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LM5121_Observed Voltage Spikes at the Output of Boost Converter

Other Parts Discussed in Thread: LM5121

Sir,


I am using LM5121 boost controller in one of my application.We have designed the converter to achieve a boost voltage of 65V from 28V, at a switching frequency of 650KHz. The part number of the switching MOSFETS used are

High side & Low side FET - IPB123N10N3 G.


But we are observing voltage spikes at the 65V output and the max spike voltage is going up to 85V. This spike voltage is coming with in a period of 900uSEC.

Is there any chance to occur spikes at the output in diode emulation mode????.... i have grounded the Mode pin to GND.

What will be the reason for the spikes observed at the output??

Kindly clarify.

Regards,

Prajeesh.

  • Hi Prajeesh

    A high-frequency switching noise can be observed at the output . Usually, it depends on yout PC board layout and the quality of the output capacitors. 

    A low frequency output ripple is most of time caused by loop stability issue. 

    Regards,

    EL

  • Sir,


    Thanks a lot for the reply.


    I have calculated the Loop Compensation values as per the required specification. Iam getting the Rcomp value in Mohms (2MOhms). But the values which has been shown in work bench design is in Kohms.

    My specification is mentioned below.

    Output Voltage - 65V

    Full Load output current - 5.5A

    Output power   - 357.5W

    Efficiency   - 0.96

    Min input voltage - 12V

    Typical Input Voltage - 28V

    Max Input voltage - 40V

    Switching Frequency - 650Khz

    Input Power - 372W

    Input current  - 13.3A

    Duty cycle - 0.57

    Input Inductor - 6.9uH

    Current sense resistor - 2milliohms

    Output capacitor - 17.5milliFarad

    R_ESR - 0.0075

    R_FB2  - 64.9K

    If Possible kindly suggest the approximate values of Rcomp,Ccomp & CHF.


    Regards,

    Prajeesh



  • Hi
    Please try Rcomp=90k Ohm, Ccomp=470nF, Chf=1.5nF
    Additionally, please select Css > 10uF.
    Regards,
    EL
  • Hello sir,

    Thanks a lot for the valuable reply.

    We have updated the value for the Loop Compensation network as suggested by you. For the first power ON boost output 65V is observed with a small ringing for 16nSEC. For the second power ON , observed that the low side switching FET got failed. We are facing the issue in this chip for the last two months.

    Low side & High Side switch MOSFET Part # - IPB123N10N3 G

    Kindly give a sloution for this. Also pls let me know what all inputs you required from our side.

    Regards,

    Prajeesh

     

  • would you please send your schematic for a review?
  • Hello sir,


    I have attached the schematics for your reference. Kindly do the needful at the earliest.

    Please consider the below mentioned component values. We have changed during testing.

    Soft start capacitor, C154  -  4.7uF

    Rcomp  R143 & R142       - 1MOhm

    CComp   C149                   -   22nF

    CHF   C153                         -    60pF 

    R150                                   -     100K

    R151                                   -     11K

    R141                                   -     8.2Ohm

    C148                                   -    470pF

    Mode pin has been pulled up.

    RC snubber has been added across the Low side FET (Q2)    Rvalue - 150Ohm/2W,  Cvalue - 100pF/200V Ceramic cap.

     Regards,

    Prajeesh

  • Hi Prajeesh
    -Try bigger Css. You can try extremely big Css just for an experiment. Try 47uF.
    -Start up without load and measure the voltage spike at SW node. DEMB pin should be grounded.
    -Increase the load while measuring the voltage spike at SW node.
    Regards,
    EL
  • Hello sir,

    Thanks for the valuable reply.

    I need one more clarification. In datasheet (Page no-22) says about the Maximum Duty cycle and the minimum input supply voltage which can achieve the target output voltage.
    The estimation of the minimum input voltage is given by the equation - Vin(min) = Fsw*Vout*(750ns+Margin)
    where margin = 100nsec
    As per our specification Target output voltage is 65V.
    Fsw = 650KHz,
    As per the above equation, the minimum input voltage is getting as 35.91V.

    Please confirm that, for a 65V target output with a switching frequency of 650Kz, 35.91V input is required.....?????
    Or, whether i can achieve the same with in 28V input.......??????
    Is there any chance of issues due to 28V input...??? If yes...,, What are the possible issues....?????

    Waiting for your valuable reply. An immediate reply is always appreciated.

    Regards,
    Prajeesh
  • Hello sir,


    Kindly reply me at the earliest.

    Regards,

    Prajeesh

  • Hi Prajeesh
    36V is the minimum Vin voltage can achieve 65V output at 650kHz
    Your Fsw should be less than 210kHz to achieve 12V to 65V step up.
    Regards,
    EL
  • Hello sir,


    I am getting 65V Boost output with an input of 28V, having switching frequency of 650KHz.  Issue iam facing is the failure of Low side FET.  Low side FET has been failed for 4 times.

    Low side FET spec mentioned below.


    1. VDS - 100V

    2. Drain current   -  58A (Contiuous)

    3. Pulsed Drain Current  - 232A

    4. Rise Time        -    8nsec

    5. Fall time           -    5nsec

    6  Input Capacitance - 2500pF


    Kindly confirm what will be the possiblilty of the failure of the MOSFET's. Waiting for your valuable reply.

    An immediate reply is always appreciated.


    Regards,

    Prajeesh

  • Hi Prajeesh
    Please check these items.
    - Peak Vds>100V ?
    - Peak Vgs>20V ?
    -Case temperature> 120degreeC ?
    -Any cross conduction?
    Regards,
    EL
  • Hello sir,


    Kindly clarify regarding the equation -15

    Vin(min) = Fsw x Vout x (750nS+Margin).


    What does this 750nSec & Margin means?


    Regards,

    Prajeesh

  • Hi
    (750ns + margin) represents the low-side switch off-time
    Regards,
    EL
  • Hii,

    On what basis you are taking 100nsec as margin. Any calculation for that...???

    I need one more clarification, It is based on the LO drive current capability.

    My switching MOSFET is having a total gate charge of 35nC, and i am giving VCC 12V from an external source. Observed rise time for the MOSFET is 22nsec. From these i am getting a drive current of 1.59A.

    MOSFET Part number -

    Q = 35nC
    T= 22nsec (Observed rise time)
    I = 35nC/22nsec
    = 1.59A.

    If i consider the input capacitance of the MOSFET, ie; 1880pF. and if iam calculating the drive current.

    C = 1880pF
    V = 12V
    T = 22nsec (Observed rise time)
    I = (1880pF x 12V)/(22nsec)
    = 1.09A.

    From these above drive current which one will be more accurate. Whether this is the right way of calculation.

    In General, Pls tell me what will be the approximate drive current of the IC LM5121 for an Input capacitance of 2500pF. with a source of 12V.

    Waiting for your valuable reply. An immediate reply is always appreciated.

    Regards,
    Prajeesh
    Hardware Design & Development
    Indus Teqsite Pvt Ltd, Chennai.