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LM5025C active clamped forward design

Siddartha koganti
Prodigy 70 points
Other Parts Discussed in Thread: LM5025C, LM431

hi,

We are designing an active clamped forward converter using LM5025C. while designing the compensation circuit, we have found reference documents for LM5025C where the application  examples have two types of compensation networks. The one in the Data sheet of LM5025C is having a dual stage opamp based error amplifier circuit and the design example has a LM431 based circuit. Can I know is there any reason for using the Dual stage opamp circuit instead of a rather simpler LM431 circuit when it comes to loop stability. Our system has an input of 18 to 75VDC and output of 3.3Vdc and a load of 12A.

Thank you in advance

  • 0
    TI__Genius 9130 points
    Siddartha

    I see the application example on page 14 of the LM5025C datasheet shows a dual op-amp arrangement for the error amp, but I cannot see the other design example you cite using LM431, where is that?

    The LM5025C design example is maybe an unusual implementation. R27/R28 form the output voltage divider, U4 is the regulation reference. R33/C30 is an RC noise filter on the ref. U3B is the main error amp. The output of U3B will go low as Vout increases, so it's output is fed to U3A, configured as a unity-gain inverting buffer, so U3A output then goes high as Vout goes higher. The U3A output then drives more current through the feedback opto, which causes COMP to decrease on the primary side, decreasing the duty cycle, to counteract the increase in Vout.

    More conventionally, the anode of opto U2 would be connected to Vout (or to the sec bias +SB), and the error amp U3B output would be connected through a resistor to the cathode of the opto - U3B would then pull more current through the opto as Vout increases, to cause COMP to fall, in order to counteract the increase in Vout.

    Both schemes are similar, and would be equivalent if the single opamp version has the opto driven from +SB. In both of these cases, the opto drive/supply rail is fixed DC, not directly dependent on Vout.

    But where a single opamp is used and Vout is used to drive the opto, there is a feed-forward effect in the regulation loop - if Vout increases, the opto current will automatically increase anyway, even before the opamp has reacted by lowering its output to pull more current. This is like having an extra zero in the control loop, and can give better transient response.


    I hope this helps explain the datasheet application schematic.

    Thanks,
    Bernard