How Much Instantaneous Current Is Required to Turn On a MOSFET?

A common misunderstanding: MOSFETs are voltage-controlled devices. There is almost no continuous steady-state current at the gate (only nanoampere leakage current). The instantaneous current during turn-on is a pulsed peak current used to charge the gate parasitic capacitance. It only exists for tens to hundreds of nanoseconds during the rising edge, including the Miller plateau. This current is NOT the current that keeps the MOSFET conducting after it turns on.

Core formula: Ipeak = Qg / tr

Qg: total gate charge from datasheet, unit nC

tr: target rise time (VDS fall time), unit ns

The Miller plateau produces the highest peak current. This phase mainly consumes Qgd (Miller charge) and dominates switching loss and EMI performance.

Engineering Reference Range (for BLDC motor controllers & lithium battery protection power MOSFETs)

Small MOSFETs (SOT23, TO252, Qg ≈ 5~20 nC)

Target rise time 50~100 ns → peak current 100 mA ~ 400 mA. Direct driving by simple logic IO is barely functional but not recommended. Insufficient driving capability creates slow switching edges and higher heat generation.
Medium-power MOSFETs (TO220, DPAK, Qg ≈ 30~100 nC, widely used for motor bridge arms)

Target rise time 50~100 ns → peak current 0.3 A ~ 2 A. This is the typical range for BLDC controllers. Gate driver ICs with 1 A ~ 2 A peak drive current are commonly selected. An external gate resistor Rg is added to limit peak current and suppress oscillation.
High-current low-Rds(on) MOSFETs (large packages, big die size, Qg >100 nC)

For fast turn-on, peak current can reach 2 A ~ 4 A or higher. This category requires dedicated high-current gate drivers. Never drive directly from MCU pins.

The gate resistor Rg is used to limit this instantaneous peak current. Larger Rg reduces peak current and slows the switching edge, lowering EMI at the cost of higher switching loss. Smaller Rg gives faster turn-on and lower switching loss, but may trigger VDS ringing and EMI violation. This is the classic engineering trade-off tied to the Miller effect.

Average Gate Drive Current (Do not confuse with peak current)

Formula for average gate current: Ig_avg = Qg × fsw

Example: Qg = 50 nC, PWM frequency = 20 kHz

Ig_avg = 50 nC × 20 kHz = 1 mA.

The average current represents total charge transferred per second and is usually small. When selecting gate driver ICs, always check peak drive capability, not average current. This is a frequent pitfall for beginners.

Simple Selection Guidelines

Low-speed applications without high efficiency requirements: reduce peak drive current and accept slower switching.
High-frequency, high-current BLDC motor controllers: reserve 1.5~2 times design margin for peak drive current.
MCU IO pins typically only source/sink 20~40 mA. They can hardly drive power MOSFETs directly, even for MOSFETs with moderate Qg, once switching speed matters.

Summary

When turning on a MOSFET, we focus on the peak gate charging current, not steady-state current. For most bridge MOSFETs in motor controllers, the required peak drive current falls in the range of several hundred milliamps up to 2 A. Quantitatively calculate using Qg and target rise time, then tune the gate resistor to strike a balance between switching loss and EMI.