• State
  • Locked Locked
  • Replies 6 replies
  • Subscribers 62 subscribers
  • Views 146 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.

ucc28780

robin gangopadhya18
Mastermind 6050 points

HEllo:

For some complicated reason,   we have to control a converter that works only in voltage control mode. So we are trying to make a discrete feedback control circuit that everyone shows classically: an error opamp, compensated for some initial P/Z, the output of the error opamp feeds a fast comparator. The comparator has a ramp.

Of course it wont simulate! Because the output of the error opamp will stay stuck at high.

How do we make the error amp not stay stuck at Vcc ? The illustration circuits given cannot work unless the error amp is made to track linearly the input difference?

any clarification on how this works will be appreciated.
r

  • 0
    TI__Mastermind 26215 points

    Hi Robin,

    This sounds very similar to what is shown on the 1st page of this white paper:http://www.ti.com/lit/an/slua119/slua119.pdf

    Is this what you are trying to simulate? 

    Best Regards,

    Ben Lough

  • 0 in reply to Benjamin Lough
    Mastermind 6050 points

    Ben

    Tried again to model Mammano's  approach to v/m.

    I put together a scheme to have sort of a soft-start circuit. Put a wire-OR diodes to have SS disconnect when the opamp output changes as the error amp kicks in.

    But the whole scheme has same behavior I got stuck forever: error output goes from high (Vcc) as long as the REF  is > vfeedback. As the Vfeedback approaches, error amp output will start going low.But it goes all the way to GND. The voltage output thus goes uncontrolled.

    Seems that there must be a suitable opamp-- pure opamp or transconductance- which will show expected error output smoothly going towards a stable output when vfeedback=~ REF

    Would you be kind enough to suggest which opamp can be used in the model?

    thnx again.

  • 0 in reply to robin gangopadhya18
    TI__Mastermind 26215 points

    Hi Robin,

    Did you add compensation to the op-amp? Typically for power supplies the op-amp is configured as some form of integrator (type I, type II, etc) to remove DC error of the output voltage. The capacitance in the compensation network should be preventing the op-amp output from going rail to rail instantaneously.

    Best Regards,

    Ben Lough

  • 0 in reply to Benjamin Lough
    Mastermind 6050 points

    Ben

    thnx for the tip.

    Can I send you a pdf of the circuit I am using?

    I am sure my opamp is not the type you use. Hence it won't start off with an output that does not hit the rail from the start.

    Again, this is an enormously important modeling effort- end result will be to know which TI opamp will model the way you indicate.

    Let me know.

    robin

  • 0 in reply to robin gangopadhya18
    TI__Mastermind 26215 points

    Hi Robin,

    Sure, I can take a look at your schematic. 

    I would say most error amplifiers will rail at startup because the output voltage is 0V. Most modern controllers will include a function to allow a soft start signal such as charging up a capacitor to control the duty cycle during startup and then let the error amplifier take over later after soft start is complete. 

    Best Regards,

    Ben Lough

  • 0 in reply to Benjamin Lough
    Mastermind 6050 points

    Thnx: let me package it so that you can easily see through the approach.