PMP7899

Marco Maioli

Other Parts Discussed in Thread: PMP7899

Good morning,

I have realized the circuit of PMP7899 project. In order to do this, I required the transformer at Coilcraft.

So I mounted the circuit, but it doesn't work correctly:
AT NO LOAD
If I supply the circuit with 9V, I have at output 27V;
If I supply the circuit with 16V, I have at output 29V.

I noted that my transformer (that I requested at Coilcraft as NA6264-AL), has the secondary only at two pins: 8 and 11.

If I see the PMP7899 schematic I note that the transformer has the secondary at three couples of pins: 8-11; 7-10 and 9-12, and the 7,8,9 and 10,11,12 are respectively cortocircuited.It seems that there must be three parallels secondaries..

Can my problem be this?
What is the exactly transformer that I must to use? Can I have the correct datasheet of transformer?

Best regards,
Marco.

over 8 years ago

0 Darwin Fernandez over 8 years ago

TI__Mastermind 40100 points

This is currently being looked into. An answer will be posted when available. Thanks!

0 David Baba over 8 years ago in reply to Darwin Fernandez

TI__Mastermind 26320 points

Are you using the specified coilcraft transformer? if not, what transformer are you using? if applicable, please can you share what transformer you are using?
Thanks.

0 Darwin Fernandez over 8 years ago in reply to David Baba

TI__Mastermind 40100 points

Hi Marco,

Do you have a schematic of your design that we can see? Thanks!

0 Marco Maioli over 8 years ago in reply to David Baba

Prodigy 170 points

I'm using the specified Coilcraft transformer..

In order to avoid problem I requested the transformer at Coilcraft directly.. I attach the transformer datasheet..

Coilcraft_NA6264.pdf

0 Marco Maioli over 8 years ago in reply to Darwin Fernandez

Prodigy 170 points

I started from the PMP7899 project, and I attempted to use the same components..

As you can see in the attached schematic, the most difference between my schematic and the PMP7899 is ISO1, that has same features of isolator used in PMP7899..

AiLux_PoE_PSE.pdfAnyway I attach my schematic..

0 David Baba over 8 years ago in reply to Marco Maioli

TI__Mastermind 26320 points

A CTR of 400% may be too high? Can you please try modifying R59 to 1kohm? Thanks.

0 Marco Maioli over 8 years ago in reply to David Baba

Prodigy 170 points

Good morning,

As I wrote in first post, I follow exactly the PMP7899 project schematic, and I used the same devices.

The only difference is the optocoupler: I use the ISP817CXSMT/R, but it has the same caracteristics of PS2501L-1-L-A (used in PMP7899 project)..

Anyway I try to change R59 from 499ohm to 1Kohm, but nothing changed..

I don't understand what is the problem, I exactly followed the original project..
There are sensitive points that I must check?

Best regards,

Marco

0 Darwin Fernandez over 8 years ago in reply to Marco Maioli

TI__Mastermind 40100 points

Hi Marco,

I agree the schematic looks the same as the reference design. Since this is a known working solution, you may want to double check all the connections and components/values on the board just in case the PCB is not the same as the schematic.

Have you checked the reference voltage? At the voltage divider feedback? It should be 1.24V.

0 Marco Maioli over 8 years ago in reply to David Baba

Prodigy 170 points

Good morning,

I have done some proofs.

I bougth a transformer made by an industry near us and I tried to use it. The transformer has the same caracteristics than NA6264-AL.

There were positive and negative results:

- At no load the power supply works correctly
- As soon as I attached a load, the output voltage falls at 22V, and it doesn't rise to 48V..

I measured the work frequency and I seen that is about 120kHz. I don't ubnderstand why.. I mounted a 49k9 resistor at RT/SYNC pin, which corresponds to 338kHz of resonant frequency..

Can you explain me why I have this anomaly?What is the element that change the work frequency?

However from the tests I note that with the new transformer, at no load, the circuit works correctly, while with the NA 6264 the circuit doesn't works..

Have you got suggestions?

Best regards,
Marco

0 David Baba over 8 years ago in reply to Marco Maioli

TI__Mastermind 26320 points

Please can you capture a scope shot of the Switch node in the conditions you are describing? loaded and unloaded?

Thanks.

0 Marco Maioli over 8 years ago in reply to David Baba

Prodigy 170 points

good morning,

we have checked the pcb, we have done some changes and it seems that the situation is improved..

Now we do some proofs with different loads in order to analize the behaviour at load..
If need be I'll write again..

Thank you for availability,
Best regards,
Marco

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