From: John Popelish
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Subject: Re: Ringing in Discontinuous Buck regulator
References: <3D8F348B.D8BBCEA7@singnet.com.sg> <3D90941F.7A4AB009@singnet.com.sg>
Date: Tue, 24 Sep 2002 17:21:37 GMT
NNTP-Posting-Date: Tue, 24 Sep 2002 13:21:37 EDT
> Hi Bob,
> Regarding your answer....
> > But the non-conducting diode has capacitance to ground (and also there is
> > other stray capacitance as well). So once it goes discontinuous, you have a
> > series resonant tank circuit. The output side of the inductor still has the
> > low impedance of the load connectod to it, and the input end of the inductor
> > is connected through the series capacitance of the diode to ground. So, this
> > L/C tank sinply resonates at its own natural resonance frequency.
> At the instant that the current goes to zero in the inductor, will it stop at
> zero or continue to go to negative? If it goes to zero exactly, how does it
> charge the stray capacitance? Don't you need some energy for oscillation to take
> Am I making sense?
As long as the inductor current is continuous, it keeps the flyback
diode conducting which keeps a constant voltage across the inductor
(output voltage minus the diode drop). That voltage is also what
drives the inductor current toward zero.
But the moment the inductor current passes through zero, The diode
turns off, and you just have a capacitor (all the stray capacitance of
the diode, transistor inductor, etc.) in series with an inductor. but
there is voltage across the inductor, since the capacitor loaded end
is near zero volts and the other end is at output voltage. So the
inductor current starts to build as that current charges the
capacitance up towards output voltage. The moment the cap gets up to
output voltage, the inductor has no drop to drive its current up, so
its current levels off. But that current keeps driving the cap
voltage up, above the output voltage, creating a growing reverse
voltage across the inductor, which begins to back the inductor current
back toward zero. This continuous until the inductor current again
reaches zero, with the capacitance charged up to almost twice the
output voltage, where the process reverses. Thus is a classic series
LC resonance with the cap pre charged to some offset from the final