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Instability in the Boost Converter using LM3481 above 18W of output power.

Other Parts Discussed in Thread: LM3481

Hello,

           I have designed a Boost Converter which operates at 325KHz, 3V-6V of input voltage to get 6V 6A (36W) and i am using a 5m ohm current sense resistor to input Isense pin of LM3481. when ever the load is increased the output voltage drops to <4V, i have added output voltage waveforms below 18W (Fig1), above 18W (Fig2) and the schematics (Fig3). F Is there any problem with the compensation circuitry and current sensing ? What can I do to overcome this issue?        

                            

                                                                     Fig 1 

                     

      Fig 2

                                                                                                                     Fig 3



  • Hi, can you share yor schematic please and I'll get back to you.
  • 3481_Big_Boost_Schematic Prints.pdfHello Jashan,

    The attached schematic may help. I've succeeded in high power boost converters, up to 140W but layout gets tricky when you have low value sense resistors coupled with the associated high currents. The attached schematic attempts to illustrate that the IC sits to one side of the power path. The power path is a circular loop that based on physical placement of the Cin /Inductor/ Switch/ senese resistor/ diode and Cout - should all form a circular loop either clockwise or counter clock wise. The direction ends up being determined by the placement of the IC lines attached across the sense line. Essentially the IC must be Kelvin connected across the sense resistor. Any unintentional added trace resistance or inductance will increase the sense voltage and cause collapse of Vout at a lower current than specified by the calculated value of RSense. Next is that gate drive may be limited and you need to chose a FEt with low gate charge, all other specs being equal. Gate helper transistors can assist also. A PNP pull down being shown here.

    Alan Martin

  • The above question corresponds to the below schematics

  • Hi, it looks like the output is current limiting (fig2),your 5 mOhm Rsence looks ok, can you check the saturation current of inductor, max drain current of FET, your schematic doesn't give part numbers so I couldn't check this. regards John