PMP4478: RCD snubber...

Andre Strul

Part Number: PMP4478
Other Parts Discussed in Thread: UCC28740,

HI to all,

We have slightly modified the design .

The RCD snubber was modified (we used 20k resistance) and output filter as well . (higher inductor value and more capacitance)
One issue we have, on 1A our voltage output is 5.35A , 2A , its 5.1V and 3A we have 4.2V
Is it a correct to assume that this is a optocopler feedback issue ? What should we modify on it.

Thank you

Andre

over 7 years ago

0 Yingyang Ou over 7 years ago

TI__Prodigy 605 points

Andre, could you list all of the modifications on your design? Meanwhile, is it possible to see the output voltage? We would like to know whether the output voltage is stable. Thanks!

0 Andre Strul over 7 years ago in reply to Yingyang Ou

Expert 1330 points

hi again,
we found the problem for the 3A , in fact we made a mistake on the sense resistor. Now we have the unit working at 1A (5.3V) , 2A (5.15V) and 3A (5.0v)
One more question... how can we minimize the voltage difference between the different loads. We were thinking to use better precision R (0.1% instead of 1%) somewhere in the feedback network
Can you advise us if that's the correct approach and what exactly should be improved ?

Thank you
Andre

0 Yingyang Ou over 7 years ago in reply to Andre Strul

TI__Prodigy 605 points

Andre, in this design, UCC28740 is used as a secondary side regulator. It can achieve a very good output voltage accuracy since the reference and feedback compensation network are all located at secondary side. As you can see in our PMP4478, there is no voltage difference varied with the load. So, my question is where the feedback network ties in your design? In PMP4478, it is the point connecting Vout and R11. The close loop can only regulate this voltage as 5V as much as possible.

0 Andre Strul over 7 years ago in reply to Yingyang Ou

Expert 1330 points

Thank you
Our feedback network (octocoupler) is the one from PMP4478 design. In that FB we were thinking to use better % resistors to achieve less voltage difference between 1A and 3A load.

0 Yingyang Ou over 7 years ago in reply to Andre Strul

TI__Prodigy 605 points

Andre, I don’t think changing higher accurate resistor can help to fix this unless the current resistors have bad TCR, which means the resistance would change much with the temperature. Can this issue be observed on other boards? If they are the same, it should be not caused by the FB resistors.

I am think where the feedback point connects to? Before the inductor? If so, that’s normal since the DCR of the inductor will cause a DC voltage drop with the increased load current.

0 Andre Strul over 7 years ago in reply to Yingyang Ou

Expert 1330 points

yes it connects before the inductor, we will change the feedback point and will advise you

0 Yingyang Ou over 7 years ago in reply to Andre Strul

TI__Prodigy 605 points

Ok, Andre. That makes sense. The voltage difference is caused by the DCR of the inductor in this design. You can try to select one with as low as possible DCR for this if you don't want change the feedback point. If the feedback point is connected after inductor, the close loop would be taken care since the LC stage is involved in the close loop and will introduce a dual-pole at 1/(2*pi*(L*C)exp*1/2). Larger inductor generates lower pole frequency which may make the close loop unstable easily.

0 Andre Strul over 7 years ago in reply to Yingyang Ou

Expert 1330 points

hello to all,

please see attached

We built a SMPS unit as per datasheet, however we are observing some noise at 5.5Khz and 11Khz (multiple)

Can you tell us where is this noise coming from ? (we suspect the FB circuit)

Results were taken with a differential amplifier

0 Yingyang Ou over 7 years ago in reply to Andre Strul

TI__Prodigy 605 points

Hi Andre

Shall we know more info about your figure? It is for output voltage noise? Which node you detected? Shall we know what the used equipment is to get this figure?

0 Andre Strul over 7 years ago in reply to Yingyang Ou

Expert 1330 points

yes output noise, we used AudipPrecision tester (so very precise)
We don't understand where this 5.5Khz noise is coming from

0 Yingyang Ou over 7 years ago in reply to Andre Strul

TI__Prodigy 605 points

Hi, Andre

We cannot see the figure clearly. Could you please send this figure via mail to us (yingyang.ou@ti.com)? BTW, please also help to capture the voltage ripple monitored by oscillation via the voltage probe. Thanks!

0 Andre Strul over 7 years ago in reply to Yingyang Ou

Expert 1330 points

hello again.

see here.

i tried to email you but it bounced.

thank you

0 Yingyang Ou over 7 years ago in reply to Andre Strul

TI__Prodigy 605 points

Hi, Andre

Sorry, I typed a wrong mail address. Please re-send it to yingyang-ou@ti.com. Thanks!

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PMP4478: RCD snubber...