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UCC28950: Phase shift PWM Converter design

AHMED OKILLY
Prodigy 170 points
Part Number: UCC28950
Other Parts Discussed in Thread: UCC28951

Hello Dear after greetings 

I have inquiry about IC (UCC28950), I want to use this IC for generating PWM signals for the phase shift DC-DC converter with synchronous rectification , but in my design already i designed soft starter circuit for the DC-DC converter , I know that UCC28950 IC designed with soft starter pin (SS/En) but i want to disable this built in soft starter of this IC to test the performance of my designed soft starter circuit, to disable this pin just not connected to any capacitance or resistance?? or what I should do to disable the soft starter of the UCC28950 IC??

Many thanks in advance dear,

Ahmed Hassan 

PhD researcher at korea university of technology and education 

south korea

 

  • 0
    TI__Mastermind 25570 points

    Hello Ahmed

    The SS/EN pin is designed to provide the soft start function and I wouldn't advise not using it. The voltage at the soft start pin is used to control the duty cycle during the initial start-up period. This can be done simply by placing a capacitor at the SS/EN pin.

    What I would suggest is that your external circuit generates a voltage ramp that you then apply to the SS/EN pin. You can make the ramp as slow as you wish, or even make it non-linear to get a particular output voltage ramp if you want. Do make sure that the ramp output impedance is high enough so that the Hiccup feature of the controller can still operate -

    Please let me know if you need further information - feel free to send me a schematic or description of your idea if you want. (We can do this privately via email if you wish)

    Regards

    Colin

  • 0 in reply to Colin Gillmor
    TI__Mastermind 25570 points

    Hello Ahmed

    I forgot to mention that I would suggest that you use the UCC28951 device instead of the UCC28950. The UCC28951 device is an enhanced version of the UCC28950. It is a fully compatible drop-in replacement for the UCC28950. Refer to application note SLUA853 at http://www.ti.com/lit/an/slua853/slua853.pdf for more details. The UCC28951 is typically the better choice, especially for applications where the system may have to operate simultaneously at a duty cycle> 90% and current limit.

    Regards

    Colin

  • 0 in reply to Colin Gillmor
    Prodigy 170 points

    thanks a lot dear colin for your replying 

    I'm sure that soft starter of this IC is important for the design and all the time my lab team use this IC for converter circuit design, but just for check ,, i want to test my soft starter to be general for any PWM converter (not only for UCC28950 circuit) but nowadays in our lab UCC28950 phase shift PWM IC only is available , so i need only PWM signals for the converter and protection , soft starter and any other requirements for converter operation will be external circuits from our design. so if i disconnect the capacitor with SS pin ,, it will be disable???

    i hope that you are understand me well

    thanks a lot dear

  • 0 in reply to AHMED OKILLY
    TI__Mastermind 25570 points

    Hello Ahmed

    You may tie the SS/EN pin to VREF to defeat the Soft Start function. Do this through a 10k resistor - that gives you the possibility to quickly pull the SS/EN pin to ground to disable the IC. The controller will start to operate immediately the VCC rises above the UVLO threshold -

    A note to other people reading this: Tying SS/EN to VREF in this way is acceptable in a Laboratory environment for experimental purposes BUT TI will not recommend it for production designs.

    Regards

    Colin

  • 0 in reply to Colin Gillmor
    Prodigy 170 points

    really thanks a lot dear colin for your brief answer and your helping 

    thanks and all my regards