Re: Software support Request for Full QSK BITX40

Jerry Gaffke


Here's some notes on a standard flyback converter:
Notice the diode in the secondary, though the text states that in some cases the diode is left out
and the result is just called a flyback transformer. 

The Bitx40 output stage has no diode, so the secondary winding of T7 has a balanced AC current with no DC component. 
The bottom of L8 is bypassed to ground by C153,154, so pin 6 of T7 is just a well filtered 12v DC rail.
T7 is an autotransformer where the primary winding is shared with the secondary making it a bit
harder to think about, but those windings could just as well have been kept separate.

Here's an interesting description of flyback vs forward transformers:
A flyback transformer can be thought of as an inductor with an extra winding. The energy is stored in the inductance of the primary winding during the on time of the drive circuit (similar to a switching regulator) which then is transfered to the output winding when the drive circuit is off. A flyback transformer thus has a net DC current through the windings and the core must be designed to handle this without saturating.
A normal forward transformer connection does not store energy but simply transfers the energy from the input winding to the output winding. For a typical switching converter, the transformer is driven by a push-pull driver or by alternately turning off each half of a tapped primary. There usually is no significant off time for the transformer waveform. This transformer type does not have to handle any significant DC current because of the alternating nature of the waveform.

He's assuming it's a flyback converter with that diode in place when he says that the windings (plural, including the secondary) have a net DC current.
In our case, the primary winding has a DC current component, but the secondary does not.
So according to wikipedia it's a flyback transformer, but not a flyback converter.
Not that names matter, except that it helps when reading any reference material.

The output transformer of the uBitx is driven by a push-pull pair, and would be considered a forward transformer. 
Nothing flyback about it, the energy at the input is transferred immediately to the output
without being stored temporarily in the magnetic field.
Output AC voltage is determined by the input AC voltage and the turns ratio, just like a filament transformer

All half baked yet, I'm still trying to sort this out.
One major conundrum: 
When the Bitx40 IRF510 turns off and the magnetic field collapses that energy needs some place to go,
the induced emf in T7 will create voltage spikes that rise as high as it needed till it finds a load, 
perhaps arcing across the transformer.  But my LTSpice simulation of the Bitx40 final with no load
shows no such spike, the output voltage remains pretty much determined by the turns ratio
as if T7 were a straight up forward transformer.

I'll let you worry about those reflections.

Jerry, KE7ER

On Tue, Jan 30, 2018 at 11:01 pm, Jerry Gaffke wrote:
While I'm dim on transmission lines, your description of the reflection is easy enough to follow and makes good sense.
Magnetics I'm even dimmer on.
I had been assuming that T7 was primarily a forward transformer, not a flyback transformer.
Hadn't really thought about it.
But you are probably right that the source of energy when the IRF510 turns off
would be coming from the magnetics of T7, and thus it is a flyback transformer.

I once ran a simulation of the Bitx40 final with a 50 ohm load, and without any load.
The output voltage with no load increased by maybe 20%, perhaps because of lower IR losses in the IRF510.
Would be interesting to see what the output voltage does when the duty cycle at the IRF510 gate changes.
No idea what happens to your reflected wave, will have to think about that one.

These are really interesting questions, and fundamental to how this all works.
I may have to get out the scope, and maybe cut a quarter wave of coax.

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