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LM5021: LM5021

Sambhav Deshmukh
Prodigy 60 points
Part Number: LM5021


Hello Dear,

We have an application where we developed a power supply using LM5021. Its input voltage range is 90 VDC to 280 VDC and output is 13VDC.

Power supply is designed for 20W. In our application we do not need the skip cycle feature so, we have bypassed this feature by adding 175mV offset using VCC pin supply. But, still it is observed that there is a skip cycle at higher voltages. (220-280VDC).

Can you pls suggest how to overcome tjis issue or completly bypass skip cycle.

  • 0
    TI__Guru 74115 points

    Hi,

    The duty required seems so small that LM5021 cannot produce so no pulses as what you see skip. You need to redesign your transformer with a bit higher turns ratio and also add some more load to maintain the minimum pulse width to allow LM5021 able to generate so to avoid skip.

  • 0 in reply to Hong Huang
    Prodigy 60 points

    Hello,

    Alone increasing the load will work or we have to consider both extra load and redesigning. transformer?

    Can you pls suggest the minimum load required not to see the skip if the power supply is designed for 20W.

    Thanks

  • 0 in reply to Sambhav Deshmukh
    TI__Guru 74115 points

    Hi,

    Re-design transformer is preferred to help minimizing the load addition.

    I need to have your design details to make suggestions for minimum load addition. But a rule of thumb seems you need to add some 1% to 5%, i.e., try 0.2W load to see if ok, or slightly higher.

  • 0 in reply to Hong Huang
    Prodigy 60 points

    Hey,

    Before making changes in transformer design.

    I would like to address one more issue that we encountered while testing.

    Actually the design is such that there is a 175mV of offset  added at the CS pin with a 432K and a 12K resistor still it is observed that the CS pin voltage goes low at particular freq. and then again up. So a kind of high low signal is seen. Further resulting in to skip cycle.

    Can you suggest potential solution to avoid this or the problem which is causing this issue.

  • 0 in reply to Sambhav Deshmukh
    TI__Guru 74115 points

    Hi,

    You need to remove that 175mV offset as it cannot help stopping the pulse skipping.

  • 0 in reply to Hong Huang
    Prodigy 60 points

    7.4.4 Operation in Skip Cycle
    During periods of minimal output power demand, the controller will operate in a skip cycle mode to reduce power
    consumption and increase efficiency at lighter loads. Skip cycle mode is entered when in normal operation the
    voltage on COMP is reduced by the external error amplifier to the point that the voltage on the PWM comparator
    falls below 125 mV. This will typically be about 1.625 V or lower at the COMP pin. When this mode is entered,
    the controller inhibits pulses on the output until the error amplifier and compensation circuit requires
    approximately 130 mV at the input of the PWM comparator. This is approximately 1.64 V at the COMP pin. The
    number and frequency of pulses in the skip cycle mode is dependent on the load and response time of the
    external error amplifier and compensation circuit. Skip cycle operation may be disabled by adding a 125-mV DC
    offset to the CS pin.

    This is mentioned in datasheet. So we thought that is the way to bypass the skip cycle.

  • 0 in reply to Sambhav Deshmukh
    TI__Guru 74115 points

    Hi,

    if your load current is very small or no load, the required duty becomes small so the pulses cannot be produced due to minimum pulse width capability as keeping pulse width will cause Vout over voltage. 

    What is your feedback circuit ? You need to check your feedback circuit to see COMP voltage can reach the citation and you need to follow the whole description.

    If you intend to use load current to solve the issue then remove that 175mV to try.

  • 0 in reply to Hong Huang
    Prodigy 60 points

    This is feedback and the compensation network from our design. pls go through this and suggest any changes that needs to be done.

  • 0 in reply to Sambhav Deshmukh
    TI__Guru 74115 points

    Hi,

    I will take a look and will reply to you by Monday, Dec 11.

  • 0 in reply to Hong Huang
    TI__Guru 74115 points

    Hi,

    The circuit structure looks ok to me.

    The parameter values cannot be determined unless you provide your schematics including power stage and transformer specs detail. 

    Note, the feedback loop itself cannot eliminate pulse skipping. It requires a minimum load on the converter output. You can also lower down the fsw, so equivalent min duty can become smaller. You can also adjust transformer turns ratios to demand bigger duty to help.