UC3825BN driving mosfets of push pull converter

Discussion in 'General Electronics Chat' started by abhinandrd, Jul 16, 2011.

  1. abhinandrd

    Thread Starter New Member

    Jul 16, 2011
    I am using uc3825bn for driving mosfets of push pull converter. When I tried and tested (on bread board) the open loop configuration given in the datasheet, I got perfect outputs on the pins 11 and 14 with desired dead time between them. But on many occasions the outputs on this pins weren’t stable the disappeared without any reason. Also I used polyester capacitors rather than ceramic since they have steady capacitance. And when I interfaced it to power circuit the all the waveforms were distorted including the PWM pulses from pins 11 and 14. Can anybody help me in this regard as what needs to be done?
    Thank you
  2. SgtWookie


    Jul 17, 2007
    Datasheet: http://focus.ti.com/lit/ds/symlink/uc3825a.pdf
    Open loop test circuit, Figure 14, is on the bottom of page 11.
    The introductory paragraph, starting from the 2nd sentence, reads:
    So, you say that you built the open loop configuration given in the datasheet, but if you built it on a breadboard, you didn't build it according to the recommendations in the datasheet.
    You really do have to follow the recommendations, or your results won't be what you're hoping to see.
    Breadboards have far too much parasitic capacitance and inductance to use for broadband circuits like this.
    Metal poly film caps are very good. But, if you don't trim the leads to a minimal length, you add a lot of inductance to the circuit, which negates the benefit that the capacitors would otherwise supply.
    Have a look at the attached very simple schematics and simulation results. That is what parasitics will do for you.

    If you want to have better results from testing your 3825 IC, then either make a circuit board, or try using the "dead bug" prototype technique on a copper ground plane.