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Old 31st August 2005, 06:51 PM   #1
Danko is offline Danko  Hungary
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Default Soft start with TL431 - HOW?

Hi, again! :-)


Another TL431 question ....
I would like to make my PSU soft-start. I was looking at the TL431 datasheet, to figure out how can I make this IC to "do" soft start.

Will that work, if I put a capacitor and a resistor paralell with the resistor, which is between the cathode, and the "reference" pin? Or this will caouse problems in the feedback loop? If I put a relay series with the R-C, and that relay "disconnects" that R-C after a short time, will this work?

BTW, I found a schematic in TL431's datasheet, on figure 27, but I don't think that will be good for me. That's only "delay", not soft-start.

Thanks in advance!
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Old 31st August 2005, 08:34 PM   #2
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May I ask why you want a soft start? It's very seldom the best solution to have a slowly rising supply voltage. It's better to mute the output in some way.
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Old 31st August 2005, 09:02 PM   #3
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Default SS for what?

SoftStart for what? A linear or switcher?

If you're doing a switcher most controller chips (SG3525, SG1846, MC33025, etc.) have dedicated SoftStart pins on them. SoftStart can be added to the TL494 very easily. I have never heard of SS'ing the TL431.

If you're doing a switcher, is your topology Input/Output ground-isolated, or non-isolated?

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Old 31st August 2005, 09:11 PM   #4
Eva is offline Eva  Spain
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You should implement soft-start in the control IC by means of duty cycle limiting (clamping error amplifier output to a slow voltage ramp).
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Old 1st September 2005, 01:37 PM   #5
Danko is offline Danko  Hungary
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I would like to make a SMPS, with output voltage 460V DC, to a tube amplifier. The slowly increasing B+ does not shorten the tube's life, reduces inrush current.


Eva: Thanks, I will dig up the UCC2808's datasheet, to figure out how to make a soft-start.

N-Channel: I have used TL494, and with that IC I could easily make a soft-start, but this UCC2808 isn't so simple. It's a 8pin packaged device. GND, OUT_A OUT_B VCC FEEDBACK Current_Sense RC COMP <-= theese are the only pins it has.
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Old 1st September 2005, 02:24 PM   #6
Eva is offline Eva  Spain
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UCC2808 already has a built-in soft-start circuit, but it may be faster than desirable.

Anyway, you can just clamp COMP voltage to a low value during the first few seconds of operation, this will limit start-up peak current and provide a smooth output voltage ramp.
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Old 1st September 2005, 03:09 PM   #7
Danko is offline Danko  Hungary
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Eva: Yes, the UCC2808 already has a soft-start function, but lasts only for a few milliseconds. I would prefer it to be about 10 sec long.

But for now, I have to "slow down" my electronics-stuffs, becouse on september 5th the school will begin.

In my lodge, i'm afraid, I cant solder anything
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