Energy recovery (reset) winding on isolated forward converter

[nicor]

Hi all,

I´m studying for my Power Electronics Uni course, have a small doubt with Forward converter.
The topology is the same as in: https://en.wikipedia.org/wiki/Forward_converter.
Of course primary winding has a magnetizing inductor which is not drawn there.
The question is: During the time the switch is off, the energy stored at the core (magnetizing inductor) is "recovered" or sent back to the source by means of the auxiliary reset winding. Is this correct?
But if the circuit is powered by a non-rechargeable battery, I don´t think that´s a good idea. How is it implemented in that case? I think a diode and large capacitor between the battery and the circuit will help (because one can recharge a capacitor, indeed), but really don´t know. Am I right?

Thanks in advance.

 

[BradtheRad]

During the time the switch is off, the energy stored at the core (magnetizing inductor) is "recovered" or sent back to the source by means of the auxiliary reset winding. Is this correct?

Apparently the energy is reduced to zero as it circulates in the diode and the primary.

Explained at the website:



Quote: "During the off-time of the transistor, the magnetic flux of the transformer has to be reduced to zero. The transformer core is demagnetized with N1' via D1 to Vin."

(The names of the components are different from the Wikipedia article.)

But if the circuit is powered by a non-rechargeable battery, I don´t think that´s a good idea. How is it implemented in that case? I think a diode and large capacitor between the battery and the circuit will help (because one can recharge a capacitor, indeed), but really don´t know. Am I right?

From the above it does not appear that current would flow 'backward' either back to an installed capacitor or to the battery.

The website below is an article about converters that are suitable for battery power.

'Regulator Topologies for Battery-Powered Systems'



Quote from article:
"The buck regulator with a flyback winding is the superior-performance topology for many battery-powered applications. The configuration has excellent stability, low peak currents, and low output ripple."

The article points out that peak current is a chief concern when using battery power. If the battery has internal resistance, then the converter will be limited in its power capability.

Of course efficiency is also important when running from batteries.

As to which topology to use, it seems inevitable that there will be tradeoffs to consider. One type might be better for one reason, and another type might be better for another reason.

 

[nicor]

Thanks so much for your answer.

From the above it does not appear that current would flow 'backward' either back to an installed capacitor or to the battery.

I´m pretty sure the current flow back to the source. I attached a page from Mohan´s Power Electronics book explaining forward converter. It´s funny because the figure description states "practical forward converter". :grin: Not practical enough to draw the complete source circuit.

**broken link removed** sorry, had to use .pdf format instead image.

My question is how is it implemented the sinking of energy from demagnetization cycle on the source side?, which I think they didn´t draw there.
I suppose a diode in series and a shunt capacitor will work, because the capacitor can store the demagnetization energy. I don´t know if the same works with other types of source (like batteries).

Regards.

 

[BradtheRad]

I´m pretty sure the current flow back to the source. I attached a page from Mohan´s Power Electronics book explaining forward converter.
...
**broken link removed** sorry, had to use .pdf format instead image.

Yes, I see now how the diode is hooked up so it sends current backwards to the supply positive. I had the mistaken idea that the diode acted like a freewheeling diode only. (Forgetting that a freewheeling diode as such is not a good idea across the primary in a switched transformer converter.)

Anyway I'm not familiar enough with the concepts of the forward converter to understand how to answer your question.

As for using non-rechargeable batteries...

It used to be possible to recharge the zinc-carbon type, though only to a limited degree.

When a new alkaline battery is depleted, it can also be recharged to a limited degree. However it makes them much more prone to ooze caustic liquid unexpectedly. Older alkalines are even more of a risk.

This could mean that the demagnetizing cycle results in reduced drain on the battery's life.

 

[FvM]

I´m pretty sure the current flow back to the source.

Yes, and it's strongly wanted not to lose the stored energy. I also agree that this isn't a problem for a battery which is acting as a capacitor on short terms.

Nevertheless you would want to place a bypass capacitor of sufficient capacitance in parallel to the battery that sources converter peak current and sinks the circulated current without causing avoidable voltage drops at the batterie's internal resistance.