Ferrite beads get a lot of attention in application notes and a lot of criticism from EMI/EMC experts. Regardless of how you feel about ferrite beads, they do have some useful applications depending on how the beads are selected. One of these uses is as a filtering element for slowly changing or DC loads. The resonant behavior of a ferrite bead is not useful for filtering digital power rails, but it is very effective for wideband filtering of DC loads and slowly switching components or circuits.
One of these use cases is the use of a ferrite bead as a filter on the gate of a MOSFET. It is normally not advisable to place an inductive or resistive component on a MOSFET gate, but slowly driven gates that require filtering on the driving signal can benefit from a ferrite bead on the gate terminal. We will explore this use of ferrite beads in this article.
How a Ferrite Bead Impacts MOSFET Gate Driving
Ferrite beads exhibit inductive, resistive, and capacitive impedance in different frequency ranges. The impedance spectrum of a ferrite bead shows a steady inductive increase until a resonant frequency is reached, at which point the ferrite bead has maximum impedance. The ferrite bead then becomes capacitive, and the impedance steadily decreases to a very low value. This is often modeled as a parallel RLC network with a series resistor that sets current limiting at very low and very high frequencies.
Impedance curve for part number BLA31AG102SN4.
To understand how MOSFET driving is affected by the presence of a ferrite bead on the gate terminal, we need to look at each of the three main impedance regions. We'll start with the resistive section, as that relates to the primary reason for using a ferrite bead on a gate drive: to remove noise.
Driving in the Resistive Range
The main reason for placing a gate resistor on a MOSFET is to control the turn-on time. This is important in applications such as isolated switching regulators where a MOSFET's capacitances interact with the leakage inductance of a transformer coil, creating some resonance. Applying just a small amount of gate resistance and some additional circuitry (RCD clamp and RC snubber) can help control the turn-on time and reduce resonances in the peak voltage and current, which could damage the MOSFET or create problems like cross-conduction in some switching converter topologies.
MOSFET capacitances and the gate resistance will determine the turn-on time. Placing a ferrite with large resistive impedance can also exhibit slow turn-on time.
This is effectively what happens in the resistive range: we're applying a moderately sized resistance to the MOSFET gate. Given the typical listed impedance values of ferrite beads at resonance, the resistive impedance on the MOSFET gate is much larger than what is typically used in a gate drive circuit. Assuming no other factors, such as excited resonances, are present, we would expect the gate's turn-on time to be significantly slowed.
Of course, you are not always driving in the resistive range alone. You are often driving with a wideband signal, such as a PWM pulse stream. This means you are also driving in the inductive range in typical situations.
Driving in the Inductive Range
In the inductive range, some of the driving energy interacts with an inductive reactance and any capacitance in the circuit. The capacitance could come from the MOSFET's terminal capacitances, which are unfortunately unavoidable. The inductive contribution from the ferrite can create the same problems found when driving an inductor with a parallel capacitor: you get an LC resonance. This could result in transient excitation, which manifests as ringing observed on the gate terminal. This could modulate the on-state resistance of the MOSFET if it is still operating in the linear range.
Ringing on a MOSFET gate can arise when there is excess inductance in the switching node. The excess inductance interacts with MOSFET terminal capacitances and could create an underdamped oscillation.
Is there a way to prevent this kind of LC resonance? As it turns out, there is. The solution is to add some damping by adding a small series resistance with the ferrite bead. The ferrite bead's internal series resistance might be enough to dampen any LC resonance, depending on the MOSFET's terminal capacitance values and ferrite bead inductance.
In the Capacitive Range
Above the ferrite bead's impedance peak, the bead appears capacitive and will have very low impedance. While it is an uncommon situation, a MOSFET could be protected with a ferrite bead but then driven with a very high-frequency harmonic signal. This would provide resistive low-pass filtering on the MOSFET gate. Additionally, because everything above the resonance appears capacitive, there is no concern of exciting LC resonances.
Expected Turn-On Time with a Ferrite Bead
Since the main effect of placing a ferrite bead on a MOSFET gate is to slow down the MOSFET turn-on time, it's worth estimating or simulating how long that turn-on time will be.
First, let's do an estimation based on a typical peak impedance value for a ferrite bead. The turn-on time for a MOSFET is proportional to its Coss value. The ferrite bead resistance and the Coss value define an RC time constant:
The 90% turn-on time will be approximately 2.2 time constants, which is the typical case for an RC circuit.
Next, the switching losses can be estimated based on the switching frequency and the switching time:
This highlights another point about ferrite beads: they don't just slow down the turn-on time; that slower turn-on time also creates more losses in the MOSFET. If you're trying to switch a MOSFET at high PWM frequency, it's best to avoid using a ferrite bead to filter high-frequency noise from the gate terminal.
The next approach for estimating the turn-on time for a MOSFET with a ferrite bead is to simulate the circuit in SPICE. You could use the parallel RLC circuit model to model the bead; however, manufacturers also provide simulation models for their ferrite bead components that are compatible with common simulation applications, such as LTspice. Extracting RLC parameters for a circuit model from a ferrite bead datasheet is another matter, and we will publish an article about it in the future.
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