flyback transformer ratio

[xaccto]

flyback transformer turns ratio

It has me puzzled how exactly do you work out the turns ratio of a flyback transformer?
its not sinusoidal waveform afterall, more like crazy pulses.
I'm just not getting the loaded secondary voltage I want with my experiments.
I'm doing a step up from 12V to 30V, unloaded the secondary does have huge voltage spikes, but not if I put a load on it, more like only 10V,

this could be purely the fault of the switching controller not regulating properly,
but still would like to know if I got the turns ratio correct.


Also, is a full wave rectifier (centre tap type) of any use in a flyback ? I've never seen a published circuit with one, allways 1/2 wave rectifier (ie one diode)

 

[FvM]

Flyback, by definition, doesn't transmit in forward mode and thus has no full wave rectifier. A single ended, forward+flyback converter may be build.

With a flyback converter, the winding ratio don't need to be designed very exactly. To achieve a 50% duty cycle, the winding ratio would be equal to intended voltage ratio, as in a forward converter. But the flyback converter can achieve a large voltage variation range.

It sounds, like you are operating a flyback transformer with a too high inductance, that don't store sufficient energy. A flyback transformer (if not using e.g. low permeable iron powder cores) always has to be gapped.

 

[xaccto]

flyback transformer design

Yes, its a RM8 core with a gap, N41 material,

the gap is in the centre core, is that enough?, or should the whole core be gapped?

I undid the clips, so can vary the core.

I've got the output to a ~1AMP load ( 2x7805 + 5ohm resistor). Monitoring the input current.
If I move the core apart, input current goes up, and secondary oscillations noticed on the switching mosfet waveform. The output increased by 1volt, input current doubled.
Somehow I think this isn't great.

 

[grittinjames]

how to build a flyback transformer

ds it have feedback?
If not your output will reduce with respect to load.
In no load condition flyback output voltage will be high in both primary and secondary.
It is quite similar to opening a charged inductor

 

[xaccto]

design flyback transformer with gap core

it does have f/b.
however in comparison to using the controler chip in a say buck regulator topology, and the same chip in a flyback circuit, the difference in regulation ability
is huge, is it just my luck or is there a reason for that, I don't know.
in either case f/b is a resistor divider network, so ought to have similiar ability.

the primary has 8Turns, the reset winding 8T and the secondary 17T (i think)
I'll retry this again, should I go more turns or less with the primary ?

By changing Ct, cap I experiment with different freq from about 20khz, to about 120khz


so far i've stuffed 2x uA78S40, (well got them free)
there is something wrong with 78S40, specifically f/b and it supposedly regulating - maybe thats why its obsolete. Its available voltage ref pin, and addition opamp is
handy tho.
More success with MC34063.

Somehow maybe its frequency control regulator blocks so far using and having not much luck or i'm just unlucky.
I have some TL494 lying around, so i'll try next, so perhaps pulse-width control has better ability ?

Looking at buying some sw control chips, SG3525 - can directly drive mosfet,
or I should maybe look at something that can do specifically current mode control ??
Suggestions ?

 

[FvM]

flyback turn ratio

I don't know your design template, but a flyback converter don't generally need a reset winding. In some cases, off-mains converter have additional primary auxilary windings to limit primary flyback overvoltage, cause it may be build with a lower leak inductance than the isolated secondary.

In your design, the said reset winding must be expected to absorb most of the flyback energy in 12->30 V step-up operation, respectively to limit the secondary voltage to about 25 V. Is this the intended operation?

 

[xaccto]

simulation of flyback transformer

Have a reset winding with a diode, and I do notice a drop in the back emf peak on the primary when connected, so I feel thats better than using some snubber on the primary, even though the mosfet appears to handle it ok anyway, but anything that
makes less stress is surely good.

can't say I really know what the diff is between a flyback or forward converter,
just to say that this is a single primary winding and one transistor switch driving it.

 

[grittinjames]

fly back primary reset circuit

Reset winding is used only for forward convertor

Added after 32 seconds:

please post yr schematics