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Original question:

UCC28780: Output Filter Calculation

UCC28780: About the calculation formula of ΔB

Tak
Intellectual 480 points
Part Number: UCC28780

The calculation formula of ΔB is described in P40 of the data sheet. (8.2.2.2.3-2)

Is this formula true for all output conditions?
For example, consider a rated output of 192W (32V, 6A).
In that case, I think I should enter 192 in Po (fl).
As a result, I think Im +, ΔB, Fsw, D, etc. will be calculated.
If I want to know Im +, ΔB, Fsw, D under the condition of 14W (1 / 8Po (fl)), should I input 14 to Po (fl)?

Also, can I think of it as a formula that can be applied to both AAM and ABM?

Also, please advise us about the air gap of the transformer used for the evaluation board.
When I use the small core (RM8), the gap is over 1mm. If the inductance value is set to about 30uH, it will be about 1.5mm.
I'm worried that it's too big, but is it acceptable for a small high-frequency flyback? (Please tell us the common air gap value.

If it is a normal flyback (100 kHz or less), I will redesign it if it becomes 1 mm or more)

However, if the leakage inductance is intentionally used in this circuit method, is it OK to consider a large gap value?
There is a concern that the gap exceeds 1 mm, but if there is a tendency peculiar to this circuit, prototype it and evaluate it.
Of course, if you recommend a gap of 1 mm or less, redesign. (Increase the core size, etc.)

B.R.Tak

  • 0
    TI__Guru 66135 points

    Hello,

    I just wanted to let you know that an applications engineer is looking into this and will get back to you within the next two days.

    Regards,

  • 0 in reply to Mike O'
    TI__Guru 66135 points

    Hello,

    You select Bmax based on the highest current when selecting your transformer core. You should also intended on selecting a material where you primary magnetizing inductance does not swing significantly with load current.

    In regards to DB it will move with magnetizing current.  In these equations you would select it for the highest magnetizing current possible.  Even though it will move the UCC28780 controller will adjust the turn on and turn off delays of the FETs to ensure the design continues to achieve ZVS down to light loads.

    I would suggest increasing the minimum frequency to 200 kHz.  This should decrease your Lm by a factor of 2, which should decrease your Bmax, by a factor of 2.  This should help reduce the gap in the transformer.

    Regards,

  • 0 in reply to Mike O'
    Intellectual 480 points

    Is the formula given in the data sheet only applicable at maximum load current?
    I want to know the frequency under a certain load condition. For example, at 1/8 rating.
    Because I assume the audio amplifier as a load condition.
    Therefore, the average power is about 1/8 and there is no problem.
    If the frequency of 1/8 rated output is less than half of the maximum frequency, we want to select the optimum core material for that frequency.
    Also, it is clear that increasing the frequency will reduce the gap, but I do not know if the frequency should be increased easily.
    Is there a target gap value?
    For example, if the center gap is 1 mm or more, we have design guidelines such as increasing the core size and increasing the frequency. .
    Generally, I think that a gap of 1mm or more should not be selected due to loss and radiation problems. Is it ok to design the transformer in the same way as a general flyback method?
    Or is it okay to prioritize miniaturization and increase the gap? Since this method uses leakage inductance, isn't some loss of the magnetic flux of the transformer exciting?
    I would like to confirm how to select the optimal core material and size.
    Is there no problem with the same idea as general flyback (PWM)?

  • 0 in reply to Tak
    TI__Guru 66135 points

    Hello,

    When it comes to designing the transformer.  I would suggest that you consult with magnetic core manufactures on designing the optimum transformer.

    To reduce losses some transformer designers will not wind over the gap.

    I found this seminar paper on designing flyback transformer that you may find helpful.

    Regards,

  • 0 in reply to Mike O'
    Intellectual 480 points

    Thank you for attaching the material. I will read it carefully and use it as a reference.
    Anyway, while consulting with the transformer maker, I will try once.
    By repeating it several times, we will gain know-how.
    （This is because I have no experience with transformers with such high frequencies.）

    Thank you.