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Help with tl494 pwm circuit design

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jackbob:
So I'm looking to make another pwm control board but want to use something other than the usual 555 or arduino. I thought of the tl494 after seeing it in a little cheap dc to dc converter. I tried wiring one up on a bread board many different ways and tried a few schematics from the Internet and could not get it to work for the life of me. Every time I wired it different a new problem came up. Could anyone help me with a simple schematic on this chip? I know how to set the oscillation frequency but not the pwm duty cycle. I just need a simple schematic or explanation using this ic as a variable pwm driver. I will either drive a fet or pnp transistor with it in single ended configuration. I know the tl494 is a little outdated but I'm not worried about that. If there is another pwm chip anyone knows about that will get the job done with less pins and much less pain in design let me know. I just happen to have a few 494's on hand so I thought I could use them.

Psi:

--- Quote from: jackbob on February 26, 2012, 10:36:38 am ---I know how to set the oscillation frequency but not the pwm duty cycle.

--- End quote ---

Pretty sure it's all feedback. The chip calculates the duty cycle automatically based on the two error amplifiers.
You can use the error amps for anything, but normally you'd have one for voltage limit and one for current limit.

As one of the error amps approaches 'zero error' (voltage difference)  the duty cycle becomes very small.
So once your output voltage is correct the duty cycle will be just enough to keep it there and no more.

As soon as you attach a load it starts to drain the output capacitor and the voltage falls. This 'error' produces more duty cycle to counter-act the voltage drop.

The TL494 is quite a nice chip once you understand how it works.

jackbob:
That's a very useful feature I like how it adjusts the duty cycle as the voltage drops. I still need to do a little research to fully understand how this chip is to be wired for certain applications. Thanks for your help on the subject, now I understand how the duty cycle adjustment works for the 494.

Psi:
It's not a special feature of this chip.
It's how most DC/DC supplies work.
They monitor the output and feed it back into the switching logic to vary the duty cycle and produce a regulated output.

The TL494 just gives you raw access to the positive and negative input of both error amps, so you can wire them up how you want.
You're not limited to building typical constant voltage supplies, you can build constant current as well.
Hell, if you really wanted to you could add a temperature sensor and build a power supply that outputs a constant voltage (or current) based on room temperate :P

electroguy:
jackbob, if you are going to use the pwm for psu, google "ATX power supply circuit"
I had a good link to a site that had about 100 different circuits on it with TL494 and KA7500 but i can't find it right now... If i find it, i will post up the link.

the TL494 and KA7500 were very popular and used in nearly all ATX power supplies at one stage... (maybe they still are?! i haven't opened up a new ATX PSU for a while...)

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