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LM5023: Issues with Switching Frequency and Mode of Operation in LM5023 QR Converter Design

Jaydeep Borkute
Prodigy 10 points
Part Number: LM5023
Other Parts Discussed in Thread: TINA-TI

Tool/software:

Dear Texas Instruments Support Team,

I am currently working on a design using the LM5023 QR Converter IC, rated for 65W, 19V output, with a universal input supply. While simulating the system in TINA-TI, I have encountered a couple of issues that I would greatly appreciate your assistance with:

  1. Switching Frequency Deviation: According to the datasheet, the system should operate in Discontinuous Conduction Mode (DCM) at a switching frequency of 130 kHz. However, during operation at half load and maximum input voltage, I am observing a switching frequency around 107 kHz. Could you kindly advise on what might be causing this deviation?

  2. Mode of Operation at Full Load: When the system is operated at 325V and 375V DC supply at full load, the expected operation is in QR (Quasi-Resonant) mode. However, the system appears to be running in DCM instead. Could you help identify why this might be happening?

For reference, I have based my design on the LM5023-2EVM Notebook Adapter, which shares similar ratings.

LM5023.tcs

  • 0
    TI__Guru* 86266 points

    Hello,

     

    Your inquiry has been received and will be answered in the order it was received.

     

    Regards,

  • 0 in reply to Mike O'
    TI__Guru* 86266 points

    Hello,

    Please see my comments below.

    1. Switching Frequency Deviation:According to the datasheet, the system should operate in Discontinuous Conduction Mode (DCM) at a switching frequency of 130 kHz. However, during operation at half load and maximum input voltage, I am observing a switching frequency around 107 kHz. Could you kindly advise on what might be causing this deviation?
      • I reviewed the data sheet and the design will operate in QR at the maximum frequency you selected Lpm and decrease frequency with load until the design reaches 130 kHz. Where it will operate in DCM mode.
      • By looking at figure 13 in the data sheet the transformer needs to be designed for a frequency of 58 kHz at maximum load low line.
      • So you might have designed for a lower switching frequency at low line. Selecting the primary magnetizing inductance will cause the design to more track what the data sheet has suggested.

     

    1. Mode of Operation at Full Load:When the system is operated at 325V and 375V DC supply at full load, the expected operation is in QR (Quasi-Resonant) mode. However, the system appears to be running in DCM instead. Could you help identify why this might be happening?
      • If you design is running in DCM more than likely your transformer turns ratio is too low.
      • I would suggest resetting your turns ratio based on the data sheet recommendations.

     

    Regards,