[SOLVED] Auxiliary Flyback Supply

Hello,

Please would someone help me to understand how to design the auxiliary supply for a UC38xx based flyback converter?

How do you choose how many turns the winding has?

How do you prevent the output capacitor on the auxiliary winding from overcharging?

Doesn't the output capacitor "steal" energy from the inductor when the secondary winding is lightly-loaded?

Many thanks

Hi,

V_aux = V_in * sek_winding_count / prim_winding_count - 0.7V.


Klaus

You don't sound like to understand the workings of a flyback power supply. In essence it is an energy pump, in that each cycle where you build up a current in the primary winding, which follows V = L.dt/dt becomes an energy pulse which is .5*L*I^2. Multiplied by the operating frequency becomes the power throughput when in discontinuous mode. It is also important to understand that the transformer in a switch mode is not in fact a transformer, it is a coupled inductor.

When you come to design the coupled inductor, you must not overflux the core when in its charging phase, or at least flux it to a point where it is happy at the operating frequency you are running it at. There are lots of articles that go through how to design a switch mode coupled inductor. Using the correct search terms is the key.

KlausST said:

V_aux = V_in * sek_winding_count / prim_winding_count - 0.7V.

Click to expand...

This can't be right because there is no reference to the auxiliary winding at all!

Thanks for your reply brushhead. I've got a working flyback that's using a separate DC supply to power the controller. I want to change it to work from an auxiliary winding, like most flybacks do. Problem is that the circuit shown in reference designs and application notes doesn't seem to work for me in simulation.

It doesn't make sense intuitively, either. The "transformer" is being driven according to feedback from the secondary voltage. The auxiliary winding has a different winding inductance and a different output impedance, so the auxiliary voltage will have no relation to the secondary voltage.

I think the idea is to put more than enough energy into the auxiliary capacitor to power the controller and regulate the voltage to a suitable level. I just want to know if there's a way to do this without major power losses in the regulation.

Right I see what you mean now. Can you post your circuit? I've used the UC38xx loads of times but normally the rectified mains runs on the same 0V as the low voltage side, as my background is AC drives. Normally you would regulate off the +5V rail and use other winding(s) for other rails and possibly use post linear regs in order to trim the other outputs to what you want.

Sometimes you would power the chip from an aux winding as you say, and have an isolated feedback path from your (say) 5V rail. You would not need regulation on the aux winding as it will tend to regulate itself magnetically because of its operation as an energy pump.

If you have a look at LTSpice there is a Linear Technology version of the chip and I think there is a demo circuit for you to play with. I do have a circuit I did a while ago using this chip and i'll post it for you tonight if I remember. It generates loads of other rails for SELV outputs and gate drives etc etc.

The "transformer" is being driven according to feedback from the secondary voltage. The auxiliary winding has a different winding inductance and a different output impedance, so the auxiliary voltage will have no relation to the secondary voltage.

Click to expand...

The "auxiliary voltage" has a strong relation to the secondary voltage, but by the working of unavoidable leakage inductance, it doesn't track the secondary voltage exactly.

A flyback transformer must have a tight coupling of primary and secondary winding (e.g. k >= 0.98), a similar coupling should be expected for the auxiliary winding. The flyback voltage of windings is respectively proportional to the number of turns (or squareroot of main inductance). Flyback voltage of unloaded windings mostly follows this relation, the loaded voltage is reduced by a leakage inductance caused voltage drop. Regulating the loaded secondary voltage raises the voltage of less loaded windings.

V_aux = V_in * sek_winding_count / prim_winding_count - 0.7V.

Click to expand...

Ratio of ton_prim and ton_flyback is missing in the equation.

The simple way is to initially assume similar volts per turn (during flyback) in the auxiliary winding as the main secondary winding.

This will not hold exactly true as FvM has pointed out in the previous post. But its a quite good initial starting point.

You will obviously need to take into account voltage drops across rectifier diodes, especially where the dc voltages involved are quite low.

Usually your auxiliary winding will be within one turn usig the above guesstimate. But you will find it quite difficult to get an exact dc voltage, as one whole turn up or down could give a fairly large voltage step.

If the auxiliary power is much lower than the main output power, you might get away with an extra half turn to get your auxiliary voltage closer. Selecting either a silicon or shottky rectifier is another trick for fine tuning the voltage.

Hi,

I now how flyback works.
But I assumed the auxiliary supply works in forward converter mode.(reversed auxiliary winding)

I just checked the datasheet. I was wrong, here it works in flyback mode. The formula in post#2 is wrong.

*****
I'm very sure I' ve seen the auxiliary supply in forward converter mode.
The forward converter mode has the benefit, that it works even when output is short circuit, and it has predictable output voltage.

Sorry for the confusion.

Klaus

The auxiliary winding can be made to work either in forward or flyback.

Its far more common to use it in flyback, because presumably the flyback main output is what is being voltage regulated.

If you use forward mode for the auxilary, the auxiliary dc voltage will be pretty much tied to the dc input voltage by direct transformer turns ratio.
As the dc input voltage may vary over a wide range, the auxiliary voltage will also vary, which may not be what you want.

Much better to run it in flyback mode where it will tend to be much better regulated along with the main output.

Sometimes the auxiliary voltage can be so well tied to the main output, it can be used as the source of overall voltage feedback. That eliminates the need for an opto isolator in the feedback path, and can greatly simplify things.

Thanks for the replies guys.

If a flyback converter didn't have any feedback the capacitor voltage would go up and up.

If the capacitor was small the voltage would rise quicker than a large capacitor.

On high-power flyback windings the output capacitor will be much larger (~1mF) compared to the auxiliary winding (~100uF?). Therefore one would expect the auxiliary capacitor voltage to exceed the main output capacitor.

Also the ripple voltage (drop in voltage per cycle) on the high-power capacitor will be much greater than the auxiliary ripple voltage because there is a lot more power on it. Again, the auxiliary capacitor voltage will be greater than the output capacitor.

I don't see how the auxiliary winding can track the main output voltage unless it has a similar load and similar sized capacitor.

What happens at the onset of flyback is the voltage very rapidly rises on all windings.
It goes on rising until the stored inductive energy causes one of the rectifier diodes to conduct. The output capacitor on that winding caps the further rise in voltage, and the capacitor is charged by the flyback energy.

Where there is more than one secondary winding, most of the flyback energy goes into the output capacitor with the lowest relative voltage. That is because the rectifier associated with that winding conducts first (and highest).

The result is that most of the output power discharges into the most heavily loaded output winding, rectifier, and capacitor.
It also equalises the voltages on ALL of the output capacitors, assuming identical volts per turn during flyback.

So your 12 turn auxilary winding might have a flyback voltage of 12v. And your 100 turn main winding 100 flyback volts.

The result is the auxiliary winding has proportionally the same voltage, and tracks the main output voltage reasonably well. Its not perfect, but its often good enough if the supply mostly operates with a constant steady load.

If the main output has very sudden and dramatic step load changes, this system leaves a lot to be desired and probably will not be usable.

Another way of looking at it is there may be multiple output windings, and the auxiliary is just one more. You cannot regulate every output, just one.
And that one could just as easily be the auxiliary as one of the others.

Thank you for your explanation, Tony.

Warpspeed said:

If the main output has very sudden and dramatic step load changes, this system leaves a lot to be desired and probably will not be usable.

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What alternative would you suggest?

You will need to take your voltage feedback direct from the main dc output to ensure the best possible output voltage regulation.

That may be very simple if the main output shares a common ground with the control system.

If the main output must be galvanically isolated from the control system, an opto isolator will be required.