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UCC28C45 Vs UCC38C45

Raju E
Prodigy 80 points
Other Parts Discussed in Thread: UCC38C45, UCC28C45

Hello,

I have designed a flyback power supply to work from 20V to 190V using UCC38C45 and my design has completed.


I the data sheet i have seen that it can work only from 0 to 70 degree centigrade.but i need my power supply to work from -30 degree centigrade

So i am planning to replace with UCC28c45 which to make it work from -30 degree. is this right method to do. Please suggest what are things need to be taken care before i proceed.

Thanks

Raju E

  • 0
    TI__Mastermind 40580 points
    Hello Raju,

    Certainly it is the correct step to use the UCC28C45, which is guaranteed to operate within its datasheet parameter limits from -40C to +105C.

    Aside from the proper temperature-range rating of the controller, I suggest to thoroughly review the parameters of the other components used in your design to verify that they, too, are rated to maintain their parameters within the operating limits necessary for proper function of your application over the full temperature range. Aluminum electrolytic capacitors, for example, tend to decrease capacitance and increase ESR at cold temperatures. Other components' properties may change considerably when very cold. Even if they warm up during normal operation, start-up may be an unexpected problem when they are frozen. Be sure the parts you specify for your design can deliver the performance you need at -30C.

    Regards,
    Ulrich
  • 0 in reply to Ulrich Goerke
    Prodigy 80 points
    Hello Ulrich,

    Thanks For your replay

    One quick query,

    I have designed my power supply using WEBENCH tool to work from 20V DC to 190V DC, output is 5V,1.5A as per WEBENCH the transformer ratio is 0.463 (Ns/Np) and the maximum duty cycle of UCC28C45 is 0.48 .

    As per my calculation when input is 20V the output voltage is

    Output Voltage= input voltage* turns ratio* Duty cycle =20*0.463*0.48= 4.44V

    But i need my output should have max of 5 V +/- 5% (4.75 to 5V) .

    So to maintain the output voltage i am planning to increase a turns ratio.

    Please suggest your opinion.

    Thanks
    Raju E
  • 0 in reply to Raju E
    TI__Mastermind 40580 points

    Hello Raju,

    Sorry for my delayed reply; I was out of the office last week.

    Your equation for output voltage is applicable to a forward-mode topology, but you indicated earlier that you are designing a flyback converter. 
    This equation does not predict Vout for a flyback, so I recommend NOT to change the transformer turns-ratio based on your equation.

    In a flyback design, the Ns/Np ratio predicts the reflected voltage at the primary-side winding which adds to the input voltage, and is seen across the drain (or collector) of your primary switch.  The main concern, assuming this is a discontinuous-conduction mode design (due to the low power), is that Volt-second balance is preserved.  Since the UCC28C45 is a fixed-frequency (user-programmed) controller, the reflected voltage must be high enough to fully demagnetize the transformer during the "off-time" to match the V-s product at the lowest input voltage during the primary "on-time" at peak load. 

    If this is maintained, the voltage feedback-loop will take care of the regulation, independent of the ratio.    

    There may or may not be other reasons to fine-tune the turns-ratio during the course of your prototype debug, but don't do it because of that inappropriate equation.  

    Regards,
    Ulrich