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From: "Fritz Schlunder"
Subject: Re: Problem about using TL494
Date: Tue, 22 Oct 2002 04:36:56 -0700
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"duqizhang" wrote in message
> |-------------| _| __diode____ L______output
> | | | | | | | |
> | TL494 | R transformer / \ C load
> | 8--| | | --- | |
> | | / | | | |
> | 9--| Q1 ---------------------
> | | \>
> _______1______ |
> |__________| a
> | R1
> --- |
> --- |
> | |
> Look at the circuit above,which is the main part of my problem circuit.
> Pin 8 connects to vcc through a resistor. Pin 9 connects to Q1's base.
> And point a provides feedback signal to Pin 1. The input voltage(changable
> DC voltage) is fed to Pin 2. R1 is 1 ohm.Our purpose is to let the ouput
> voltage to be proportional to the input voltage.
> Our problem is the output voltage is less than what we want. For example,
> if we give 4V to the pin 2 (the circuit's input) and the duty of Tl494 is
> 50%,the output will be 2V,100mA across the load.And the current across
> R1 is 400mA. The efficient of the switch supply is so low.
> Furthermore,when the input of pin2 goes up, current across R1 rises
> quickly to above 2A,which is so strange.
> Any help will be appreciated.
Ummm... Clearly you misunderstand how the TL494 is supposed to be used.
For one thing the TL494 is a voltage mode PWM controller. The built in
error amplifiers are not meant to see any significantly AC signals at their
inputs at all. You must input DC signals to the error amplifiers for them
to function properly, otherwise unpredictable behavior will likely result...
From the looks of your diagram you seem to have somewhat confused the TL494
as a current mode PWM controller, which is most certainly is not. If you
want to sense instantanteous switch current like you are with your low side
current sense resistor R1, I might suggest you use a current mode controller
such as the UC3844 (or any number of the UCx84x series).
In the mean time you need to learn how the TL494 works before you will be
successful trying to use it... Unfortunately the datasheets for that
particular IC are extremely skimpy and don't really leave the reader
understanding how it functions. This document helps tremendously at
understanding the TL494:
I suggest studying the TL494 block diagram with extremely close scrutiny
until you fully understand how it would function if you build it out of
discrete IC's and components. Also one more tidbit to keep in mind. The
error amplifiers on board the TL494 need feedback resistors just like any
op-amp does for closed loop fixed gains. Without using feedback resistors
from pin 3 (FB) to pins 2 and 15 (inverting inputs), you will never be able
to get the TL494 to operate with stability since the gain will be much too
high. Also, if you aren't using one of the error amplifiers, make sure to
tie the inputs appropriately such that the output of the specific error
amplifier goes low.
From the sounds of it you also would greatly benefit by understanding switch
mode powersupply basics some more as well. Go to your local libraries and
gets some good books; read them and understand them as much as possible. If
all you need is an output voltage proportional to the input voltage, all you
really need is a transformer (with appropriate turns ratio), and a fixed
duty ratio driving your switching transistor (use a MOSFET for low voltages,
they are much better/higher efficiency than BJTs in many applications,
assuming they are selected appropriately), provided the output load is
sufficiently high enough to cause the output filter inductor to operate in
continuous conduction mode. It is more complicated than that, but that is
basically what it amounts to.
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