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What determines the minimum switching frequency of the UCC29950?

Otto Hunt
Expert 1475 points
Other Parts Discussed in Thread: UCC29950

A LLC resonant converter must operate above circuit resonance to keep the mosfets switching at zero voltage. In the design suggested in the UCC29950 data sheet, a circuit resonance of 120 kHz was used. The UCC29950 lowers the switching frequency as low as 70 kHz in response to increased load. How does the UCC29950 know when to stop lowering the switching frequency so as to stay above circuit resonance? Other resonant controllers have pins to set the minimum switching frequency, but no such pins exist on the UCC29950.

  • 0
    TI__Mastermind 25570 points

    Hello Otto

    Thanks for taking a look at the UCC29950 PFC/LLC combo controller - this is a new device with several novel features which make using this device simpler than previous devices - a fixed LLC operating range is one of them.

    The UCC29950 LLC stage has a fixed operating frequency range of 70kHz to 350kHz and will operate anywhere within this range depending on the voltage at the FB pin. The user of this device has to design the resonant tank so that it always remains in the inductive region of the gain vs frequency characteristic.

    We used a circuit resonance of 120kHz in the design example. This resonance is the 'loaded' resonance where the RL effectively shorts out the transformer magnetizing inductance, ie. one at f0 = 1/(2*pi*sqrt(LR*CR)) where LR is the resonant inductor and CR the resonant capacitance.

    The 'unloaded' resonance (where the RL term high enough that it can be neglected) is at a much lower frequency and is the one at fp = 1/ sqrt((LR+LM)*CR) where LR is the resonant inductor. This resonance is at 50kHz. (1/(2*pi*sqrt((LR+LM)*CR)) which is lower than the minimum operating frequency of the UCC29950.

    I've seen conflicting uses of the term 'circuit resonance' as it applies to the LLC topology (I've been guilty of this myself) but this is the one we have used in the UCC29950 design.

    The UCC29950 EVM is a highly efficient and robust design - it would be even more efficient were synchronous rectification to be used. The LLC stage output will withstand repeated hard short circuits, and will operate safely down to the minimum 70kHz frequency during system shut-down.

    The datasheet describes the design process for both the PFC and LLC stages. The LLC stage design process is based on the conventional first harmonic approximation approach and is satisfactory for most applications. A new, numerically based design approach for LLC stages is shown in the application note at

    I hope this answers your question.

    Regards

    Colin

  • 0
    TI__Expert 3095 points

    Hi Otto Hunt,

    The minimum switching frequency of an LLC controller can usually be adjusted to match the resonant frequency of a given power stage. In the case of UCC29950 the minimum switching frequency is fixed within the controller and the designer is expected to adjust the resonant frequency of the power stage to match the minimum switching frequency of the controller.

    To help designers match their power stage to the fixed controller minimum switching frequency we have published a design process for this device that you can find on the Web:

    http://www.ti.com/lit/pdf/slua733

    We trust this application note will assist you in designing your power stage to operate with UCC29950. If you feel the internally programmed minimum switching frequency does not address your requirements, please let us know.

    Thank you

    Joe Leisten

  • 0 in reply to Joe Leisten
    Expert 1475 points
    Thanks, Joe and Colin. I am pretty clear on this, now.
  • 0 in reply to Otto Hunt
    Expert 2845 points
    Hello, all

    Please give me how to remove PFC burst mode of UCC29950.

    Because UCC289950EVM-631 have two problems ( checked two units )
    Load 0.00A AC 0->100V Output Voltage overshoot to 12.7V
    AC100V Load 0.00A->10A Output Voltage down to 9.5V (few mS)
    AC100V Load 0.06A->10A Ouyput OK. No drop
  • 0 in reply to Hiroshi Doi35
    TI__Expert 3095 points

    Hiroshi Doi,

    Unless memory fails me there is no burst mode on the PFC of UCC29950. If the internal power demand is zero (or very close to zero) the PFC will stop switching. This avoids problems of the bulk capacitor being over-charged due to minimum duty cycle when the PFC stage has no load.

    The LLC has a burst mode that is triggered when the output voltage goes too high. This is done because the LLC topology has difficultly limiting output voltage when it has no load.

    The limited transient performance that you report is a feature of the LLC topology. The Burst mode that is entered when completely unloaded will affect transient performance. If you need very good transient step performance I would suggest leaving a small static load (resistor) on the output so the output is never completely unloaded.

    Thanks for your interest in this part,

    Joe Leisten