Solution Finder Get Tech Support

2 outputs flyback (5V/12V) - 5V at high load goes down

Posted by: marcin on

Hello,

I'm doing second approach with 2 outputs flyback - quite high loads. First out 5V 5A max, second 12V 4A max + active shunt regulator. This time I switched the shunt regulator off and put only 100mA load at 12V. I wanted to focus only on 5V output. Here are the results:

Vout=4,89 @ 0,96A (5,1Ohm load)
Vout=4,78 @ 1,87A (2,5hm load)
Vout=4,5 @ 3,36A (1,27Ohm load)
Vout=4,42 @ 4A (1Ohm load)

As you can see 5V is going down too much. The whole project is based on DER-094 but recalculated for my requirements. Anyway flyback_sch.pdf is schematic and flyback_top/bott.pdf is PCB for this project. I also attached xls file exported from pxlbg file.

There are two things which bother me:
1) PCB - my current PCB provider didn't do it well. I mean specially around TOPSwitch. There is big area of GND on which TOP and other elements are placed - not only signal elements but also return pin from BIAS winding. Could this be the problem? Should I rather avoid areas and do it using single connections and Kelvin point as stated "many" times in examples?
2) Transformer construction - perhaps this is a place where I should look for improvements?

Am I right with these 2 points or there is something else/additional to consider and to change in this project?

Many thanks,
Marcin

Files

Attachment Size
Flyback_5A5V_4A12V_1_b.xls 21.5 KB
flyback_sch.pdf 23.2 KB
flyback_top.pdf 68.89 KB
flyback_bott.pdf 74.14 KB

Comments

Submitted by PI-Sarek on 08/31/2009

Hi,

Cross regulation in a multi output power supply relies on the coupling between the independent outputs. As such a weighted feedback scheme such as the one used by you (R7 and R17) provides improvement in load regulation of the additional output at the expense of slight degradation in load regulation of the main output.

You can adjust the regulation by adjusting the value of R17 and R7. In your example if increase in value of R17 improves load regulation of the main output (5V) that should solve your problem.

The load regulation in a multi output flyback is also dependent on the base load on the outputs. It is important that you use a base load on your 12V output equal to the one that is expected in your target application. a higher base load on the additional output with the adjustment of weighted feedback (R7 to R17 ratio) will help you to improve performance.

To get the best regulation it is important that the feedback circuit R7, R17
R22 and U3 are connected as near the output terminals as possible and the voltages should be measured on the output terminals. A copper trace between the point from where feedback is obtained and the output terminals will often drop voltage as a function of load current and result in poor regulation. Please review this in your layout as we could not read your layout files as the silk screen is not clearly visible.

Please refer to our application note AN-22 for additional and detailed information about adjustment of weighted feedback and transformer design and construction to improve performance of a multiple output design.

http://www.powerint.com/sites/default/files/product-docs/an22.pdf

Regards

PI-Sarek

Submitted by marcin on 09/01/2009

Hi PI-Sarek,

I followed your suggestion (about changing R17 value) and removed R17 from PCB completely. I wanted to see if in this case 5V output will be "more stable". Results are still the same. To me it seems like something else is a problem. Regarding base load, DER-094 is my "mentor" for this project. When I look at cross-regulation table there I can see that 5V output is so damn stable even 12V output load is varying from 0 to max. Based on this I came to thing that feedback circuit is probably not a "problem" in my case, right? I can be wrong but....

I'm doing second approach (as PCB project bothers me all the time) but speaking about multi-outputs flyback could you please advice about VOR and KP for such project. I heard that VOR in such projects should be as low as possible - I took 90V and KP should also be low - I took 0,7. Is this correct? Anything to adjust here?

Thanks for your time!