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UCC256404: LLC magnitizing current

fred cheng
Prodigy 140 points
Part Number: UCC256404

Hi team,

normally we want magnetzing current small to have "the smaller loss",

copper loss obvious is dominated,

1.but is smaller magnetizing current also bring smaller core loss?

2. is larger Lm tends to have larger wiring coil resistance ?

  • 0
    TI__Genius 16155 points

    Hi Fred,

    Magnetizing current reduced by increasing the Lm value. This would improve the light load efficiency. However, during full load, the variation of frequency will be very high in order to achieve the required gain. So, ΔB will increase as the frequency of operation will be lower. So, core losses will increase. 

    Also, to achieve the larger Lm, you need to have a greater number of turns. Due to that Rac will increase.

    Regards

    Manikanta P

  • 0 in reply to Manikanta Pallantla
    Prodigy 140 points

    Hi Manikanta,

    really appreciate your illustration, about full load, I have two questions.

    1. want to confirm on one thing,  since with same full load condition, higher Lm would leads smaller Fs which means longer discontinuous time.

    so in order to compensate the longer discontinuous duration, RMS current need to be higher than the one with lower Lm to meet same load current, right?  

    but how about freewheel current during discontinuous time, is it also increased as the duration time extend?

    。

    2. also since the gain of Mg should be exactly 1 for nominal input voltage, so if my input voltage at my UCC256404 is fixed.

    ideally there won't be frequency shift (Fs=Fr) and the above scenario we discussed won't happen either , right?

  • 0 in reply to fred cheng
    TI__Genius 16155 points

    Hi Fred,

    fred cheng said:

    1. want to confirm on one thing,  since with same full load condition, higher Lm would leads smaller Fs which means longer discontinuous time.

    so in order to compensate the longer discontinuous duration, RMS current need to be higher than the one with lower Lm to meet same load current, right? 

    You are correct. I would strongly recommend you to simulate the LLC with different tank parameters to see this effect.

    fred cheng said:
    but how about freewheel current during discontinuous time, is it also increased as the duration time extend?

    As the frequency goes lower and lower, ILM (the ringing current) current goes towards zero which causes ZCS.

    fred cheng said:

    2. also since the gain of Mg should be exactly 1 for nominal input voltage, so if my input voltage at my UCC256404 is fixed.

    ideally there won't be frequency shift (Fs=Fr) and the above scenario we discussed won't happen either , right?

    Thats correct.

    Regards

    Manikanta P

  • 0 in reply to Manikanta Pallantla
    Prodigy 140 points

    Hi Manikanta,

    thanks, 

    by your answer to question 2,

    you mean by increasing Lm, the "loss" of freewheel current would still be lower even though the duration is extended, right?

  • 0 in reply to fred cheng
    TI__Genius 16155 points

    Hi Fred,

    By increasing Lm, the rms value of magnetizing current will reduce. So, copper losses will reduce. However, Increasing LM too much will lead to loss of Zero Voltage switching. So, though the conduction losses are reducing, switching losses will be increasing. 

    Again, please do the simulations to see these effects.

    Regards

    Manikanta P