On a sunny day (Fri, 24 Jul 2020 21:23:42 -0700) it happened snipped-for-privacy@highlandsniptechnology.com wrote in :
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.
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.
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.
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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?
I haven't simulated it or dug deeper, but is seems the positive rail is sta ndard flyback, regulated with Ipeak squared like normally, while the negati ve path is a forward style converter, with no inductor in the buck part, so more or less just the input voltage and turns ratio?
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
The turn-on phase isn't a buck, it's a forward converter. Turnoff is conventional flyback. It might like a snubber on the primary, mostly for cosmetics.
I admit the dynamics is interesting, especially at startup. If I decide to use it, I'll certainly Spice it first.
Anybody with another idea could post a sketch.
--
John Larkin Highland Technology, Inc
Science teaches us to doubt.
Claude Bernard
This is a charge pump. Vout = Vin * N2/N1. Regulation not possible. Transistors sink huge peak currents during startup.
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(this is an automotive amplifier DC-DC converter, a very typical example)
This is a forward converter. Vout = Vin * D * N2/N1 (give or take DCM/CCM). Eminently regulable. Transistor sinks trapezoidal current corresponding to inductor charge.
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This monstrosity might be better deserving of such a name,
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but I don't know why anyone would ever build it. (The effect is to drive L1 with two pulses per cycle; in analogy to steam engines, it might be called double-acting. The forward pulse amplitude is set by VIN - Vds(sat); D1 is required to set the flyback pulse amplitude to some maximum value, otherwise you would have two inductors fighting it out, and a waste of voltage. Like the half-wave forward converter, D must be limited to 50% or less.)
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