MOSFET Parameters
From datasheet, at your Vgs
Typically 10-15V for standard MOSFETs
From datasheet, typical value
Peak source/sink current of your gate driver
Results
Gate Resistor (Turn-On)
25.0 Ω
Gate Resistor (Turn-Off)
30.0 Ω
Peak Gate Current
480 mA
Gate Driver Power
24.0 mW
Actual Rise Time
45.8 ns
Actual Fall Time
54.9 ns
Common MOSFET Reference
| MOSFET | Qg (nC) | Vth (V) | Type | Package |
|---|---|---|---|---|
| IRLML6344 | 5 | 1.5 | N-ch Logic | SOT-23 |
| SI2302 | 8 | 2.0 | N-ch Logic | SOT-23 |
| IRF3205 | 20 | 3.0 | N-ch Standard | TO-220 |
| IRFZ44N | 44 | 4.0 | N-ch Standard | TO-220 |
| IRF540N | 71 | 4.0 | N-ch Standard | TO-220 |
| IRFP260N | 130 | 4.0 | N-ch Power | TO-247 |
| IRFP4568 | 180 | 4.0 | N-ch Power | TO-247 |
Click on a row to use those values. Values are typical from datasheets.
Understanding MOSFET Gate Resistors
Gate resistors are critical components in MOSFET switching circuits. They control the rate at which the gate capacitance charges and discharges, directly affecting turn-on and turn-off times, switching losses, and circuit stability.
Choosing the right gate resistor involves balancing competing requirements: faster switching reduces losses but increases EMI and risk of oscillation, while slower switching is more stable but increases power dissipation in the MOSFET.
Gate Drive Theory
Gate Charge and Capacitance
MOSFETs have significant input capacitance (Ciss) that must be charged to turn on the device. The total gate charge (Qg) specified in datasheets represents the charge needed to fully enhance the MOSFET at a given gate voltage.
Switching Time Calculation
The gate resistor limits the current available to charge/discharge the gate capacitance:
Where:
- Rg = Total gate resistance (driver output + external + internal)
- Ciss = Input capacitance
- Vdriver = Gate driver voltage
- Vth = Gate threshold voltage
Gate Driver Power
Power dissipated in the gate drive circuit increases with frequency:
This power is dissipated in the gate resistor and driver IC, not in the MOSFET itself.
Design Guidelines
Separate Turn-On and Turn-Off Resistors
Many designs use different resistors for turn-on and turn-off. A diode bypasses the turn-on resistor during turn-off, allowing asymmetric switching times:
- Slower turn-on — Reduces inrush current and voltage spikes
- Faster turn-off — Minimizes shoot-through in half-bridge configurations
Minimum Gate Resistance
Never use zero gate resistance. A minimum of 2-10Ω is recommended to:
- Prevent high-frequency oscillation from gate-drain feedback (Miller effect)
- Limit peak gate current within driver capabilities
- Reduce EMI from fast switching edges
- Damp parasitic inductance ringing
Gate Resistor Placement
- Close to gate pin — Minimize loop area for reduced inductance
- Use wide traces — Keep inductance low in the gate drive path
- Consider ferrite beads — For high-frequency oscillation suppression
Power Rating
Gate resistors dissipate power during switching. Calculate power dissipation:
Use resistors rated for at least 2× the calculated power for reliability.
Common Issues and Solutions
Gate Oscillation
Symptoms: Ringing on gate waveform, multiple switching edges, excessive heating.
Solutions:
- Increase gate resistance (start with 10-22Ω)
- Add ferrite bead in series with gate
- Reduce gate drive loop inductance
- Add small capacitor (100pF-1nF) from gate to source
Slow Switching / High Losses
Symptoms: MOSFET running hot, poor efficiency, waveform shows slow transitions.
Solutions:
- Decrease gate resistance
- Use a stronger gate driver
- Choose a MOSFET with lower gate charge
- Increase gate drive voltage (within MOSFET limits)
Shoot-Through in Half-Bridge
Symptoms: High current spikes, excessive heating of both MOSFETs.
Solutions:
- Use faster turn-off (lower turn-off resistance)
- Add dead-time between high and low side switching
- Use MOSFETs with well-matched threshold voltages
Frequently Asked Questions
Why do I need a gate resistor if the driver has output impedance?
Driver output impedance alone may be too low, leading to oscillation. External resistors provide consistent, controllable impedance and can be easily adjusted during development. They also protect the driver from gate-to-source shorts.
Can I use the same resistor for turn-on and turn-off?
Yes, a single resistor works for many applications. Separate resistors with a bypass diode are used when you need different rise and fall times, common in half-bridge designs to prevent shoot-through.
How do I measure actual switching times?
Use an oscilloscope with adequate bandwidth (at least 5× the switching frequency). Measure gate-source voltage for gate timing and drain-source voltage for switching performance. Use a current probe to verify peak currents.
What about logic-level MOSFETs?
Logic-level MOSFETs have lower gate threshold voltages (1-2V) and can be driven directly from 3.3V or 5V logic. They typically have lower gate charge but the same principles apply. Gate resistance may need to be lower due to the reduced drive voltage.
Should I add a gate-source resistor?
A 10kΩ-100kΩ resistor from gate to source ensures the MOSFET stays off when the driver is in high-impedance state (during startup or fault conditions). It's especially important in noisy environments or when using long gate drive traces.