Pulsed DC through magnetics?

[eeka chu]

I realise this goes against the AC grain, but will pulsed mains DC through a transformer begin significantly saturating it with a DC component?

The application is very obscure, I know the diodes usually go after the magnetics. :P There won't be any filtering prior to the rectified mains going through, so it'll be coarsely pulsed DC with no static DC component in effect. But I'm less sure about how the iron will respond to that once it's storage and latency are taken into consideration.

[dave w p]

If you put DC into a transformer the main effect you'll get is heating the coil, you shouldn't get any DC out. The pulses however will be transferred to output at a different AC voltage. Transformers work by applying changing voltage or current to the primary. Any potential DC will quickly fall to zero volts.
I hope this helps you.
The HIFI site

[aardvarkash10]

it will saturate unless its gapped for the current. The output will still be AC.

What you are desribing is an automotve ignition system btw...

[star882]

It is very common to pulse DC at high frequency through transformers. All single ended isolated switching power supplies do just that. The automotive ignition system just happens to be a form of flyback converter.

[c2cthomas]

Transformer types - Wikipedia, the free encyclopedia

Used pulse transformers in radar's - 100KV @ a couple of amps and presto - fried tech!!!

[eeka chu]

Thanks for the help!

I realise any form of constant DC component will saturate the core to some extent, but was wondering how it starts behaving when the DC is constantly varying. Since then there is a constant polarity but never a constant current.

In the same way that putting the rectified mains across any amount of parallel capacitance will begin flattening the peaks, the core of the transformer would seem to be similar to putting a choke in series with the rectified mains.

However, I have seen things like National Semi's SMPS designer and flybacks where they seem to be doing just that, with the intent being to massively reduce the size and cost of the magnetics. If saturation was an issue, that would seem to counter the logic at work.

Forgetting the reduction in the size of the magnetics, is this why they tend to use such high frequencies when doing this?

The wiki section on pulsed transformers seems to be with regards to squarewaves, where there would be a significant component of constant DC.

I'm thinking specifically of the mains, for example, just moving the rectifiers to before the transformer with zero filtering in the way. The frequency will double, the waveform will never be sat still and will be constantly changing, but it will be a single polarity.

In the squarewave example, the duty cycle will fall, a lot. Is that going to happen with a rectified sinusoidal form? Something is telling me, having it at one polarity is not making use of half of the capacity of the core, to swing in the opposite direction. So it's duty cycle would still half?