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UCC28910: Is UCC28910 suitable for a low power offline flyback convertor with 140 VDC output

Part Number: UCC28910
Other Parts Discussed in Thread: UCC28704, UCC28C42

I’m working on the design of an offline flyback power supply generating ~140VDC and capable of supplying ~ 20mA of current. Efficiency is not a great concern and the output voltage doesn’t need to be too tightly regulated  but I would like the solution to be small and simple. My first thought was to use a flyback controller with an internal FET (since it is low power) and primary side regulation to keep the component count down. The UCC28910/11 seemed to be a good fit but when I try to use this device in webench the output voltage is limited to 25V. Is this limitation intended because the VDS of the internal FET (700V) is not high enough to support the 140V output reflected back through the transformer?

Playing around with Webench some more I found that the only primary side regulation option that it would suggest for a 140V output is a UCC28704 which uses an external FET - Is the UCC28704 my best bet?

  • Hello,

    It may be the reason.   The switch node voltage will be VIN max + Vout*Np/Ns + inductor leakage spike.  You can double check your numbers to verify this.

    But this is most likely the case.

    The UCC28704 will work for this application using an external FET.  Please note that PSR fly back converters require a very clean aux signal that uses a transformer with low leakage inductance and inner winding capacitance.  It is suggested that the transformer that are used are machine wound for repeatability.

    The other option you have is to use a UCC28C42 in a fixed frequency fly back converter with opto isolated feedback or regulate the VCC voltage to the controller.  This design can tolerate a nosier aux winding compared to the UCC28704.

    Regards. 

  • Thanks for the information Mike. I think with a decent snubber I could control the leakage spike particularity given how little primary current I'll need. Actually I'm finding that my primary current is so low that it's giving rise to a relatively large primary inductance. I do have a need in this product for a power supply to power some other circuitry maybe 200  mA at 5V. Do you think I'm better off designing the convertor to regulate a 5V  output then add a second secondary to give me a high voltage limited by a zener or LDO to get my 140V? I recall seeing a reference on E2E to a design methodology to add a second winding but I was unable to find it in my searches yesterday. Given how little power I need on the 140V rail I'm wondering if designing regulation for a 5V rail and blindly adding a winding with turns to give ~140V will get me close enough. 

    Your point about the very clean aux signal gives rise to some concern. I have struggled in other designs getting the leakage inductance low enough in flyback transformers with high isolation specs  - it's not easy. I'll take a look at the datasheet for the UCC28C42.

  • Hello,

    You can design a multiple output fly back with one output 140V and the second 5V.  You just needs to size the primary magnetizing inductance for the full output power.

    Regards,

  • Thanks again Mike. I had a look at the UCC28C42 which will certainly increase the design complexity and part count given the need for the compensation network and the opto feedback. I will speak to a magnetics designer about the feasibility of building a flyback that will work with the UCC28910 or UCC28704 and see where that take me.