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TL2843: About TL2843D-8 PWM output wave

cyan zhou
Expert 1500 points
Part Number: TL2843

PN: TL2843D-8

I use TL2843D-8 like this: TL2843D-8 pin6 pwm output conect to PWM:

when I link the load, PWM OUTPUT wave as below(CH1 yellow), Do you know why there is a drop down(about 1V). How can i fix it?

  • 0
    TI__Mastermind 31580 points
    Hi Cyan,

    The current flowing in the transformer primary is balanced around 0, see pink trace. for the first half of the pulse the current is recovering from the negative peak and no current is been delivered from the PWM gate drive circuit. The voltage of the transformer primary is clamped by the base emitter of Q1 to the gate pin of the controller. Only when the current exceeds 0 does the gate start to deliver current through the top transistor Q2 and the voltage drops by a Vbe.

    Regards

    Peter
  • 0 in reply to Peter Meaney
    Expert 1500 points

    Hi Peter

    Thanks for your reply.

    Maybe my description is not clear enough,

    Now I marked test point as below:

    CH1: YELLOW - TL2843 OUTPUT PIN

    CH2: GREEN - Voltage between transformer primary

    CH3: PINK- R1,R7,R9 pad

     

    As you said, Only when the current exceeds 0 does the gate start to deliver current through the top transistor Q2 and the voltage drops by a Vbe.

    How the current can affect the Voltage of test point on CH1 trace?

    BTW, I tried to adj freq, the result is: only when freq above 500Khz,  voltage drop will not be appeared. whereas, it appears.

    CH1: Yellow - TL2843 OUTPUT PIN

    CH2: Green - Voltage between C11,C12

    CH3: Blue - Voltage between transformer primary

    CH4: Pink - Current of transformer primary

    Look forward to your guidance, thanks very much.

    Cyan Zhou

    Arrow

  • 0 in reply to cyan zhou
    TI__Mastermind 31580 points
    HI Cyan,

    When the current is below 0 the current is flowing in the reverse direction and some of the current will flow back through Q1, the gate drive voltage can increase to a diode drop over Vcc, please check to see if this is happening.

    To prevent damage to the controller you can put schottky diodes between the gate pin and GND and Vcc to clamp the gate pin to a diode drop above Vcc and below ground. This will prevent unwanted currents from flowing into the IC itself.

    Also the DC black caps and primary inductance of the transformer will form a resonant circuit so the waveforms will change depending on the operating frequency.

    It should be simple to simulate the circuit so you can study it in more detail.

    Regards

    Peter