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LM5023-2NBEVM: How can I use it in a buck topology (Vin =300- 1500Vdc , Vout=300V dc)?

Aurora Craciun
Prodigy 90 points
Part Number: LM5023-2NBEVM
Other Parts Discussed in Thread: LM5023, UCC28700, UCC28C42, UC2844, UCC28704

Can I use Lm 5023  in a buck topology (Vin =300- 1500Vdc , Vout=300V dc, 200W output power)?  If yes,  how do I limit the lower frequency above 25KHz ?

  • 0
    TI__Intellectual 880 points
    Hey Aurora,

    We do not recommend using the LM5023 and its opto-based feedback is not optimal in a buck configuration so we do not recommend using it as such.

    In buck mode we would recommend using the UCC28700 controller like in this reference design: www.ti.com/.../PMP20345

    The switching frequency is determined by the power level. So you'd have to have an output that would limit the frequency from going below the 25kHz switching frequency you desire. This part has similar attributes to the LM5023 like the valley switching and less generated EMI.

    Alternatively, if you find that you cannot find the converter switching at the desired frequency, we do have fixed frequency switching controllers like the UCC28C42 used in a reference design like this in a buck configuration: www.ti.com/.../PMP10833

    Regards,
    Davit
  • 0 in reply to DavitKhudaverdyan
    Prodigy 90 points

    Hi Davit,

    The reason I've chosen this part is the start-up using depletion mode FET.  Why is a problem using  optocoupler ?  The converter is a floating buck .  I would send you a block diagram  but I'm not sure how. 

    I've built the converter but I have issues with low line and full load. The switching frequency is below 15KHz. In order to increase it, I need to reduce the inductance. If I reduce the inductance, the switching frequency at 1500V and full load  goes up ( above max. frequency specified in the datasheet ). What is .happening in this case ?  Do I  still get the full power or the IC is limiting the frequency and the power ?

    Thank you ,

    Aurora 

  • 0 in reply to Aurora Craciun
    TI__Intellectual 880 points
    Hey Aurora,

    Just wanted to let you know that I'm discussing this with a colleague and that I will get back to you on this question.

    Regards,
    Davit
  • 0 in reply to DavitKhudaverdyan
    TI__Guru* 86286 points

    Hello Aurora,

    The LM5023 is a QR controller which means the frequency will change based on loading and peak current. Avoiding 25 kHz under all load conditions may be difficult.  You can used the following equations to estimate the switching frequency.

    Ipeak = Iout*2

    ton = L*Ipeak/Vin

    Duty Cycle = Vout/Vin = ton*Frequency

    Frequency = Duty Cycle/ton

    I looked at your input and output requirements and your duty cycle requires a 100% to 20%. The LM5023 is a QR controller and the maximum duty cycle is generally around 50%.  I don’t believe this is the correct topology or correct switch mode controller for your application.  100% duty cycle is not typically possible in any switch mode power supply design due to magnetic saturation and switch mode controllers duty cycle limitation.

    A transformer based peak current mode switch mode power converter could handle the 5 to 1 input range. A UC2844 peak current mode controller with a programmable fixed frequency would work for this application.  You could design for a fixed switching frequency and a maximum duty cycle of 50% and a fixed frequency of 50 kHz.

    Forward Duty Cycle (D) where Np/Ns if the transformer turns ratio.

    D = (Vout/Vin)*(Np/Ns)

    The forward converters turn’s ratio can be calculated with the following equation based on volt second balance. The transformer turns ratio is set with the minimum input voltage (Vin) and the maximum duty cycle of 50%.  The minimum duty cycle can be calculated by algebraically manipulating the transformer turns ratio.

    Np/Ns = Vin*D/(Vout*(1-D))

    I read through the thread and the reason why you chose the LM5023 was for the depletion mode startup FET configuration. This startup can be implemented with the UC2844.  The UCC28704 use a similar circuit that you can find in figure 20 of the data sheet.  The circuit is form by RLIM, the depletion mode MOSFET QST1, RST1, RST2, QST2, CST and RST3.  The only difference is in the UC2844 application RST3 would be tied to VREF instead of the NTC pin.

    http://www.ti.com/product/UCC28704/datasheet

    Regards,

    Mike