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Apparent tl494 glitch

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Mosaic

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Has anyone noticed that after some time running (hours) Tl494's can ignore the output control (pin 13) and cause the output from a single drive transistor to = the oscillator frequency?

Normally the drive transistor = 1/2 the osc freq with pin 13 held high, to pin 14, Vref.
What I see on the 'scope is that the pulse starts to get twitchy, cutting by half the period in a random manner, creating a lot of obvious audible noise rather than a fixed hum. Osc. Freq = 1Khz. Normal drive tranny pulse freq = 500hz, pulse widths around 25 µS normally

Then it can lock onto half the normal period with a consequent half pulse width , ultimately maintaining the same load control.

Powering down for a few minutes and then restarting stops the prob for a while, then it restarts.
 
have you used decoupling capacitors close to the tl494?..it could be noise causing your issues...have you got this on a well layed out pcb or a wire-wrap board?......cuzz it sounds like you have noise issues.....you get a big enough vcc cap on it?
 
Yes, dbl sided PCB layout with a 0.1uf smd cap sitting on the power pins directly under the dip chip.
The supply is 12V from an LM317 based on the low noise spec sheet design. It uses a 10uF cap. at its output and a 10uF on the adjust pin.
Perhaps I should place a cap on the TL494 Vref (pin14) output as well. It is in a 'noisy' environment.
I'll bump the 1Khz freq up a bit as that is the bottom of the spec'd freq range for the chip. It might be too close to tolerance as the chip ages.

I have noticed that 'sometimes' on soft start boot up the TL494 starts @ 1Khz output (transistor drive = osc freq) and in a second or so it switches to 500Hz pulse out. The RC osc. combo is a 0.1uF with 12.7K resistance, stable sawtooth.
 
Well i couldn't replicate the fault today, but by upping the freq. to about 1.4Khz the pulse stability seems improved. Jitter is halved to 0.5µS vs about 1µS previously.
Time will tell, I guess.
 
I've seen circuits for a 1khz version, however you'd better check the 'sheet to see if it can go that low, if it does also make sure the r's and c's your using are within the reccomended range also in the 'sheet.
Did you decouple the vref too, as you know its used as a logic level for the mode i/p, noise might make the chip switch mode.
 
I think I some running at 15,750 once and had noise problems.
You may need real good grounds.
 
Well, I've put on about 12 hours run time since the change to 1400Hz and it's holding up ok. No more frequency toggling.

Most of the control loop is via the DTC input, not the error amps. I have had to use HF neg. feedback and RC filters along the DTC line to isolate out the 'noise' and stabilize the control loop. It's a current loop with a sizable RC filter to produce an average current out of the pulsed currents. Because of the staged RC filters, balancing the loop can be come tricky as phase shift instability can occur.
 
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Being voltage mode compensation for the '494 can be an issue, current mode control seems to be easier to manage.
 
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