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LM5021: the waveform of drive output pin is abnormal

user6342601
Prodigy 20 points
Part Number: LM5021

Switch Power :  85~265VAC input ,  12V 5A 60W output , f = 132kHz .

The waveform of drive output pin is abnormal , the time of rising edge and falling edge is very long , up to 1us . The data of datasheet is 25ns and 10ns . 

Don't connect the follow circuit of drive pin ,  measure the waveform , it's same , has no improvement . 

The waveform as follow:  C2 is driving waveform . 

Hope receiving the reply as soon as . 

Thank you !

  • 0
    TI__Guru* 85506 points

    Hello,

    How much capacitance loading do you have on the gate DRV? 

    What are you driving with the LM5021?  The waveform does not look like it's a FET.

    Regards,

    Mike

  • 0 in reply to Mike O'
    Prodigy 20 points

     Hello,

     The driving circuit as follow : 

    The point of measureing waveform is the left of R11 .

    Thank you.

     

  • 0 in reply to user6342601
    TI__Guru* 85506 points

    Hello,

    The drive is capable of driving 0.7A.  Is your waveform measure at the output.  If you are measuring to the left I would expect that waveform to be fast around 50  ns.

    I am looking at your schematic and I am wondering if Q4 is loading down VCC preventing the gate drive from operating correctly.  Could you remove D5 and ground Q5 base and retake your waveform?

    Please note the VCC capacitor need to be at least 10 times the gate capacitance that your are driving.  I am not sure why you just did not tie output to Q6 and Q7's bases?

    Regards,

    Mike