Lowpass Filter (LC) resonance problem

[[email protected]]

Hello everyone!

I am looking to design a low pass filter that has a cutoff frequency of 20 kHz. The filter needs to be able to handle about 20A of current so the direction I have taken is an LC filter. (Inductor with a high current rating).

I simulated the low pass filter tonight (LTspice) using the values of 7.9 uH and 15 uF suggested from some of the online lowpass filter calculators.

From the simulation I see 0 dB in most of the passband but then I see a huge resonance point of +32 dB at 14.49 kHz!


Here is a link to the simulation result:


http://i1001.photobucket.com/albums/af140/mikemillernow/LP_filter_79uH__15uF_zps5421958c.jpg


1.) Is there a way to eliminate this resonance?

2.) Is there a different filter that would have less peaking but still ableto handle the high current for this application (20A)?

3.) There is an H-brige that will be modulated at 200 kHz (pwm) and this will going through the 20kHz lowpass filter to given a DC voltage without allthe pulses. If the resonance point is kept as it is what type of issues would this cause? Is this even a problem?

4.) From the simulation I see a cutoff frequency of 22.7 kHz instead of 20 kHz. What is the best way to tune this? Maybe to keep the capacitor fixed (15 uF) and then sweep the inductance value in simulation?

Thank you you so much for your thoughts.

-Mike

 

[Phil Allison]

I am looking to design a low pass filter that has a cutoff
frequency of 20 kHz. The filter needs to be able to handle
about 20A of current so the direction I have taken is an
LC filter. (Inductor with a high current rating).

I simulated the low pass filter tonight (LTspice) using the
values of 7.9 uH and 15 uF suggested from some of the
online lowpass filter calculators.

From the simulation I see 0 dB in most of the passband
but then I see a huge resonance point of +32 dB at 14.49 kHz!

** You need to specify a load impedance for a simple LC filter to work
sensibly.

The values you came up with suggest a load of about 0.5ohms.

There is an H-brige that will be modulated at 200 kHz (pwm)
and this will going through the 20kHz lowpass filter to given
a DC voltage without all the pulses. If the resonance point is
kept as it is what type of issues would this cause?

** Depends if the PWM modulation contains frequency components near 15kHz.

Cos that LC network presents a dead short at resonance.

Is this even a problem?

** Try an LC filter that matches your load at resonance.


..... Phil

 

[miso]

Probably add some series resistance to the inductor as well.

 

[Phil Allison]

"Vladimir Vassilevsky"

From the simulation I see 0 dB in most of the passband but then I see
a huge resonance point of +32 dB at 14.49 kHz!


Is this for class D amplifier?

** With a 0.5 ohms load ?

This is well known problem.

** Avoided by setting the filter resonance higher than 20kHz.

Speakers are very much inductive at high frequency.

** That is not a correct generalisation.

Until the frequency exceeds 50kHz.



..... Phil

 

[Phil Allison]

"Vladimir Vassilevsky"

Could be a car audio, for example.

** Bose once made 0.5ohm drivers for car audio, but that was unusual.

The norm is to use step-up inverters as part of the amplifier and regular 8
ohm speakers.

Until someone checks frequency response with signal generator.

** Any smart designer includes a sharp low pass filter at the input to the
power amp - with a turnover frequency about an octave lower than the output
filter resonance.

Good point.

** FYI:

Speaker voice coils are poor inductors due to severe eddy current losses in
the iron pole piece - some even refer to them as "semi inductors" since the
impedance only doubles for each TWO octaves rise in frequency.

Phase angle of the current is about 45 to 50 degrees rather than 90 .


.... Phil

 

[Phil Allison]

"Vladimir Vassilevsky"

The 0.5 Ohm woofers are not uncommon

** Bullshit - they are very uncommon.

as step-up guarantees problems with EMC unacceptable for OEMs.

** Total bullshit.

Practically all 12V car amplifiers use step-up supplies.

But even retail stepped-up car amplifiers 8 Ohm is too much.

** I mentioned 8 ohm speakers - not 8 ohm rated amplifiers.

I'd say 2 Ohm is usual.

** 2 ohm rated amplifiers and 4 ohms rated in bridge mode with *hundreds* of
watts of output.

That does not happen without a step up inverter.

You stupid, over-snipping fuckhead.



..... Phil

 

[RobertMacy]

Speaker voice coils are poor inductors due to severe eddy current losses
in
the iron pole piece - some even refer to them as "semi inductors" since
the
impedance only doubles for each TWO octaves rise in frequency.

Phase angle of the current is about 45 to 50 degrees rather than 90 .


... Phil

Is that phase angle caused by the eddy current losses? Is there any
contribuion at all from launching audio energy into the 'room'?

 

[Phil Allison]

"RobertMacy"

Is that phase angle caused by the eddy current losses?

** Yep

Is there any contribuion at all from launching audio energy into the
'room'?

** Nope.



.... Phil

 

[[email protected]]

The 0.5 Ohm woofers are not uncommon as step-up guarantees problems with
EMC unacceptable for OEMs. But even retail stepped-up car amplifiers 8
Ohm is too much. I'd say 2 Ohm is usual.

You're right. Ignore what Phyllis has to say. He's off his meds,
again.

BTW, dual voice coils are quite common, as well. 2x2ohm voice coils
gets close to 100W without a boost. That's good enough for 90% of the
market. 2x1ohm buys more power but causes other problems. It's done
but not often.

 

[Phil Allison]

<[email protected]>


** You are a retarded, ASD fucked **** head who

CANNOT FUCKING READ !!!!!!!

( means incapable of following the context )

Piss the HELL OFF !!



.... Phil

 

[[email protected]]

<[email protected]>


** You are a retarded, ASD fucked **** head who

CANNOT FUCKING READ !!!!!!!

( means incapable of following the context )

Piss the HELL OFF !!

Get back on your meds, Phyllis. You're wrong.

 

[miso]

Have you noticed how these people show up with a question, and then they
never follow up?

 

[Anthony Stewart]

The 20A coil will need to be a large air core to avoid saturation effects. THe simulation needs to be shown with a 8 Ohm load or whatever. The peak gain can be estimated easily as the ratio of Reactance to Resistance. Since there is no load shown impedance of L or C at resonance to the source resistance must be almost 100 to get almost 40 dB gain. Once loaded with 8 Ohms this also reduces the Q or peak gain.

 

[Phil Allison]

The 20A coil will need to be a large air core to avoid saturation effects.

** Why is that - Tony ??

You read it in a post on a web forum somewhere ?

You plucked it out you arse ?

THe simulation needs to be shown with a 8 Ohm load or whatever.

** Yeah - whatever ..........

PISS OFF you PITA Google Groups cretin.




..... Phil

 

[Anthony Stewart]

The reason ferrite and iron core chokes are NOT used in high current audio filters such as speaker crossovers is prevent the THD and IMD products from their non-linearity.

Inductors drop in value with rising DC current or Bass freq current. The rated current is where the Inductance is 50% L at 0 Amp current.

This is why ALL crossovers for Bass will use air core coils often with 4" diameter.

{Phil u should know better than to question my judgement.}

 

[Phil Allison]

"John Fields"

Anthony Stewart


The new kid in town shows up begging for a fight, huh?

** Nothing like a fair contest, at all.

Be like punching a ripe tomato .....






...... Phil

 

[Mike Miller]

Thanks for the comments on the resonance. The response is much better with the load!

-Mike