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For some reason my SG3524 is giving me a sine wave

Pranav Bhatkhande
Prodigy 10 points
Other Parts Discussed in Thread: SG3524, UC2524

Hello People, we're making a step up circuit using PWM using the SG3524 for our college mini project. We're using the SG3524 and are unable to get a PWM output. What could be the reason for this.. ? I've swapped IC's and no luck..we're getting a sine wave on the CRO!

  • 0
    TI__Guru*** 143166 points

    I do not know what "CRO" means.

    Nevertheless, I have a theory about sine wave generation on the SG3524 collector output when used in a standard boost circuit.

    When the SG3524 output transistor turns on, the collector voltage will be close to ground. During this time current in the inductor will increase.
    When the transistor turns off, the collector voltage will increase very quickly and the inductor current will flow through the diode to the output.
    After a short time the current in the inductor will drop to zero and the diode will turn off. The collector will drop to VCC level which starts an LC oscillation.
    The capacitor is the capacitance of the off diode, off transistor, and board.

    Regards,
    Ron M.

  • 0 in reply to Ron Michallick
    Prodigy 200 points

    CRO in this contex is Cathode Ray Oscilloscope.

    some of my older scopes (Brittish brand name Cossor)  actually had this written on the nameplate

     

    The frequency of intended boost operation and of the sinewave might give a hint whether we are dealing with parasitic capacitance and coil inductance.

    one must connect both collectors and emitters of the SG1524/2524/3524 family of ICs

    also one may use either only one of the output transistors in boost circuit, or both choice depends on the powerlevel and conversion ratio.

    use of one transistor conveniently limits switch dutycycle to tad less than 50%

     

  • 0
    Prodigy 200 points

    it would be helpful if you told us from what voltage to what are you converting

    most of us suppose that you are using boost topology are you using Boost or capacitor doubler topology with inductor input?

    are you using only the internal transistors for milliampere level output or do you use external power switch?

     

    (I have used SG1524 in push pull  boost using 1:1 inductor/transformer in an 100KHz  28V to 60V conversion using external MTM1225 mosfets at 100W level)

    it was so long time ago(1982-1984 or thereabouts),

    one possibility might be way too small inductance in the coil or transformer.

     

    the SG3524 (and UC2524) output stage has built in antisaturation network which limits output transistor current to 100mA

    and if frequency is too low to keep the current increasing to 100mA within one cycle (or half cycle in the case of push pull)

    one whitepaper using SG3524 as boost controller for external mosfet is:

    http://www.cohortviii.com/~matthes/projects/boost_conv.pdf

    it is also having sinewave appearing in the output but it might be due to instability/unstability or measurement method dependent.