Stability of buck regulator with variable (3rd party) load

[Yerfel]

Hi,

I have a SMPS I am designing and I need to verify the stability – I’m worried that 3rd party loads could cause problems. It will be an isolated supply, nominal 14V – 20V in, 12V out with type II compensation, and a loop bandwidth of around 40kHz (dictated by the optocoupler required for the isolation). It will have a max power output of 3W.

My concern is that it will be powering external 3rd party devices (possible over several feet of cable). There is a possibility that the load could provide a significant capacitance and / or noise. I will design the compensation network to use a specific value of capacitance on the output choke. However, could the additional load of the 3rd party device make the system unstable? What can I do to guard against this?

Dito for the noise. There is the possibility that noise entering the system could make the system unstable. The SMPS output will be low impedance, so I wouldn’t expect a big problem here. Also I can add a filter on the output to prevent noise at a higher frequency than the compensation network entering the system.

Does anyone have practical experience in these situations? Any comments on the effect on stability, or other possible issues, welcome.

Many thanks.

 

[sreid]

Your concerns, while genuine, may be be excessive. Most Power Supplies are connected to unknown loads.

1) Try to have as much Phase Margin as is possible in the design.

2) Specify the maximum external capacitance that can be added. However, what ever capcitance might be add will likely have a high series resistance [Electrolytic Caps]and thus have a high freuency pole which will preserve your phase margin at high frequencies.

3) Filter the supply feedback [say 5x bandwidth] to minimize the possibility of your supply being the victom of customer induced noise. Mostly I've seen noise getting into Safety Circuits and causing the supply to trip when it didn't really need to.

 

[IRstuff]

This appears to be partially, a problem with specifications. Once you specify the expected load range, and design for it, there should be no further issues, unless the user exceeds the specified range.

The only big issue is where to draw the line, which is then, a function of the cost/performance for the product.

TTFN

FAQ731-376

 

[sreid]

In my first reply under 2) I should have said "High Frequency Zero."