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Selection of isolated flyback controller IC

Chirag Panchal2
Prodigy 100 points
Other Parts Discussed in Thread: UCC28742, UCC28740, UCC24636, UCC24612

Hi There,

I am interested to design an isolated flyback controller (Apppro. 50W). I would be grateful If anyone could help me to select (give suggestions) a controller that can provide/support below features:

Vin= 550V-1000V (Max)

Isolated Output= 8  ( each Vout=24V and I out= 250mA (Max)

continuous Short circuit protection

Constant voltage

Overvoltage and overcurrent protection

Thanking you in advance.

Thanks and regards,

Chirag

  • 0
    TI__Expert 8210 points

    Chirag

    Our flyback controller UCC28742 would work well here. This controller is designed for high voltage input like the one you are designing for and includes all the fault protections you're looking for.  To help with your design we recommend you use our excel design calculator https://www.ti.com/lit/zip/sluc652 

    Best Regards,

    Eric

  • 0 in reply to Eric Faraci
    Prodigy 100 points

    Hi Eric,

    Thank you for your suggestion. After I send you my query, I came across UCC28740. What do you think about that? 

    Thanks and regards,

    Chirag

  • 0 in reply to Chirag Panchal2
    TI__Expert 8210 points

    Chirag

    UCC28740 is very similar to UCC28742.  The main difference between the two is that UCC28740 has an integrated HV startup function while UCC28742 relies on an external resistor for startup.  What this means at a system level is that UCC28740 will have lower standby/no load power consumption than UCC28742.

    If you would like to eliminate the startup resistor in UCC28742 for any reason (such as standby power, eliminate temperature hot spot, etc.) then UCC28740 would be the best device.  If this does not matter for your design requirement, then UCC28742 is better.

    The HV startup pin on UCC28740 is only rated for 700V.  For your input voltage, it will either require a resistor and zener clamp or to use the center point of a stacked capacitor (if you have one) to insure that the voltage on the HV pin never exceeds the 700V abs max.

    Best Regards,

    Eric

  • 0 in reply to Eric Faraci
    Prodigy 100 points

    Hi Eric,

    Thank you for your detailed explanation. Does UCC28740 provide short circuit protection like UCC28742  as I have not found anything in its datasheet?

    In addition, I have recently come across the above design and wondered what is the benefit of using UCC24636 in the secondary side instead of a diode and some filter circuitry? 

    I am expecting to design a flyback converter with higher efficiency (at least greater than 85%).

    I would be grateful If you could provide any suggestions to achieve such efficiency.

    Thanks and regards,

    Chirag Panchal

     

  • 0 in reply to Chirag Panchal2
    TI__Expert 8210 points

    Chirag 

    Yes, UCC28742 has output short circuit protection.  For UCC28742 if IOCC (which is set by RCS) ix exceeded for 120ms, UCC28742 will go into fault shutdown.  Section 7.3.4 (page 16-17), specifically figure 14, of the datasheet has details on this.

    UCC24636 is a synchronous rectifier (SR) controller.  What it does is allows the freewheeling diode to be replaced with a FET.  Primary due to a FET not having a forward voltage drop like a diode, an SR generally will enable higher efficiency compared to a diode rectifier.  Details about this are in the following PSDS paper https://www.ti.com/seclit/ml/slup378/slup378.pdf

    The efficiency improvement from using an SR depends on the output current, with higher output current leading to greater efficiency improvement.  With your requirement of 250mA the efficiency benefit from using an SR may be less, but it would help you achieve your efficiency target.  We recommend you test your design with an SR to validate the performance and see if it works for you.

    For our SSR flyback like UCC28740 and UCC28742 we recommend you use UCC24612 SR controller instead, this device uses a VDS sensing operation so the design is simpler.  If you do not have a space on your current PCB for UCC24612, you can retrofit it on any power stage to measure the performance with our daughter card EVM https://www.ti.com/tool/UCC24612-1EVM

    Best Regards,

    Eric