Help with LC filters for Buck Converter.

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ttttrigg3r

Joined Oct 14, 2016
20
Hi everyone and happy new year. Would you please help me with this power supply I'm using for my circuit.
I'm using a switching regulator to step 28VDC down to 5VDC to power my PIC controller and ADC as well as an RTD sensor.
This is the switching regulator I'm using. It recommends using an LC filter, and the inductor should be 10-47 uH. What kind of inductor should I be using, and value? There's power inductors, high frequency and so on. When I search on Newark, there's about 10 categories.

Thank you
 

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BobTPH

Joined Jun 5, 2013
2,190
Power inductor comes to mind. But really, what is important is the current carrying capability and the, not unrelated, internal resistance of the inductor. How much current are we talking about?

Bob
 

Bordodynov

Joined May 20, 2015
2,468
I use LTspice

--- Bill of Materials ---

Ref. Mfg. Part No. Description
L1 Wьrth Elektronik 74437324220 WE-LHMI 4020 inductor, 22µH, 1A pk
L2 Bourns, Inc. SRR7045-220M inductor, 22µH, 1.15A pk
L3 Murata MBH7045C-220MA=P3 inductor, 22µH, 1A pk
L4 Bourns, Inc. SRU1048A-220Y inductor, 22µH, 2A pk
L5 Bourns, Inc. SRU1048-220Y inductor, 22µH, 2A pk
L6 Murata MDH10060C-220MA=P3 inductor, 22µH, 2.8A pk
L7 Murata MBH12575C-220MA=P3 inductor, 22µH, 3.1A pk
L8 TDK SLF12575T-220M4R0 inductor, 22µH, 4A pk
 

MrAl

Joined Jun 17, 2014
6,930
Hi everyone and happy new year. Would you please help me with this power supply I'm using for my circuit.
I'm using a switching regulator to step 28VDC down to 5VDC to power my PIC controller and ADC as well as an RTD sensor.
This is the switching regulator I'm using. It recommends using an LC filter, and the inductor should be 10-47 uH. What kind of inductor should I be using, and value? There's power inductors, high frequency and so on. When I search on Newark, there's about 10 categories.

Thank you
Hello there,

Even with a small value inductor you will find that a post LC filter knocks the ripple down quite a bit.
You can do some simple simulations to show how this works just by generating a sawtooth or even a sine at the switching frequency, then pass it through an LC filter.
Even 2uH works good enough in many cases if the switching frequency is 100kHz or better. You can even wind this by hand air core and get decent results.
For example, 2uH and 100uf knock the 100kHz switching frequency down to just 1 percent of what it was pre filter.
If the load ever gets disconnected it is also good to check for an inductive spike. Keep the capacitance high enough, 10uf knocks harmonics down to only 10 percent vs 1 percent for 100uf.
It is also a good idea to keep the feedback at the output of the regulator and not the output of this new post filter, if you do have control over where the feedback line connects to the output of the converter.
 
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