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LM5021: Cycle Skipping

Loc Ngo
Prodigy 20 points
Part Number: LM5021

Hi, my name is Loc Ngo, an electrical designer of high voltage DC-DC at ABB Plano Texas. I am using LM5021 for my small bias power to convert 360V-400V to 12V in primary side and 12V for secondary side, currents for each side approximately 100mA of each. Switching frequency is 500Khz, transformer turn ratio is 33/5/5 and total inductance is 425uH. I am getting cycle skipping and operating duty cycle is just 2% at Vin of 360V, already tried to add the offset voltage to the current sense pin but still not help, cycle still skipping. 

Would you please help to resolve this issue? 

my email address is loc.ngo@us.abb.com

Thanks

Loc

  • 0
    TI__Mastermind 40090 points

    Hello Loc,

     

    Thank you for your interest in the LM5021 off-line PWM controller.

    The main reason for cycle-skipping is that the energy content of each switching cycle is higher than the load requires for regulation at the target switching frequency. Too much energy per cycle means that the controller must suspend switching over some cycles to balance the energy throughput for the load power being consumed in order to prevent overvoltage. 

     

    Cycle skipping is a normal behavior of this controller, provided that the test load during the skipping is much lower than the full load.   Based on your posted comments, I gather that full load appears to be about 2.4 Watts ( 2 x 12V x 0.1A).  My “back of envelope” calculations indicate that at full load , based on 360V input, 425uH, and 500kHz switching, the peak primary current should be about 0.168A and on-time should be about 200ns. 

     

    500kHz = 2us period. 2% of this period = 40 ns, so I assume that your reference of 2% operating duty cycle refers to the actual on-time versus the apparent low-frequency skipping period.

    Your posting does not contain sufficient information to go much further, but some possible causes of the skipping are:

    1. Current sense resistor value is too low or CS filter is too heavy,

    2. MOSFET turn-off delay is too long (due to high Coss),

    3. Test loading was actually lighter than you thought it was,

    4. Switching frequency is set higher than expected,

    5. Some multiple combination of the above possibilities, and/or something else.

     

    Possibilities 1 & 2 are quite likely because these items (contributors to time delay) are often overlooked during design.  Turn-off delay not accounted for results in higher than expected peak current which delivers too much energy per cycle.  At 360Vin, di/dt = 360/425u = 0.85A/us = 0.085A/100ns.  So, even only 100ns extra delay can allow the primary current to increase 85mA higher than the expected 168mA peak.  Since power conversion is proportional to the square of Ipk, this increase delivers almost 2.3X as much energy per cycle as expected.  Longer delays are even worse, so cycle skipping is a likely consequence by the LM5021 to avoid output OV.  But at this time, this possibility is only speculation.

     

    More information would be useful to narrow down or pin-point the root cause, or uncover additional possibilities. Please provide full input and output specifications and a schematic diagram of the LM5021 flyback power stage, if possible.  If you wish to avoid posting this in the public E2E forum, you may send it to me by email to ulrich_goerke@ti.com and I’ll review it privately.  Meanwhile, feel free to investigate the possibilities that I listed above.

     

    Regards,

    Ulrich