Help with tl494 for boost converter

Thread Starter

brunounreal

Joined Apr 23, 2010
26
Hello everyone,
this is my first post in this forum ! I’m an electrical engineering student and I have a project which is a dc-dc boost converter controlled by a tl494, but I’m having some trouble with it!
I’ve already studied the behavior of the ic and tested (with success) part of the control!
But I’m having troubles with the comparators! If I understood well the circuit, when I set a reference at pin 2 , when the voltage at pin 1 is higher, the pwm should decrease and if is the voltage is lower it should increase to compensate the variation on the output right?
And what’s the influence of the resistors that set the gain?
Also, does anyone knows a good simulations software? I’m using multisim but it doesn’t have the ic!
thanx
 

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SgtWookie

Joined Jul 17, 2007
22,210
Hello Bruno,
Here is Texas Instruments' web page for the TL494:
http://www.ti.com/product/tl494
After the page loads, click on the "Technical Documents" button.
I suggest that you read not only the data sheet, but ALL FIVE of the Application Notes, as they will help you to understand how the IC can be used in several different ways, and how to troubleshoot in case of difficulty.

As far as a SPICE model, look here:
http://forums.ni.com/t5/Circuit-Design-Suite-Multisim/I-NEED-SUBCIRCUIT-MODEL-FOR-TL494-PWM-OSCILLATOR-AND-TC4421/m-p/522982
 

Thread Starter

brunounreal

Joined Apr 23, 2010
26
I've already read it and try to understand!
but i can't make it work as i need to ( when the voltage is higher than the reference thw pwm drecreases and when lower increase)
any ideas why doesn't it works ?
please help! i'm going crazy over this!
 

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SgtWookie

Joined Jul 17, 2007
22,210
Take a look at the datasheet:
http://www.ti.com/lit/ds/symlink/tl494.pdf
and build the test circuit on page 6.

Try feeding the IC the sample waveforms shown in the datasheet, and see what comes out of it.
Use all of the same parameters; same VCC, everything.

And when you post a circuit, please post it in .png format rather than .PDF; as the .png files can be viewed without downloading and without having to load Adobe Reader.
 

Thread Starter

brunounreal

Joined Apr 23, 2010
26
unfortunely i only have acess to test equipament a few hours a week so i simulated the circuit that you told me with multisim.
i've obtained similar results to those on the datasheet but still dont understand how to solve my problem!
i can provide the printscreens of the waves if it helps!
i'm completely out of ideas of what could it be the problem! the circuit works fine! i've understood how the DTC works! i think i've understood the FB! but i cant get any configuration that works as i need it to!
 

SgtWookie

Joined Jul 17, 2007
22,210
OK then, did you try any of the example circuits?

The circuit you posted in the .pdf file looked sort of random. You did have valid values for Rt and Cd, but not quite sure what you were expecting to see?
 

Thread Starter

brunounreal

Joined Apr 23, 2010
26
the values of Ct and Rt were calculated for a 20kHz frequency.
at In1 i have 2.5V from a voltage divider for setting the reference!
at In1- i have a potentiometer to simulate the voltage reading at the output of the step up.
About the gain resistors i am not really sure! i used 2,5k and 500 because they were the ones who gave the best results!
I had the In2- conected to 5v with a resistor and In conected t the ground!( because it isnt relevant for now )
what i was expecting was if i change the value at the potentiometer for a value higher than the reference , the pwm decreases to compensate that variation and if i decrease the value at the potenciometer the pwm should increase!
at least thats what i'm trying to do !
 

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SgtWookie

Joined Jul 17, 2007
22,210
Alright, the test circuit worked as advertised, correct?

Then why don't you start from the working test circuit, and just change one item at a time towards your schematic, and when things stop working, go back a step and figure out where you went wrong.
 
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