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FudFly converter

Started by Unknown July 25, 2020
I guess this is pretty obvious, but when the fet turns on, it's a
forward converter charging C1, and when it turns off it's a flyback
charging C2.

https://www.dropbox.com/s/4vsi3a5r3ky0wv0/FudFly_Conv.jpg?raw=1



-- 

John Larkin         Highland Technology, Inc

Science teaches us to doubt.

  Claude Bernard
  
On a sunny day (Fri, 24 Jul 2020 21:23:42 -0700) it happened
jlarkin@highlandsniptechnology.com wrote in
<2mcnhftioe6o3qsp0lh71gdnm2if4nltps@4ax.com>:

> >I guess this is pretty obvious, but when the fet turns on, it's a >forward converter charging C1, and when it turns off it's a flyback >charging C2. > >https://www.dropbox.com/s/4vsi3a5r3ky0wv0/FudFly_Conv.jpg?raw=1
Yes, in principle. One thing to take into account is that for a high ratio of flyback to forward so a very short flyback pulse, is transformer winding heating A shorter pulse for the same power means more current during the flyback into the output. i * tflyback However the ohmic losses in the coil go up with i^2 * Rcoil * tflyback This heating limits what you can do with reasonable wire gauge. To get a decent size transformer a few more turns and a bit less flyback is simpler. Slower switching is easier on the driver too. ?
jlarkin@highlandsniptechnology.com wrote:
> > I guess this is pretty obvious, but when the fet turns on, it's a > forward converter charging C1, and when it turns off it's a flyback > charging C2. > > https://www.dropbox.com/s/4vsi3a5r3ky0wv0/FudFly_Conv.jpg?raw=1
LOL, I have just invented something similar and wanted to share. My idea is to use the L output pin of a synchronous buck converter to supply a floating MOSFET gate driver elsewhere in the system using the same charge pump principle. Provided the PWM duty cycle is reasonable, it works like a charm -- just prototyped that to source 1mA. It needs one more part than a flybuck (two diodes and one capacitor instead of an additional winding and a diode), but a cheap off-the-shelf inductor can be used, which is a big advantage. Best regards, Piotr
Piotr Wyderski wrote:

> using the same charge pump principle.
I was referring to the doubler network connected to the drain, not to your secondary. Best regards, Piotr
On Fri, 24 Jul 2020 21:23:42 -0700, jlarkin@highlandsniptechnology.com
wrote:

> >I guess this is pretty obvious, but when the fet turns on, it's a >forward converter charging C1, and when it turns off it's a flyback >charging C2. > >https://www.dropbox.com/s/4vsi3a5r3ky0wv0/FudFly_Conv.jpg?raw=1
You get very peaky current in the forward converter if there's no output inductor, and it's unregulated. There IS a converter that regulates both, with a conventional filter on the forward section. The forward stage is PWM, while the flyback section is PFM. There's some interaction between the two, if current mode is employed. This was documented in the mid-80s by Steigerwald at GE and I think there's a control chip for it. Another uses a fixed frequency, but modulates the active clamping network for the forward converter, from the mid 90s. https://www.researchgate.net/publication/274370275_Designing_a_Cost_Effective_Single_Switch_Isolated_Multiple_Output_Flyback_Converter A third uses two transformers, which is sort of silly if the switch and controller costs less than the magnetics - which is the current reality. At low power, of course, you can get away with murder. Your circuit appears to not regulate only the negative voltage generated by the flyback period - your waveform suggests complete energy transfer; so a designed rather than fudged magnetic part. If so, then why not just add another flyback winding? RL
On Fri, 24 Jul 2020 21:23:42 -0700, jlarkin@highlandsniptechnology.com
wrote:

> >I guess this is pretty obvious, but when the fet turns on, it's a >forward converter charging C1, and when it turns off it's a flyback >charging C2. > >https://www.dropbox.com/s/4vsi3a5r3ky0wv0/FudFly_Conv.jpg?raw=1
I guess I missed the point - this is a single output, so one regulation is as good as another. Current mode control, or even simple limiting may be an issue. RL
Does this really work

I haven't simulated it or dug deeper, but is seems the positive rail is standard flyback, regulated with Ipeak squared like normally, while the negative path is a forward style converter, with no inductor in the buck part, so more or less just the input voltage and turns ratio?
*Charge pump.

You're missing two diodes and inductors for that.

Tim

-- 
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Design
Website: https://www.seventransistorlabs.com/

<jlarkin@highlandsniptechnology.com> wrote in message 
news:2mcnhftioe6o3qsp0lh71gdnm2if4nltps@4ax.com...
> > I guess this is pretty obvious, but when the fet turns on, it's a > forward converter charging C1, and when it turns off it's a flyback > charging C2. > > https://www.dropbox.com/s/4vsi3a5r3ky0wv0/FudFly_Conv.jpg?raw=1 > > > > -- > > John Larkin Highland Technology, Inc > > Science teaches us to doubt. > > Claude Bernard >
On Sat, 25 Jul 2020 10:09:48 -0500, "Tim Williams"
<tiwill@seventransistorlabs.com> wrote:

>*Charge pump. > >You're missing two diodes and inductors for that. > >Tim
Don't follow that. Got a sketch? -- John Larkin Highland Technology, Inc Science teaches us to doubt. Claude Bernard
I think Tim hints at that you need an inductor to have a buck part and diodes for the freewheeling part. Otherwise it won't act as the state space model dictates