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Todor 4th July 2005 02:46 PM

SMPS noises with TL494
Hello, I am making a +-40V car amp PS with the 494 and as I can say i experience some problems with making it work normally.
First of all if someone could tell me what noises should be heard from the PS when not loaded? Should the sound be smooth and regular because i can say i hear some "misbehaviors" around the center frequncy. Also i have a very big startup peak of about 10-15A in the 14V. I also doubt about the toroid i am using.It is around 40x25x10 with M 2000. I dont know if it is suitable for the job. I cannot adjust the frequncy somehow cus when i load it it makes awful sounds and the Amper meter jumps where it is not supposed to go for that load. The FETs are IRFP150N.
I would be gratefull If someone could add anything to my problem and if unsufficient i could bring some more info about this project. thank you all!

RCBandwidth 4th July 2005 03:02 PM

We need more information
Hi:You will need to provide a lot more information if you want any help.First of all what is your switching frequency,are you using push pull configuration?,how did you determine the proper number of turns for your core?...

If you can post a schematic of your design it would help towards getting the ball rolling. RC

jackinnj 4th July 2005 03:23 PM

You don't want to run a switching power supply "unloaded" -- the value of the inductors are inversely related to the load!

Send me an email privately and I will send you the article on how to mod an ATX supply -- it describes the theory well and uses the TL494. The author wrote it for a ham radio magazine -- in their applications the current demand will run from a few hundred MA to tens of amps -- and they need very quiet performance.

djQUAN 4th July 2005 03:47 PM

in a normal SMPS there should be no sound. is the sound that you hear more like a whistle? if so, the cores or the windings are vibrating which is not a good thing. have you seen the input/output waveforms of the transformer?

RCBandwidth 4th July 2005 03:56 PM

My Guess
If I had to guess I would have to say that his switching frequency is set too low..the lower harmonics of the square wave will be very audible!!..I do suspect he may have other design issues.
Bob C.

Todor 4th July 2005 08:47 PM

What should be the value of magnetic permeability of the ferrite core? Noone mentiones this, because here in my country ferrite toroidal cores are not widespread I can even say they are hard to find, so i haven't got much choice.

RCBandwidth 4th July 2005 09:25 PM

Need more info
If you want help you need to supply information.A schematic can overcome the language barrier.I asked you a few questions in an earlier post but you have not responded to them.

Todor 4th July 2005 11:04 PM

maybe i have to give up with this and just repeat someone other's design which at least is known to be working.The problem is that i want to use 494 and the schemes i found with it somehow didn't like them. Some of them were without voltage regulation, something i need. I am now searching for sg3525 or 24 for they are more often used. I haven't got any luck of finding it so far! It may sound weird but when some electronic component is not made in your country it's sometimes hard to find it as the only way is import.
Anyway if i have any further experience with the PS i would post here

Eva 5th July 2005 09:49 PM

You are trying to do too much design and development in a single step.

Audible noises in regulated SMPS are almost always due to instability in the regulation loop. Anyway, you should gain experience with unregulated circuits first.

Todor 5th July 2005 10:11 PM

Thanks for the advice about the unregulated PS Eva, I will try it.

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