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Old 3rd November 2005, 10:49 PM   #1
Bryan is offline Bryan  United States
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Default LC vs. CLC Theory

I was wondering about LC vs CLC design. I know it is a common practice to add a small resistor in front of the first L in the filter to "fine tune/increase" the final voltage of a PS. Now, at what point (speaking in terms of uF) does this start to lean more towards a CLC vs the LC filter... That it, will using a 1-2uF cap in front of the LC push the topology towards CLC?

Also, what specifically are the advantages to the LC vs CLC filters....

Thanks,

Bryan
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Old 4th November 2005, 03:46 AM   #2
kmtang is offline kmtang  Canada
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LC is so called choke input filter. The output voltage is more stablised than the CLC capacitor input filter. However, it needs to maintain a minimum current for it to maintain the stabilized voltage. The voltage will increase sharply if the current draw is less than the critical point.

The CLC maintains reasonable voltage from minimum to maximum current draw. The first input capacitor usually limited by the rating of the rectifier tube that range from 4uF to 47uF.


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Old 4th November 2005, 07:31 AM   #3
dhaen is offline dhaen  Europe
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Download the free PSUD from Duncan Amps . It's an easy to use program that simulates all toplogies. This will allow you to see what's going on.
LC has the advantage of more even current through the rectifiers, rather than the charging spikes of C(LC). It suffers on regulation and choke buzz if the choke is not specifically designed for that.
As you say, adding a small input cap is a great way to trim the output voltage. Whether it affects the topology is arguable.
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Old 4th November 2005, 01:07 PM   #4
MRupp is offline MRupp  Germany
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Just a word of caution: I built an LC supply, albeit for a SS amp (55V, 2A average current draw). I then put a small capacitor (8,2uF) ahead of the input choke. However, I noticed that there was substantial ringing in the power transformer at diode switch-off, even though I had RC snubbers accross the secondaries.

Removing the capacitor (or rather replacing it with a much smaller 75nF) substantially reduced the ringing. Btw, the Duncan PS designer also showed some ringing in the simulation, which is why I had a closer look in the first place.

Depending on your specific setup this may or may not be an issue, but I would still suggest to either build a choke input or a capacitor input supply instead of trying to create something inbetween.
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Old 4th November 2005, 08:29 PM   #5
SHiFTY is offline SHiFTY  New Zealand
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I tried LC with some UF4007s, with a 1uF cap in front of the first choke to stop the mechanical buzzing.

However it caused massive voltage spikes which blew the fuse and turned the diodes into a dead short!

Since then I just use CLC and IN5408s... No worries!
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Old 4th November 2005, 09:33 PM   #6
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Just to concur, Bryan.

You can get massive switching spikes at the choke-input side with a small C depending on the other parameters, or even without a C there if the rectifiers are semiconductor. (It works a little like a car ignition!) That is why you often find a serie R.C pair over the choke of an LC filter (common values 10K and 10nF 1.5 KV). It is always wise with any LC design to check there with a scope (HV probe!).
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