The dual diode thing on the grounded side makes it feed back on the primary peak-to-peak voltage, which tracks the secondary, which also rectifies peak-to-peak.
The LTC3803 is a peak current limiter, so the startup current is pretty much always the same, regardless of topology.
Interestingly, at current limit in the flyback topology, it is pretty much constant power vs voltage. That can be useful.
That's just terminology. I call the non-flyback side a forward converter, because fet turn-on drives the load with no specific energy storage.
I think of "switched capacitor" as something else, an actual switched capacitor.
--
John Larkin Highland Technology, Inc
Science teaches us to doubt.
Claude Bernard
Shorting the drain capacitor and using a slow diode on the primary peak clamping/regulation node could allow clamp current to be included in the transformed secondary flyback delivery, through the slow diode's Trr, rather than just being burnt in the clamp's load.
Active clamp circuits use a switch in that diode position and the induced reverse/recovery current that it produces allows potentially lower switch turn-on losses as the drain voltage falls naturally - but it won't work in a discontinuos (complete energy transfer) circuit, at fizxed frequency.
If a switch is used, it can be translated to a ground-referenced pmos, using the ~ same drive as the main switch (with suitable negative dc offset).
The lack of the inductor is a regular implementation at low power, e.g. see the SN6501. For lower ripple level is commonly being implemented as two interleaved forwards, i.e. the push-pull. :-)
I was also considering that topology in my multi-output slew-rate limited low-power auxiliary PSU; perhaps you remember the thread. But the added complexity and dependence on the transistor model turned out not to be worth further exploration. The LT3439 did it better. But then I am still using to feed two groups of independent, interleaved half-wave rectifiers. The loads are quite symmetric, so there is no noticeable flux imbalance, all is within the current-mode regulation capabilities of the controller. It works very well at power level
200-500mW per channel.
Just to note: there *is* a topology called flyback-forward.
Or a high-side depletion-mode N FET due to its correct body diode direction. I did it once and the results were very OK. The V_IN range was just right to use this trick.
It occurs to me that my circuit is functionally equivalent to a discontinuous boost or flyback converter driving a voltage doubler or a C-W multiplier. Fet ON makes a low impedance voltage pulse of Vcc*N volts into the capacitive load, and fet off is flyback in the opposite direction. All the sophisticated objections to my circuit apply to the flyback/C-W case too.
I guess my only innovation is the p-p detector in the lowside feedback loop, which only a few thousand people invented before me.
Never mind.
--
John Larkin Highland Technology, Inc
Science teaches us to doubt.
Claude Bernard
The transformer, tied to the positive rail, already supplies half of the pk-pk being sensed, without the need (or interference of) the CD parts in question.
The fet current waveform, is a combination of the series Llk/Cout (reflected) and the primary mag current. The former portion of that peaks early and is resonant. Though the chip may have a spike suppressor on it's sense pin (a delay), it can't regulate on a waveform with current that varies early in this manner.
It's one of the basic features (and hazards) of a flyback - the output current being uncontrolled when shorted. In your case, this current will be capacitively limited by the output structure - but may involve output (or at least output cap) polarity reversal.
When you use 'terminology', you should stick to definitions of the terms, which DO exist. We know what you mean, but you should stick to simple switch-on and switch-off terms to describe the operating periods.
Your 'forward' operation is basic switched capacitor with (ideally) a 'DC transformer' separating the elements.
It is a switch shorting two voltage sources, something usually tolerable only at very low power levels, where efficiencies below 50% can be ignored.
I've already admitted that, at low power levels, you can get away with murder. That doesn't mean that you should't admit to your crime . . .
Getting away with it is just part of the fun, and any circuit that can survive on parasitic components is bound to end up cheaper ( at thr BOMaterials level, anyways) than one requiring a purchase order and schematic entry.
Terminology is important, otherwise you cannot communicate in any sensible manor.
The FET turns on in forward fashion, with the voltage on the secondary side determined by VDC in times turns ratio. That voltage is imposed directly on the output cap. Thus switch cap, since you have a cap on the VDC input as always
Switch cap converters with non-ideal transfer ratios are not very efficient
By half, I meant the input voltage ( 'forward' ) share of the sensed voltage.
Chaotically, and not necessarily safely, if at all possible, when a silly nodal waveform appears within the regulation loop.
Input (cap) and C1.
There's a TI chip that regulates a switched cap. You should check out it's efficiency plots. TPS60121.
Switched cap light load efficiency usually depends on reduced frequency ie no load / no loss.
I stuck an old switched cap clamp sim, late, into the download. This shows the commonly used pfet transposition to common drive gnd, with common gate drive voltage offset.
I've Spiced this topology and it can be made perfectly stable. It gives decent regulation on the load side without the usual optocouplers. I'll actually build it if my customer will give me a little more info about his needs.
My Spice circuit has no input cap and it works fine. I have never considered power rail bypass caps as "switched". Output filters neither.
But use the words that you like. I just build circuits.
Of course if you have two caps at very different voltages, and slap them in parallel, energy is lost. So don't do that.
--
John Larkin Highland Technology, Inc
Science teaches us to doubt.
Claude Bernard
Zin, Llk are the main ones, but it's hard to tell without an actual model or measured waveforms from which they can be enumerated or derived.
Doodling on a pad isn't building circuits, so there's a limit to how they can be evaluated.
I doubt very much, for a start, that the chip can be induced to produce the illustrated pulse-widths, except under very specific external conditions - something that nature is very lax in ensuring.
I'm not going to flog topology or applications on newsnet - I learned some years ago that there was little profit or satisfaction in doing so.
Yes, at low power you don't care about crap efficiency and the current can be limited by relatively high Rds(on) or hFE-limited transistors. Here's another example of that:
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I know the PP drivers as MAX253, same thing more or less.
I recall a bunch of ADuM isolator parts having PP drivers in them, though I think they're actually current mode and suggest^H^Hrequire output filter inductors?
Hmm, I get very few search results. One patent that's some sort of quasi-resonant C-clamped-flyback hybrid.
It seems obscure enough that I wouldn't feel bad creating yet another definition for it...
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