J
John Popelish
Robert said:Err...
Take a look at the current and voltage waveforms in a flyback system.
Turn on the switch, inductive current increases in the standard R/L form.
Yes. And the magnetic field is instantaneously proportional to coil
current.
When the switch is opened, the magnetic field starts to collapse; the
waveform across the inductor is square-ish in most practical circuits.
During that time, the current goes rapidly to zero as the flyback
voltage pulse rises; roughly remains near zero at the flattish top, and
then goes negative as the flyback voltage pulse drops to zero.
No argument. The switch current goes to zero first, the coil current
detours first to the coil capacitance, and to snubber, if any. Then
the voltage rises till either the capacitance (and snubber) absorb all
the energy stored in the coil, or a clamp voltage is reached. If the
latter (as the op is doing) then the current detours to the clamp.
The current continues to ramp toward zero as does the magnetic field.
As the current and magnetic field pass through zero, the voltage
starts to swing back toward that connected to the other end of the
coil (toward zero volts across the coil). But to change the voltage
across the coil requires current to charge the stray capacitance
across those nodes, so this current passes through the coil and and
this causes a reversal of the magnetic field.
Perhaps I misunderstood your last post when you said, "Allowing
current to flow after the switch opens will mean that the magnetic
field will continue to exist while it collapses, and if current
continues to flow, the magnetic field will reverse."
How could the field not continue to exist while it collapses? And the
coil current must cease and change directions before the field ceases
and changes directions.
However, the voltage would continue to decrease and go negative (L-C
oscillations), but the switch (FET) internal diode conducts, allowing
the coil current to continus to flow.
The voltage pulse seen has the *same* polarity as the supply.
In a standard flyback scheme, the negative current waveform is
mirror-image of the ramp-like charging time.
This remains to be a fairly close picture of operation waveforms, even
if the core of the inductor saturates to some extent.
My only point was that the applicable generalization for this op is
that the magnetic field is instantaneously proportional to the coil
current at all times, and the field reverses exactly when the current
reverses. If you agree with this than we are on the same page.