LM62460-Q1: Buck Converter Input Filter: Purpose of Resistor in Parallel with Inductor

Hale Konopka

Part Number: LM62460-Q1
Other Parts Discussed in Thread: LM62460RPHEVM

I am puzzled by an element of the filter design found in SNVA886: Reduce Conducted EMI in Automotive Buck Converter Applications. A 2k resistor (RF2) appears in parallel with the ferrite (LF2), and is attributed to "preventing filter stability issues."

Can you provide any further information on what these filter stability issues might be? I haven't been able to find a clear answer from searching online. Based on an LTSpice simulation of this I see only a difference of 0.5dB w/ 1MOhm load, 0.32dB w/ 100 Ohm load, and ~0dB with 1Ohm load.

It also seems odd to me because the LM62460RPHEVM which I have been using as one of my benchmarks in testing does not implement this. Your answer will be helpful for us to decide if we actually need a placeholder for this on our next rev of PCBs.

7 months ago

0 Jimmy Hua 7 months ago

TI__Guru 50120 points

Hello,

I am not sure about the resistor in parallel with the inductor EMI filter, but for the LM62460RPHEVM, the filter stability is addressed by the inclusion of the damping capacitor. Explanation of EMI filter stability is detailed in SNVA489 which is what the EMI filter was designed with.

Regards,

Jimmy

0 Timothy Hegarty 7 months ago in reply to Jimmy Hua

TI__Guru 55542 points

Hi Hale,

Take a look at the "Fundamentals of Power Electronics" textbook where there is a chapter dedicated to input filter instability. A series RL in parallel with the main filter inductor is one of the three main options for input filter damping, and this is sometimes simplified to simply a parallel resistor (although it does limit/clamp the total inductive impedance, thus compromising attenuation).

Regards,

Tim

0 Hale Konopka 7 months ago in reply to Timothy Hegarty

Prodigy 50 points

Hi Tim,

Thank you pointing to this resource. In the simulation I've shown though the addition of the parallel resistor seems to only improve damping by up to .5dB. Am I missing something in this simulation setup?

Thanks,

Hale

0 Timothy Hegarty 7 months ago in reply to Hale Konopka

TI__Guru 55542 points

Hi Hale,

1kΩ seems high for the damping resistance. As Jimmy mentioned, you may be better off using an electrolytic cap, 4x the ceramic input cap value.

See parts 10 and 15 here for detailed info: http://www.how2power.com/other/EMI_Guide.php.

Tim

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LM62460-Q1: Buck Converter Input Filter: Purpose of Resistor in Parallel with Inductor