How to plot MOSFET with gate series resistor RC lowpass cutoff frequency

When driving a MOSFET with a gate series resistor, the combination of the gate resistor and the gate capacitance forms an RC low-pass filter. This filter limits the switching speed of the MOSFET and affects the frequency response of your circuit.

Using UliEngineering, you can easily plot the RC cutoff frequency of this gate drive circuit vs the gate drive voltage. Since the gate capacitance of a MOSFET varies with the gate voltage, the cutoff frequency also changes with voltage.

In this example, we’ll plot the RC cutoff frequency for different gate resistor values (10Ω, 22Ω, 33Ω) using a IRF540N MOSFET with a total gate charge of 94 nC.

from UliEngineering.Electronics.MOSFET import mosfet_gate_capacitance_from_gate_charge
from UliEngineering.Electronics.Filter import rc_cutoff_frequency
from UliEngineering.EngineerIO import normalize_numeric
import matplotlib.pyplot as plt
import numpy as np

# Define the MOSFET parameters (IRF540N)
Qg = normalize_numeric("94 nC")
# Plot starting from 2V min threshold voltage
Vg_th = normalize_numeric("2 V")
Vgs = np.linspace(Vg_th, 20, 1000) # V

# Calculate the gate capacitance
Cgs = mosfet_gate_capacitance_from_gate_charge(Qg, Vgs)

# Gate resistor values
resistors = [10, 22, 33]  # Ohms

# Plot the RC cutoff frequency for each resistor
plt.style.use('ggplot')
plt.figure(figsize=(10, 6))

for R in resistors:
    # Calculate cutoff frequency for each gate voltage
    fc = rc_cutoff_frequency(R, Cgs)
    
    # Convert to MHz for better readability
    fc_mhz = fc / 1e6
    
    plt.plot(Vgs, fc_mhz, label=f'{R}Ω gate resistor')

plt.xlabel("Gate drive voltage (V)")
plt.ylabel("RC cutoff frequency (MHz)")
plt.title("MOSFET Gate RC lowpass cutoff frequency vs gate drive voltage")
plt.legend()
plt.grid(True, alpha=0.3)
plt.xlim(Vg_th, 20)

plt.savefig("MOSFET-Gate-RC-Cutoff-Frequency.svg")
plt.show()

This script demonstrates several important concepts:

• Gate capacitance variation: The gate capacitance increases with gate voltage, which decreases the cutoff frequency
• Resistor impact: Larger gate resistors result in lower cutoff frequencies, slowing down the MOSFET switching
• Frequency range: The cutoff frequencies are typically in the MHz range, which is important for high-frequency switching applications

The plot shows that at higher gate voltages, the RC cutoff frequency decreases due to the increased gate capacitance. This is why MOSFET switching speed can vary depending on the gate drive voltage level.

For a relatively large 94 nC gate charge, the cutoff frequencies are in the range of a few MHz, or even below 1MHz especially if using larger gate resistors.

When used in current regulator circuits, this low-pass filter effect can significantly impact the performance of the circuit, including leading to oscillations due to the phase shift introduced by the RC filter. Therefore, it’s crucial to consider the gate drive resistor value and the MOSFET’s gate capacitance when designing the gate drive circuit.