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UCC28780: Optimal leakage inductance for high efficiency

Tak
Intellectual 480 points
Part Number: UCC28780

Dear Ulrich

I designed a 200W power supply using the UCC28780 for audio amplifiers.

The power supply is working fine, but the power efficiency is worse than expected.
At least a few percent lower than the EV that you offer.

There are several possible causes, but I don't think the switching frequency and transformer design are optimal.

The specifications are as follows

AC85V~AC276V  、200W OUTPUT (+32V) 

The minimum frequency is set to 200kHz, and the transformer specifications are as follows.

Lm=23uH

Ll=2.5uH

RM10 TP5B

Lm is around 23uH to realize 200W output.
Is the Ll value of 2.5uH too large?
Since it is an active clamp method, I intentionally made it a large Ll, but I feel that this has an adverse effect.
Please tell me how to determine the optimum leakage inductance value.

Also, the frequency may be a little higher. (FET is Si)

B.R. Tak

  • 0
    TI__Mastermind 40350 points

    Hello Tak-san, 

    Based on the promise of ACF, it is tempting to want to allow the leakage inductance to become very high compared to the magnetizing inductance, but there are still some drawbacks to high leakage inductance. It represents energy not coupled to the output which must be captured and reclaimed by the active clamp.    
    This is energy that still causes second-order losses in the winding DC and AC resistance and the clamp-switch on resistance.  2.5uH out of 23uH is more than 10% which is rather poor coupling in my opinion. 

    In typical flyback designs, the engineer often makes extraordinary effort to minimize leakage inductance.  The use of ACF allows a more relaxed approach to winding design, but a practical amount of leakage minimizing is still beneficial to reducing losses.  Still, I feel that leakage loss alone does not account for the lower efficiency that you are experiencing.  In most cases I've seen, the overall transformer losses are higher than expected, especially the core loss.  Proper choice of Litz wire (if bobbin-wound) minimizes the Rac loss.  Planar winding losses are more difficult to minimize because of pcb limitations.  Check the core and winding temperature rise to determine if one or both have excessive loss. 

    Regards,
    Ulrich  

  • 0 in reply to Ulrich Goerke
    Intellectual 480 points

    Dear Ulrich  

    Thank you for great advise.

    I'll check what you mentioned.

    Lastly, let me check only one point !
    What do you think is the appropriate leakage inductance for 23uH?

    (It's fine from experience points. I'm not looking for the best)
    However, you have at least more experience than I do.

    B.R.Tak

  • 0 in reply to Tak
    TI__Mastermind 40350 points

    Hello Tak-san, 

    My previous rule-of-thumb had been to expect less than 5% of the magnetizing inductance.  3% was pretty good, while 1-2% is really good. 
    But wiser engineers than me warn that leakage inductance should not be thought of as a percentage, because it is largely a geometrical issue.
    Academic papers have studied ways to calculate it from the core shape and winding structure, regardless of magnetizing inductance value. 

    Yet, experience has shown that it still tends to result around 3-5% of Lm, when Lm > 100uH. 

    I have no significant experience with Lm around 25uH, so I am not sure if the percentage trend still holds at such low inductance.  
    In my imagination, if the transformer size also scales down with the 25uH, it seems that Lllk should also follow, but I think the geometric scaling lags behind Lm.
    In other words, the bobbin and core do not shrink proportionately by 1/4 even if you can gap the core to achieve 25uH.  As a consequence, it may be more of a challenge to reduce leakage to <5% simply because the physical winding structure prevents it from getting lower.  

    I guess I'd strive to minimize it and see what results.  If it is actually <3%, then the relationship still holds and maybe you can relax the winding structure a little.
    If it can't be made < 5%, then we'll have to live with it and accept some reduction in efficiency from the second-order resonance losses.  However, I still think those losses simply from higher leakage should not amount to another 2% of output power.  I think that some improvement in efficiency may be obtained by iterating the winding structure and trying different core materials to find the lowest loss ferrite for the flux density and frequency at which you are operating.  

    On the other hand, I don't want to assume that any further efficiency gains can only be made by changing the transformer design. There are many other contributors to re-evaluate: EMI filter loss, Input rectifier, bulk-cap ESR, switching transistors, resonant current shape (to bring the rms value as low as possible), SR-MOSFET loss, output filter loss and ESR, interconnections and cabling loss, etc.  All could bear another look. 

    Regards,

    Ulrich

  • 0 in reply to Ulrich Goerke
    Intellectual 480 points

    Dear Ulrich 

    Thank you for the really helpful advice.

    We are still planning to make several trials.
    In short, we have the opportunity to modify the transformer to find the optimum value.
    I will use the information you have provided to make it more complete.
    I also investigate losses other than transformers and aim for optimization.
    Thank you for your kindness and information every time.

    B.R. Tak