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LM51551: Advice on the design on HV flyback in DCM.

clive harvey1
Prodigy 40 points
Part Number: LM51551
Other Parts Discussed in Thread: LM5155

Hello All,  

I am working with a client that requires the following:

input voltage:  3 - 18V

Output voltage: 2Kv

Output current 0.5mA

Output voltage ripple:  <1%

I am looking at using the LM51551, but following the application note SNVA866, not only have a found multiple errors in this application note, but it only works for CCM and I need to run in DCM. Im choosing DCM due too the low power requirement and keeping the magnetic's low.  

I am looking at running multiple secondary stages in series to achieve the voltage requires, this works fine in simulation, with other controllers and free running.  But there doesn't seem to be much guidance for designing a step up flyback in DCM for the LM51551. 

any help will be much appreciated.   

  • 0
    TI__Mastermind 21720 points

    Hi Clive,

    Thanks for reaching out with your question and for considering the LM5155 for you application.

    The LM5155 can be designed, as with most low-side controllers, to operate at a flyback the operates in DCM. Currently there is not an application note for this type of operation with the LM5155 because it is not a very common design for this device.

    To operate in DCM the primary side inductance just needs to be small enough to ensure the inductor current always returns to 0A even at maximum duty cycle. This article should help with the loop calculation and understand DCM operation.

    Please let me know if you have any questions.

    Thanks,

    Garrett

  • 0 in reply to Garrett Roecker
    Prodigy 40 points

    thanks alot for your reply.  I will review that now, I can get this working fine free running, but with DCM changing the compensation networks, i was going to struggle to design this without an app note.  

  • 0 in reply to Garrett Roecker
    Prodigy 40 points

    Hello, I have now had a read through that app note.  Though it did give me a method for calculating duty cycle for DCM, it didn't seem discuss how i should compensate for DCM apposed to CCM. 

    I have put together a spreadsheet following SNVA866, im wondering is i use the DCM equation for calculating duty cycle, would i be able to follow the rest of this app note?  

    Would the method for calculating current in this app note hold true for DCM, given the lower DCM duty cycle?  

  • 0 in reply to clive harvey1
    TI__Mastermind 21720 points

    Hi Clive,

    There is not currently an application note that describes how to design the LM5155 as a DCM flyback.

    Which current are you referring to in the application note? The application note should help guide the design.

    -Garrett

  • 0 in reply to Garrett Roecker
    Prodigy 40 points

    Firstly using the app note, the rms current is either equal too or greater than the peak current.

    Also the current compensation network equations don’t work, making the slope resistor way over 1k and the shunt  value way too high.

    The currents will need to calculated differently for the DCM and the compensation network will have different pole/zero I think.

  • 0 in reply to clive harvey1
    TI__Guru* 78190 points

    Hi Clive,

    Sorry currently we don't have the tool for DCM.  All flyback using diode rectifier will enter DCM at light load.  Could you start the design with CCM for an enlarged load level, say 2x,  as the boundary condition, so in reality you circuit will always operate in DCM?  Then, you can choose the compensation there for the first cut, then fine tune the loop by test?

    Thanks,

    Youhao Xi, Applications Engineering