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LM5116 Not getting constant output.

syed hamad
Expert 1120 points
Other Parts Discussed in Thread: LM5116

Hello Tiers,

please help the analog power supply design in LM5116 here attached my schematic.i did not get constant output voltage. 

In output side without load i get only 4VDC and i connect 1K RESISTOR load on out side i get 10V pulsed dc this is not constant one.This voltage is continious

i try this input voltage 30V but in practical application i use up to 20V to 100V /1Amps .

waiting for your valuable suggestions,

5238.DC_DC_100V_V01(GENESIS 200013)_schematic.pdf

regards,

syed 

  • 0
    Genius 3295 points

    Hello Syed,

    I looked through your design and have a question and a  few suggestions. First the question: what is the significance of the E at the end of your resistor values in your schematic? For example your schematic shows gate resistor value of 10E for R3 ;and R10 mosfet gate resistors. Is this 10 ohms?

    Assuming that 2.2E means 2.2 ohms I see you have connected the current sense pin, pin 12 to two resistors R6 and R19. R6 is connected to the drain pin of the Q2 and R19 is connected to the top of the sense resistor, R18. This is not correct. You cannot use the RDS on of the bottom mosfet and a current sense resistor for pulse to pulse current limiting at the same time. You need to either remove R6 and use the current sense resistor, or remove R19  and R18 and use the RDS of the mosfet Q2. I see that in the LM5116 data sheet the "typical application" circuit they provide does show two resistors connected in this manner. However one of the resistors is labeled N/A, which I assume means not applicable, and the other resistor is a 0 ohm resistor. This done so that you can choose one method or the other. You don't use both. Maybe this is what the  E is about?

    The way you have R6 and R19 connected explains the output voltage you are observing. Because both R6 and R19 are connected across the Drain to Source pins of Q2, there is 4.4 ohms connected from your switching node to ground. So when Q2 turns off, there is still a low 4 ohm resistance present when the top switch turns on. Therefore a very high shoot through current flows through the current sense resistor and shuts down the pwmic. 

    If 10E actually means 10 ohms, then you have either the wrong FET or the wrong gate resistor or both. The IRF3710 has a very high Qg of 132 nC. This is a very high gate charge to support at 500 KHz, or even 100KHz. You need to select your mosfets using a figure of merit approach that factors in gate charge as well as RDS on.

    Hope this helps,

    V/r,

    Chuck Sampson

  • 0 in reply to Chuck Sampson
    Expert 1120 points

    Hi Chunk,

    Thanks for your inouts,

    yes the E means ohms in the above case,

    i will make changes as per the your inputs and let me check for the result..

    regards,

    syed