on) heat is the selection crux — at 200A, P=I²R=200²×0.0019≈80W, impossible without heatsinking. Real inverters keep a big margin and pick low-Rds(on) parts.
- drain current
- on-resistance (1.9mΩ)
Parasitic capacitance — source of switching speed & loss
- Fabrication leaves Cgd·Cgs·Cds; turning the gate on/off means charging/discharging them. Larger values slow switching and cause loss via I=C·dv/dt.
- Datasheet groups them — Ciss=Cgs+Cgd (input), Coss=Cds+Cgd (output), Crss=Cgd (reverse). Supplying that charge fast is beyond an MCU → a gate-driver IC is mandatory.
Body (parasitic) diode
- A parasitic diode between drain and source. Even with the gate at 0V (MOSFET OFF), current flows source→drain when the source voltage exceeds the drain.
- Its reverse-recovery time (trr) is fairly long, so an external diode (e.g., Schottky) is added if loss matters; in inverters it also serves as the freewheeling path for inductive loads.
Biggest trap — the 'fully-on switch' region is named oppositely: saturation for BJT, linear (ohmic) for MOSFET. Remember 'MOSFET switches in linear; saturation is the lossy region to pass through quickly.' The exact SQJQ466E datasheet numbers are pinned in the raw transcript's 'slide data'; package and gate-driver circuit selection come in the inverter-design lectures.