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Original question:

UCC2895: On/off operation

UCC2895: UCC2895 PWM opening time & SS time

user5794331
Prodigy 10 points
Part Number: UCC2895

Hi. Im glad to be here. I have a question about UCC2895. I want to work with UCC2895 but before I wanted to try by simulating it. I downloaded your pspice transient model and simulated in pspice like below.

As in the seen in second image, I calculated soft-start time tSS= ((22*10^-9 * 3.6 * 69800) / 3 ) = 1.84272 ms by looking from datasheet. And I assumed calculation is true. In simulation I obtained these ;

In 3. and 4. image about to EAOUT and SS pins in 0.5V so pwm output is open . 

And 5. and 6. images about to simulation in 1.8 ms .

So I realized, when ss time in 1.84, Vout is 3V that is wanted. So I wonder, if my ss time is 5ms, Will Vout  be 3V in 5th ms?

And how can I reduce pwm opening time? So how can I down EAOUT's 0.5V time?

Forgive me for my poor English. Thanks in advance

  • 0
    TI__Mastermind 35830 points

    Hello User5794331,

    Thank you for your interest in the UCC2895 Phase-Shift PWM controller.

    At start-up, the EAOUT voltage follows the SS pin voltage.  If you increase the SS time to 5ms, EAOUT will control the PWM so that the output will reach 3V in ~5ms.

    To speed up the time from zero to first PWM, you can set the SS cap C1 to a lower value at first for faster ramp-up, then add in a parallel capacitor after a delay to increase the SS time to full Vout.

    Regards,
    Ulrich