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Original question:

PMP21455: Check the part number of the transformer

PMP21455: Suggestions for implementation with minimal components

Lazar Saronjic
Prodigy 10 points
Part Number: PMP21455
Other Parts Discussed in Thread: PMP20708, UCC28881

Tool/software:

Hi,

We're designing a very compact mains powered device, and the PMP21455 specs and small form factor fit our requirements.

I've have some questions in regards to reducing the size and the amount of necessary components:

1. Are the rectification components for the unused 24V output necessary? Can we leave this secondary winding floating or just place a small loading resistor across it?

2. Can we use the same approach as in the 24V PMP20708 design variant, where the auxiliary winding is used for output voltage regulation instead of an optocoupler solution?

3. If I understood correctly, the Q1 and R7 are used instead of the internal regulator of the UCC28881 to extend the operating input voltage below 30VDC? I assume that's the main reason that 2 large 8.2uF capacitors are used for primary smoothing.

Since we're designing our device to operate with either universal mains or perhaps 240VAC only, can smaller values be used for C3 and C4 in this case? Any suggestions are appreciated.

Also, we'd be interested in approaches to fix/mitigate the high output voltage under no/small load conditions (From the test report: 5.4V with no load, 4.6V under load).

Thanks in advance, and sorry for the long post.

Regards,

Lazar

  • +1
    TI__Expert 4110 points

    Hi Lazar,

    See my answers below:

    #1: No. PMP21455 is built based on PMP20708 which has 24V output. If you only need 5V in your application, you can remove 24V winding and related rectifier components.

    #2: Yes, you can. Just de-populate D6 and populate R8. But please do expect loose regulation on 24V output.

    #3: You are right. Q1 and R7 circuit are there to ensure U1 can be properly biased with 24VAC input or 24VDC input. If in your design you only need to take care high line input (say 240VAC +/-10%), you can remove Q1 and R7 and connect Vrec node to U1 pin5. You can also reduce the capacitance on C3 and C4. Just need to make sure the valley voltage of C3 and C4 is still higher than 24V level should be fine.

    For the higher output voltage at light load issue, dummy resistor load is the cheapest solution...

    Regards,

    Sheng-Yang Yu

  • 0 in reply to Sheng-yang Yu1
    Prodigy 10 points

    Hi Sheng-Yang Yu,

    Thank you so much for the quick reply.

    Regards,
    Lazar