Loop stability

Posted by: rt60man on 03/27/2013

Hi,

Although, I am not new to power supply design, I some times fail to comprehend loop stability.

My power supply project (attached) seems working fine for sometime now, but I was just wondering how the loop stability would be affected if I had to change certain capacitor values.. in the following cases...

Case 1:

C17 increased to 1000uF, ESR value unknown

C15 increased to 470uF, ESR value unknown

Case 2:

C12 increased to 1000uF, is the ESR value critical for loop stability?... IMO ESR of C13 alone should be responsible for loop stability calculations

PI does not seem to complain if I change these as above

Thanks in advance for your insight

Files

Attachment	Size
PIDesign1 100V_2.0A_TL431 REG u3.pdf	61.25 KB

Comments

Hi,

Please go through the app note for control loop design.

http://www.powerint.com/sites/default/files/product-docs/an57.pdf

Regards,

PI-NANO

Hi Nano,

I went through AN57, treatment is only provided for TL431 based error amplifier and single output.

Can I have some info. for multiple outputs as in my case?

Also found that using a zener feedback is far more simpler using PI expert... no loop stabilising compensation at all... or is it there? can I have info on closing the loop using zener diodes?

Thx in advance

Hi,

The flyback topology enables design of multi output power supply with low parts count. Generally one of the outputs is tightly regulated and the balance outputs are cross regulated that is, they are regulated using good coupling between the winding turns of the main regulated output and windings of these additional outputs.

Although 6-10% regulation of the unregulated outputs is typical the actual results often depend on transformer design, contribution of diode forward drop and recovery cnaracteristics and base load on each output.

You can also use a weighted feedback scheme where you obtain feedback from multiple outputs and improve regulation of one output by slightly degrading the regulation of the second output.

We have a detailed application note that explains design of multiple output power supplies including design of weighted feedback. Please use the link below for reference:

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

Use of a zener needs to be carefully evaluated for zener tolerance and temperature coefficient together with load regulation of the zener circuit. Now a days not many power supplies seen with zener based feedback circuit.

By using a zener feedback ciruit from stability point of view, you will not have any control over phase, for DCM operration you may not face much problem with phase, but for CCM operation phsae drop will depend based on transformer construction and capacitor ESR (A higher capacitor ESR will bring phase back, which was dropped due to double pole effect). In addition to that you are designing with multiple outputs. So I would strongly recommond you to use a TL431 based feedback circuit instead of zener based feedback circuit so that you will have much more control over feedback circuit.