Isolated Forward converter L(sec) saturaton

[spinach]

I am using a one transistor forward converter......i am worried that my input line voltage is varying some 20%

...So if i have the secondary inductor current going up and down by the same amount at one input voltage...then when this input line voltage rises the secondary inductor current will rise up and then not fall as far back....eventually leading to saturation of the secondary inductor.

-i could solve this by ensurin that seondary inductor current always falls back as far as it has risen at highest input line......but then this will mean having a low duty cycle and power throughput will be more awkward.

Please can reades advise if i am too worried about secondary inductor saturation in one transistor forward converters.?

 

[roydm]

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[spinach]

Apologies for missing letters out of the first post, this keyboard is well past it.

Anyway, I am thinking about using less capacitance at the output of my FWB, but know that this will increase the amount that the line input goes up and down......so i want to check that with this increased input line variation, i am not going to saturate the secondary inductor....

...with a widely varying input line, it will be surely difficult to ensure that the volt seconds across the secondary inductor are the same for both on and off times of the switch.

..could this mean saturation for the secondary inductor?

 

[roydm]

Spinach,
I was unable to figure out what type of circuit you are refering to. Is it some form of switching voltage regulator?
Roy

 

[DHambley]

A forward converter should adjust it's output inductor to put out the same current to the load no matter what the input voltage is, ie, the pulse width should be inversely proportional to input voltage.

The secondary inductor should be designed to be below it's saturation level at the worse case high load. this should include the peak of the ripple current.

Did you design the inductor?

 

[spinach]

Hi

The inductor was just an off the shelf one.......this was part of a pseudo-PFC one transitor forward converter i was doing which i now know wouldnt have worked as the pulses of current drawn by such a converter cannot be passed by the supply inductance.....you can mitigate it by adding AC capacitance to the AC line in but that level of capacitance has too much leakage.

I'm going to have to quit on this one and do a flyback switcher instead....i was hoping with a forward i could use cheaper output caps.......i know its not switching for a time round the mains zero crossing but my regulation requirement is not at all great...it just has to compete with the poor, varying output voltage that you get with an auxiliary coil on a PFC transformer.