Fundamental physics of coupled inductor cross-regulation in forward converter?

[grizedale]

Fig 4, page 6-4 shows coupled inductors at the output(s) of a forward converter.

**broken link removed**

They improve cross regulation.

Is the method of cross regulation that when one output gets heavily loaded, its capacitor gets discharged, and its Vout falls.......then.......its inductor has a higher voltage across it...(Kirchoff's).........

....then...it obviously couples this increased volts/turn to the other (more lightly loaded) winding......so its inductor voltage increases...and so it "feels" less driving voltage from its transformer secondary....and so as duty cycle increases.....the majority of current goes to the heavily loaded winding....which is the desired (balancing) effect?

OR.....

Is it that the heavily loaded winding couples more flux into the common core...and this flux opposes the growth of current in the other winding.........so the duty cycle can increase, and the majority of the current will go to the heavily loaded winding, and not to the lightly loaded winding..which is the desired effect?

 

[FvM]

The advantage of coupled inductors is in no-load operation. With uncoupled inductors, an unloaded output inductor goes to DCM and is no longer averaging the input voltage, the output voltage rises up to the input peak voltage. Coupled inductors share the flux between windings and keep the CCM mode waveform as long as one output is sufficiently loaded or goes to balanced DCM in overall idle operation. Less than unity transformer and inductor coupling, winding resistance and diode voltage drops still restricts perfect cross regulation.

 

[BradtheRad]

That's an inventive method of regulation described in the article.

I think your second idea brings out the advantage of common coupling. But I believe it applies chiefly in case of heavy load at all 3 outputs.

Because consider the question when all 3 are light loads: Do they get out of regulation then? Because in that instance no one load is doing anything to 'inhibit' the other 2.

So I suspect the regulation comes into play when all three loads try to draw high current.

High current through coils generates strong flux fields. The flux fields inhibit each other. So none of the 3 gets out of regulation.

These go together: strong current, strong flux, weak EMF. This agrees with your first idea. However it applies to 1 load, or any number of loads. Hence I don't think it's the chief operating principle with inductive coupling.

That's why I think your second idea describes the action of inductive coupling in the converters.

 

[grizedale]

BradTheRed

Because consider the question when all 3 are light loads: Do they get out of regulation then? Because in that instance no one load is doing anything to 'inhibit' the other 2.

...So basically would you suggest that because of the "all-outputs-light-loaded" case, we should still use weighted resistor feedback from each output when we use coupled inductors?

 

[FvM]

I didn't comment yet your attempts to explain the effect of coupled inductors. Reviewing BradtheRad's post, I feel tempted to clarify, that both are looking at the wrong side of the problem.

The coupled output inductor is basically working a a transformer. It couples winding voltages, not currents. Individual currents are summed up to a total magnetizing current for the common core.

But other than in a usual transformer, all winding currents have the same direction (enforced by the rectifier diodes). Each winding can only contribute it's output load current to the magnetization, it can't share the load of another winding.

P.S.: Feedback with coupled inductors is a more practical problem. As said, there will be still a load dependent voltage inbalance due to non-ideal component properties. Mostly, SMPS with coupled outputs have a "main" output voltage with narrowed specification that drives the feedback, resulting in a full-regulated and one or more half-regulated outputs. If you have two symmetrical outputs, using an averaged feedback signal would balance the error.

 

[BradtheRad]

would you suggest that because of the "all-outputs-light-loaded" case, we should still use weighted resistor feedback from each output when we use coupled inductors?

I don't know about you but I think of the load as providing 'weighted resistor feedback'. Whatever amount of voltage it draws off the capacitor, it's replaced by a pulse from the coil. (This agrees with your description in your initial post).

A light load will cause weak pulses through the coil, which will cause a weak field on the coil. In that case the inductive coupling concept need not come into play. The buck converter might as well have its own separate individual coil.

A heavy load on the other hand will discharge the capacitor more which in turn will pull strong pulses through the coil. Strong pulses create a strong field. That's when it is relevant to wrap the coils on one core.

I notice the diodes in the buck converters serve to prevent their neighboring coil from being pulsed backward by a strong pulse in another coil. This makes it possible for the buck converters to coexist on a single core.

While it may not work with every type of supply, the common core idea has appeal.

Please understand I can't guarantee my approach is the correct one. I didn't read the entire article. And I see FVM posted a different conclusion while I thought over what was going on with the concept. With his experience FVM is very likely to catch something that I overlooked.

 

[grizedale]

i agree that the diode stops current being pushed back through it...though there is nothing to stop one coil's flux impeding current flow in the other coil

 

[grizedale]

Individual currents are summed up to a total magnetizing current for the common core

Hi,

-Though i keep reading (in other places) that the way to wind a coupled inductor for the dual outputs of a half bridge is to wind the two wires together, then connect the cathode of the output diode to one end , and the cathode of the other output diode to the other end..............

...this would result in each coils flux cancelling the other one's flux...

surely this cannot be right.?