• State
  • Locked Locked
  • Replies 21 replies
  • Subscribers 66 subscribers
  • Views 2283 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

buck converter

Tharanath PP
Intellectual 390 points
Other Parts Discussed in Thread: LM5085, LM10011

Hi,

    I have a buck converter using lm5085 , Vin= 72v, Vout=24v. Connection diagram is same as in data sheet. TonMin=300ns. The problem I am facing is that Inductor(12uH) is heating and melting. Can anyone help me?

Tharanathpp

  • 0
    TI__Guru**** 191445 points
    Can you post your schematic and inductor part number?
  • 0 in reply to JohnTucker
    Intellectual 390 points

             Thank you for responding sir. I attached word schematic. Please see the attached file sir.

    Vin= 24 to 72, IL= 10A Max, 

    Inductor part no: 2102-RC... Inductor is heating at 2.3 A itself.

    Please help me

    Thank you,

    Tharanath

    Doc2 BUCK.docx

  • 0 in reply to JohnTucker
    Intellectual 390 points
    Hi, John Tucker,

    Below I have attached the schematic in word. Please go through it and please let me know the issue. Inductor is heating is the issue. When I measure the voltage across the inductor its about 1. to 1.6v. Theoretically it should be zero voltage. My present inductor value is 12uH. My buck is software programmable using opamp from mcu. Vout varies from 5 to 60v through software. anyway inductor is heating. Please let me know the wayout.
    Yours sincerely ,
    Tharanath
  • 0 in reply to Tharanath PP
    TI__Guru**** 191445 points
    I don't directly support LM5085. I suspect it may be a layout issue. Can you post that as well? In the mean time I'll alert someone to look at your schematic.
  • 0 in reply to JohnTucker
    Intellectual 390 points

    Hi, John,

                I am attaching lay out of my circuit. there are two converter ,one is configurable buck and one is 12 buck , 12 v buck is working( load is less), only configurable buck has issue( left side of pdf). Please see the attached file.

    Thank you,

    Tharanath

    RelayMotorDriver_Pcb.pdf

  • 0 in reply to Tharanath PP
    TI__Mastermind 33680 points

     I am assuming that you set your Switching frequency to ~275kHz ?  Did you check to see if the power convertor is switching at your set frequency?  can you cehck the switch node and confirm the converter is stable?

     

    Thanks.

     

    David.

  • 0 in reply to David Baba
    Intellectual 390 points
    HI David,

    I checked the switching node frequency, its 300khz, and its varying according to the load, if I INCREASE the up- to 2.5A Vout is stable (currently Vout what we have set is 10v)if increase the load beyond 2.7A Vout increases to 11.1v. Switching frequency is also varying according to the load. Here issue is inductor is heating and Vout also varying.

    Thanks you
    Tharanath
  • 0 in reply to Tharanath PP
    TI__Mastermind 33680 points

     

    I suspect the variation is due to your implementation of the external amplifier. I would implement a FB Resistor network then use an injection resistor being driven by an amplifier; into the FB node.

    Having said this I do not think the voltage variation is directly causing an issue with your inductor overheating. Back to the main issue of inductor over heating. Is the inductor getting hot at Iout of 2.5A?

  • 0 in reply to David Baba
    TI__Mastermind 33680 points
    Also, how are you loading your convertor? In order to determine why your inductor is over heating I would measure the current in the inuductor. this is the easiest way to determine what is going on.
    Thanks.
  • 0 in reply to David Baba
    Intellectual 390 points
    Hi David,

    I checked with removing external amplifier and directly connected to fb pin to feedback network also. That time also inductor is heating around by 1.5A . I loaded with with rheostat .

    Thanks you,
    Tharanath
  • 0 in reply to Tharanath PP
    TI__Mastermind 33680 points
    I think we should measure the Load current and also the inductor current on an oscilloscope, not DC. If you do not have a current probe, suggest using a low ohm current sense resistor and measure differentially across that. Suggest using a 30mR Sense resistor in series with the inductor before the output capacitor. This will tell us what is going on with the inductor current. Hope this helps?
  • 0 in reply to David Baba
    Intellectual 390 points
    Hi David,
    Thank you for your help. We are measuring how you suggested.

    Thank you,
    Tharanath
  • 0 in reply to Tharanath PP
    Intellectual 390 points

    Hi David,

                           I want to know which type of current which would result inductor heating , because we kept aside the circuit for some time. Anyway yesterday we put a hat sink on mosfet and some how we could manage heat of mosfet ,but inductor still heating. 

    Thank you,

    Tharanath

  • 0 in reply to David Baba
    Intellectual 390 points

    Hi David,

                       We measured the inductor current waveform.  I am attaching the waveform. I have placed 0.025R resistor between inductor and output capacitor and measured voltage across it. Inductor current is going around  6A  in the waveform. But load current is only 3A. Its just shocking to me.  Very big ripple current in the inductor . I think thats why inductor heating up.Please help me. Red color waveform is voltage across .025R resistor,ie indicates inductor current. green is gate waveform. Please help me sir. I am using reduced ripple configuration ie, only one capacitor across Rfb1. I have following parameter,

    Vin=48V, VOUT=24V,  IL=3A, iNDUCTOR =15uH , iNDUCTOR saturation current =15A, MOSFET= FDD5614P

    Thank you,

    Tharanath

     

    Thanks you,

    Tharanath

  • 0 in reply to Tharanath PP
    TI__Mastermind 33680 points

    Hi,

     

     

    The current waveform should be triangular and not trapezoidal?  The measurement may well be distorted and not real?  In response to your previous question, the heating of an inductor is made of two components.  DC losses relating to the RMS current and DCR of the Inductor and the AC losses which is related to the peak to peak ripple current.  Just looking at the magnitude of both in your plots, I don’t see an issue (ignoring the waveform shape; if real, suggests there is an issue with your power supply).  But before we confirm that, if your MOSFETs are running cooler than your inductor then we can say your inductor is heating.  However, if your MOSFETs run at a higher temperature than your inductor, it maybe that the inductor is conducting hat from the MOSFET.  Have you ruled this out?

     

    Thanks.

     

    David.

  • 0 in reply to David Baba
    Intellectual 390 points

    Hi David,

    We check the inductor current using 0.05R also and the waveform was triangular (Without the capacitor across the RFB2) Still the FET and the inductor was heating but the converter was not stable i.e. the the frequency was not stable as you can see from the below pictures.

    After experimenting we try with the capacitor across RFB2 and the converter is stable.

    Regarding the heating of Inductor due to mosfet, we have provided a heat sink for the mosfet but still the mosfet and the inductor is heating up and i am not getting where the power is dissipated as the output current drawn is only 3A.

    Thanks,

    Tharanath

  • 0 in reply to Tharanath PP
    TI__Mastermind 33680 points

    Is the MOSFET temp hotter than the inductor?
    I see the Green waveform is pulsing when it should not be? 

  • 0 in reply to David Baba
    Prodigy 120 points

    Hi David,

    Thanks for your support we could understand the issue with monitoring the inductor current. When we increased the inductor value to 47uH the heating issue is solved. As the inductor current was increasing due to the on time and lesser inductor value.

    Also can you suggest the variable output voltage circuitry for the with the FB pin injection method, as when we driver through the op-amp the converter is not working properly.

  • 0 in reply to Anoop Varghese
    TI__Mastermind 33680 points
    place a series resistor (injection Resistor) to the Feedback (FB) node. say you have a 1V Reference on the part. and you have 2V on the input side of the injection resistor which is 1k. YOu will have 1mA driving into the FB node. this 1mA multiplied by the Top FB Resistor gives you the voltage that the amount the output voltage is reduced. It follows the principles laid out in the LM10011. Alternatively you can use this part? Now all you need is an amplifier to connect to the injection resistor to provide the proper voltage setting for a given voltage. Hope this helps?
  • 0 in reply to David Baba
    Intellectual 390 points

    Hi David,

                    We are using 10Mhz opamp for feed back to buck controller. Is load current depend on switching frequency? When I check in DSO, Increasing load switching frequency is reducing. Theory based it is independent?

    Thanks,

    Tharanath

  • 0 in reply to Tharanath PP
    TI__Mastermind 33680 points

    Hi,

     

     

    The Frequency should increase when you apply more load.  The is not a Fixed Frequency PWM Controller so variations in frequency are expected.  the COT control scheme employed in the LM5085 should hold a fairly constant frequency over the load and VIN Range.  When the load decreases to discontinuous current conduction (DCM) the frequency will decrease.