Ripples in output of SMPS

Posted by: Mahendra Karule on 01/19/2010

I have designed 3 Phase Energy Meter Power Supply Using TOP-HX252PN.
It’s a dual output power supply with outputs 12V, 1A and 6V, 0.3A.When there is no load on 6V, the O/P of 12V is stable and no ripple exist.As soon as we load the 6V with any load the ripple on 12V of 1V peak to peak appears.Also we found that, sudden increase in load at 12V ripples disappears very quickly.
While when we gradually increase this load, ripples also fades away gradually.

Comments

Hi,

Could you please provide me with the schematic, layout, transformer design and any other documentation you have from the design?

Could you also provide an oscilloscope plot of the ripple on the output with the given line and load conditions?

The more information you can give me the easier it is for me to help you solve the problem.

-Wesley

Hi,

Please find the layout and schematic files. As far as oscilloscope files are considered, I will try to provide the file as soon as possible.
As I mentioned earlier, ripples are seen on both outputs of 1V peak to peak when AC coupling mode in oscilloscope is used. This happens when I apply load of 100ohm at both output.

Regards,
Mahendra

I see nothing obviously wrong with your schematic so I am curious as to the characteristic of the ripple you are seeing.

Does it look like low frequency line noise? Since it's fed from a three-phase line I would expect the ripple on the bulk capacitors to be minimal. So I expect it to be switching noise. Is the noise around 66 kHz?

Let me know as that will help us diagnose the problem further.

Its a low frequency noise with amplitude of 1V peak to peak and of frequency in range 60-100Hz. I will try to send you snapshot of noise from oscilloscope as soon as possible.

Regards,
Mahendra

At this stage we are testing the circuit with single phase only. Will this have any effect?

Regards,
Mahendra

Yes the ripple on the DC bus may have an effect on the output voltage. If the ripple is severe enough the control loop may not be able to reject the ripple to an acceptable level. What is the ripple on the bulk capacitor?

To test this theory, try feeding the power supply from a high voltage DC supply if you have one or temporarily add large value input capacitors to the input stage to get a stable DC bus and then see what the regulation is. If it has improved then you know it is caused by the ripple on the bulk capacitor. If not we can continue looking for the cause.

-Wesley