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Author Topic: smps supply filter ?  (Read 1048 times)
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max
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« on: November 23, 2019, 12:30:46 12:30 »

Hi,

smps power supply questions,

1. how to calc the output LC filter in order to reduce the output noise or the best way?
2. for transformer bridge rectifier, how to calc the filter cap, is it same as for linear supply i.e 2uF/ma
3. is it ok to use the smps with RF circuits, what are the precautions?

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optikon
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« Reply #1 on: November 25, 2019, 04:17:31 16:17 »

Some comments

1) How much noise reduction would you like? What frequency band? LC network calculations are fairly straightforward... until you need to include the parasitics..then its best to come up with a model and just simulate it

2) There is some standard calcs - search google.. but I never use them. Decide what kind of ripple you can live with then simulate. You can find the appropriate capacitance much faster - dont forget to include ESR.

3) It depends. In general, SMPS with switching creates higher harmonics. These can easily couple into your RF circuits causing trouble. If you must, use lots of filtering and then high PSRR regulation.. and then more filtering...


Can you post a circuit and list the performance you want? You will get better answers.

« Last Edit: November 25, 2019, 04:20:41 16:20 by optikon » Logged

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vern
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« Reply #2 on: November 26, 2019, 03:21:26 03:21 »

Max,
you need to be a lot more specific about power, voltages, current etc. and also the desired noise reduction, should the device comply with FCC rules, does it need to be certified?
Best thing is to pick a SMPS IC from a company that provides some examples, like TI, Maxim or MPS, they usually include all the filtering in their designs, some even have complete BOM's.
Using an SMPS with RF circuits: of course, everyone does it. You need to be more specific here also, what kind of RF circuit do you use it for? What frequency is your RF circuit?
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bigtoy
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« Reply #3 on: November 27, 2019, 10:25:59 10:25 »

Like optikon said, simulate it. LTspice is free and easy to use.

Also, you don't say what type of SMPS you're using. Hysteretic type converters require some ripple on their output to function correctly; other converter types can be more deeply smoothed.
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max
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« Reply #4 on: November 27, 2019, 01:49:02 13:49 »

Thanks for the reply.
For the smps I am using the Chinese module based on XL4015 ic.
As for the RF, I am using the two types of transceiver modules, 2.4ghz type and the vhf type 170mhz.

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« Reply #5 on: November 27, 2019, 03:45:44 15:45 »

The XL4015 has a fixed 180kHz frequency, pretty low actually.
The caps on the board are only 47uF input and output on the module (found a picture, I think this is your module)
You need a much bigger cap on the input if you use AC and a bridge rectifier, depends on your output power.
The output cap is also on the small side, if you look at the XL4015 datasheet they use 330uF in their examples.
At least you don't have to worry much about HF distortions because the XL4015 is a slow switcher, you might just need a ferrit bead and a ceramic cap after the electrolytic cap. That could already be on the board, I can't tell with the picture I have.
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