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Can An LC Filter be Placed At The Input OF a DC-DC Converter

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ballimo

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Hi Everyone,

is it a common practice placing an LC filter at the INPUT of a non-isolated buck converter to reduce reflected noise? does LC filter add any problem?

Regards,
Ballimo
 

FvM

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does LC filter add any problem?
Possibly. It can cause excessive ringing as response to the input ripple current and/or load transients if dimensioned inappropriately.
 

BradtheRad

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An LC input filter can draw steady current from a power supply, allowing maximum power utilization from a battery or solar panel.

The inductor acts as a choke. The capacitor has lower internal resistance than the battery or solar panel.

Suppose your power supply provides only 1A continual. The LC filter can accept that, then turn it into bursts of 2A, which it feeds to the buck converter.
 

mtwieg

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It's in fact quite common to put LC filters on the input, but care must be taken to ensure that the impedance of the filter is not so high that it causes loop instability. It's a classic control problem, look for literature by RD Middlebrook on the topic.
 
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ballimo

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I have sent an email to the technical support (of the DC-DC converter manufacturer), he reply as follow:
the noise coming out of the input would be common mode and a single inductor may actually make the noise more difficult to manage; he suggest using input bulk caps with low ESR at the switching frequency and ceramic caps... and if necessary a common mode choke,not a single inductor.

I was wondering why common-mode noise issue arise with respect to a non-isolated dc-dc buck converter? usually common-mode noise occur in isolated dc-dc converter, right?
 

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I have sent an email to the technical support (of the DC-DC converter manufacturer), he reply as follow:
the noise coming out of the input would be common mode and a single inductor may actually make the noise more difficult to manage; he suggest using input bulk caps with low ESR at the switching frequency and ceramic caps... and if necessary a common mode choke,not a single inductor.

I was wondering why common-mode noise issue arise with respect to a non-isolated dc-dc buck converter? usually common-mode noise occur in isolated dc-dc converter, right?
Naturally common mode interference can come from both types. The reason it's not often discussed with nonisolated converters is because usually they have to share a common low impedance ground with the rest of the circuit, and therefore you cannot just throw a CM choke on their inputs, as doing so would create huge amounts of ground bounce. Physically, CM interference arises from parasitic components between your switching components and earth (like your FETs and the heatsink, bolted to the chassis), and nonisolated converters will have that as well.
 
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