A Colpitts oscillator uses a buffer to provide positive feedback
through a capacitive divider. In the PowerPath controller, this
is realized by the FET. Since it is in a common-drain/source-
follower configuration, it will act as an AC buffer with improved
performance at higher drain currents. The signal at the top of
the capacitive divider is injected into the middle of the divider,
causing the signal at the top of the divider to rise (and the process
repeats).
Oscillations may occur in scenarios where the FET is not fully
enhanced:
X During initial startup, when the gate voltage ramps up and the
output capacitor is charged
X When current is being regulated (if the controller uses active
current limiting)
X When voltage is being regulated (as seen in surge stoppers)
To validate the concept of the switch FET being in a Colpitts
oscillator topology, a basic circuit was constructed without a gate
driver IC (Figure 5). The FET's C
GS
(not shown as a discrete
component in Figure 4) and C2 form the divider.
Figure 5: Prototype of a circuit. (Source: Analog Devices)
Figure 4: Test circuit of NFET as a Colpitts oscillator.
(Source: Analog Devices)
A demo board is used to start into a
capacitive load. During startup, the
gate voltage ramps up with a set dV/
dt, which the output follows.
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ADI | Powering the Future
