SMPS Duty Cycle

[gearheadgene]

Is there any reason to not use a low duty-cycle design? Would 9% be too little? What ramifications, if any, does this have?

regards

gene

[darkfenriz]

For the same voltage and power, this will cause 5 times more peak current stress compared to 45%.

[gearheadgene]

Yes, I see what you mean. In simulation, on SEPIC topology, Vin = 50V, Vo=3.3V, Iout = 2.0A, duty cycle is around 6%. The ripple current current in the inductors is quite high. So I dropped the Vin to 10V and the ripple current dropped as well. Although the average in both cases is the same - as I expected it should be.

I think I will have to change topology to flyback so that the duty cycle can get higher - maybe 50%, seems reasonable.

[ifrythings]

Originally posted by gearheadgene
I think I will have to change topology to flyback so that the duty cycle can get higher - maybe 50%, seems reasonable.

50% duty cycle in a Flyback converter is the maximum it can go, the other 50% is reserved for core rest, you'll have to lower it, maybe 25%(ish)?

[Eva]

Originally posted by ifrythings


50% duty cycle in a Flyback converter is the maximum it can go, the other 50% is reserved for core rest, you'll have to lower it, maybe 25%(ish)?

This "maximum" duty cycle actually depends on the ratio between the input voltage and the reflected (reset) voltage, which is a function of the turn ratios and the output voltages. It may be lower than 50% as well as higher.

[gearheadgene]

Eva,

Can you tell me if using toroid in flyback supply is ok? I read somewhere (that I can't recall) that it is ok since there are distributed gaps in the core material. But, as I read more, most of the literature points to using a precise gap in the core, by design. How would that play out when using the toroid, where the exact dimensions of the gap are unknown?

thanks

[Eva]

A single gap in the inner leg of the core is the worst situation because it results in higher copper losses due to proximity effect. The "finging flux" leaking from the gap induces eddy currents in nearby magnet wires. The only advantage is that the windings effectively shield the gap resulting in reduced leakage flux.

Gaps in the outer legs result in lower copper losses at the expense of high leakage flux.

Iron powder cores don't suffer from increased copper losses due to fringing flux but they tend to produce more leakage flux, particularly when the windings don't cover the entire core (as in iron powder E cores).

Toroids suffer from poor coupling between windings in comparison with E cores with interleaved windings. This is a big problem in flyback topologies because more energy has to be clamped in the primary side (a two-switch flyback may solve this, though).

Good line isolation is also quite hard to achieve with toroids and usually results in even worse coupling.

Now choose yourself

[Elvee]

Originally posted by Eva
A single gap in the inner leg of the core is the worst situation because it results in higher copper losses due to proximity effect. The "finging flux" leaking from the gap induces eddy currents in nearby magnet wires. The only advantage is that the windings effectively shield the gap resulting in reduced leakage flux.

Gaps in the outer legs result in lower copper losses at the expense of high leakage flux.

This makes me think:
Old style potcores may well offer the best of both options: unlike the present construction, made of two symetrical cups, they were composed of three parts:
-A disk with a center pillar
-An outer ring
-An end disk
If the gap is implemented at the top of the pillar, it will still be enclosed and shielded, but it will largely be offset from the main part of the winding, especially so because of the presence of the coil former's flange.
Might be time to revive this design with modern materials.

[Circlotron]

When you think about it, a potcore is an inside-out version of a toroid. The winding and the core are in opposite places. The dual of a toroid perhaps?

[megajocke]

"If the gap is implemented at the top of the pillar, it will still be enclosed and shielded, but it will largely be offset from the main part of the winding, especially so because of the presence of the coil former's flange.
Might be time to revive this design with modern materials."

IIRC I did a FEM simulation of something similar and the results were even worse as there will be more layers of winding causing increased proximity effect.

Spacing the winding away from the gap works well though. You can curve it too and it will be even better. That is, winding is furthest from center leg at the center of the bobbin curving inwards at the ends of the bobbin.