LNK304 low voltage operation

Posted by: simontrimble on 04/03/2012

We have a LNK304 PSU in one of our products that we specify to operate between 85-265V AC and it's heavily based on Fig 5. (PI-3757-112103) in the LNK304 datasheet. The PSU currently works really well and outputs 12V DC with the circuit drawing anything between 18mA and 100mA.
A customer has now requested the product to work from a 24V PSU. I tested the product and it only reliably works down to around 25-30V. It starts to have trouble supplying the higher current loads resulting in the PSU restarting every second or so.

Is there a modification that can be done to allow it to operate reliably down to 20V but still give the full 12V, 100mA? Would something like the addition of the floating constant current source as per DI-152 be the solution? If it is then is there any example schematic or pointers I can use to help me integrate this into the LNK304 design?

Thanks.

Comments

For this application, it is very tough for LNK304, SINCE maximum duty cycle could be hit for 20V input. PLS modify your circuits based on the following list, hopefully the SPEC can be meted.
1. Modify D1 and D2 to a low voltage schottky diode.
2. Reduce the dummy load. Verify output with higher value resistor for R4.
3. Short D 3 and D4 for 24Vdc input, use schottky diode for D3 and D4 if input is 24Vac.
4. reduce the Value of RF1. Use a inductor with lower inductance and lower DCR for input and output inductor.

If the output can not be good enough with all the above modification, you may have to try some other solution.

Thanks for the help Cochrane, those points helped a little and brought the minimum voltage down by a couple of volts. It doesn't give me quite the margin I was hoping for so I have a feeling I am not going to be able to re-use the existing circuit and may have to look at another solution.
Best regards.

Hello,

I have the same problem.
My power supply has to work from 19.2VAC to 265VAC and at 19.2VDC. The output is 5V - 500mA.

I have plan to adapt the PI-3757-112103 with a part of the schema present in the DI-152.
Does it work if I add the "Startup circuit" ? (q1, q2, VR2, D2, D4, R4, R5 and R6) ?

Thanks

Best regards.

Hello,

I made a copy / past of parts of schematics from documents "PI-3757-112103" and " DI-152" to illustrate my idea. I really apreciate any comment about this possible solution.

Regards.

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Hello,
How can I have an answer to my problem ?

I have made the assembly on a prototyping board and until now, 2 circuits are broken so I guess this solution doesn't work, can you say me why ? Is there another schema to use LNK304 with low input voltage (19.2VDC) ?

Regards.

Your Circuit doesn't work because , As you increase the AC Input , The Zener diode in the circuit starts conducting and Pushes more than the required current into the BP pin og IC which causes the failure to Blow Up. How ever in DI-152 , there resistor divider network is connected in Series with Zener Diode. Try the Same circuit as described in DI-152.

Hello PI-KSAN,

Thanks to your feedback we have corrected our experimentation. Do you confirm that our corrected schematic is now correct ? Especially that current injector in BP Pin is correctly implemented and correctly configured ?

Thanks for your support,

Laurent

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Schema_B0_To_validate.pdf (99.17 KB)	99.17 KB