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Flybacks and Waveforms...

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Patrick

Sat Mar 05 2011, 09:07AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Does anyone know what the primary waveform is (primary wound on ferrite) if driven by a 555? And or/ the Secondary of a diode-split Flyback? Or will I have to measure this myself? A paint Jpeg, or a hand drawn diagram would be a useful approximation.

I presuppose that a square wave in will not equal a square wave out. Also, I expect the primary current to be a rising saw tooth graphed function, with the same voltage would be appraximately square?

Proud Mary

Sat Mar 05 2011, 01:24PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Patrick wrote ...

Does anyone know what the primary waveform is (primary wound on ferrite) if driven by a 555? And or/ the Secondary of a diode-split Flyback? Or will I have to measure this myself? A paint Jpeg, or a hand drawn diagram would be a useful approximation.

I presuppose that a square wave in will not equal a square wave out. Also, I expect the primary current to be a rising saw tooth graphed function, with the same voltage would be appraximately square?

Primary time constant Ï„ = RL applies. Ï„ will impose its own idea of order on any input 'square' wave and, inter alia, limit the useful upper pulse repitition rate.

Antonio

Sat Mar 05 2011, 01:24PM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

Look at this, for example:

Patrick

Sat Mar 05 2011, 02:07PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Antonio wrote ...

Look at this, for example:

Yes this helps.... but what about a secondary waveform with no secondary side components, whats theV trace like then?

Ash Small

Sat Mar 05 2011, 03:35PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Patrick wrote ...

Antonio wrote ...

Look at this, for example:

Yes this helps.... but what about a secondary waveform with no secondary side components, whats theV trace like then?

I'm just stating the obvious here, but would the output not just be a spike?

EDIT: And wouldn't the diodes prevent any ringing?

Steve Conner

Sat Mar 05 2011, 03:41PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

The simple answer is that, without any loading on the secondary side, the output is "undefined". To a naive first approximation, considering an ideal transformer and ideal switch, it's a spike of infinite voltage and zero duration.

In a more detailed analysis, the switch takes a little time to turn off, the secondary's self-capacitance rings with the transformer's inductance, and the resulting spike is more of a damped oscillation, whose peak voltage is just dangerously high rather than infinite.

Moral of the story: Flyback converters are not designed to be run without a load (or some form of control loop) and will die. Put on a load, and the waveforms change to what you can see in any EE textbook like the link posted by Antonio.

Patrick

Sat Mar 05 2011, 05:43PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Steve McConner wrote ...

The simple answer is that, without any loading on the secondary side, the output is "undefined". To a naive first approximation, considering an ideal transformer and ideal switch, it's a spike of infinite voltage and zero duration.

Your absolutly right, i feel as dumb as a box of rocks.

Steve McConner wrote ...

In a more detailed analysis, the switch takes a little time to turn off, the secondary's self-capacitance rings with the transformer's inductance, and the resulting spike is more of a damped oscillation, whose peak voltage is just dangerously high rather than infinite.

Yes, this was what i wanted to know.

Steve McConner wrote ...

Moral of the story: Flyback converters are not designed to be run without a load (or some form of control loop) and will die. Put on a load, and the waveforms change to what you can see in any EE textbook like the link posted by Antonio.

yes but i really meant what would happen with Zero installed capacity or inductors, just purely resistive, and a duration of kV out much less than that of the off time, all of antonio's and my sources show continuous output V and I. Yet this is not how we operate our HV transformers here on the forum, our output HV is most often discontinuous.

so with only a high voltage resistor in series with the output, what would the graph translations look like? Approximatley a rounded spike that tapers down with time?

Ash Small

Sat Mar 05 2011, 09:03PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

In your first post you mention 'diode split secondary'. Surely this means the output won't be a damped oscillation? (output can't go negative)

If I=V/R, the output through a resistor will still be a spike. It won't tail off gradually (unless you have a capacitor in parallel with the resistor)....or am I missing something?

Patrick

Sat Mar 05 2011, 09:16PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Ash Small wrote ...

In your first post you mention 'diode split secondary'. Surely this means the output won't be a damped oscillation? (output can't go negative)

If I=V/R, the output through a resistor will still be a spike. It won't tail off gradually (unless you have a capacitor in parallel with the resistor)....or am I missing something?

in this pic there are 3 diodes in series with 3 coils...isnt this how modern flybacks are made, to reduce insulation demands?

Ash Small

Sat Mar 05 2011, 09:21PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Yes...but the diodes prevent the output going negative, so the output can't oscillate. (at least as I understand it)

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