Parasitic Oscillation in Hot Swaps
By Aaron Shapiro, Product Applications Engineer,
Analog Devices, Inc.
Introduction
Hot swap, surge stopper, eFuse, and ideal diode controllers
that use high-side N-channel MOSFETs (NFETs) can suffer
from oscillation during startup and voltage/current regulation.
Datasheets typically mention this issue briefly, along with the
recommended fix of adding a small gate resistor. However, without
a clear understanding of the root cause of oscillations, designers
may position the gate resistor poorly in the layout, making the
circuit susceptible to oscillation. This article will discuss the
theory behind parasitic oscillation and may prevent a board
revision.
Initially, adding gate resistance may seem redundant, as the
resistance looking into the NFET's gate is infinite. Users may omit
the part with no consequence and question whether the gate
resistor was necessary. However, the 10Ω gate resistor serves as
a preventative measure to suppress ringing on the gate node. The
gate node possesses the components of a tank circuit, starting
from the gate trace itself. A long PCB trace introduces parasitic
inductance and distributed capacitance to a nearby ground plane,
creating a high-frequency path to the ground. Power FETs that are
optimized for a high safe operating area (SOA) have nanofarads of
gate capacitance, which is further exacerbated when additional
FETs are connected in parallel for increased current handling.
Zener diodes used to clamp a FET's V
GS
also contribute parasitic
capacitance (although C
ISS
from a power FET dominates).
Figure 1 shows a generic PowerPath™ controller with parasitics
drawn in.
When the circuit is rotated (Figure 2), its resemblance to a Colpitts
oscillator becomes evident (Figure 3). It's a tank circuit with added
gain, capable of generating sustained oscillations. This configuration
is found in PowerPath controllers that utilize N-channel FETs.
Figure 1: A generic PowerPath controller. (Source: Analog Devices)
Figure 2: A rotated PowerPath controller. (Source: Analog Devices)
Figure 3: A Colpitts oscillator. (Source: Analog Devices)
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