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PMP20289: Question about Reference Design of High Frequency Resonant Converter Using High Voltage GaN Devices_LG TV

Part Number: PMP20289

Team,

 

There need your help about PMP20289.

There have some new questions about PMP20289 which is a 500W(400V – 12V/42A) high frequency resonant converter using high voltage GaN devices.

 

PMP20289

http://www.ti.com/tool/PMP20289

power : 500W

output : 12V 42A

Structure : primary side is LLC with GaN, Secondary side SR with FET (2mohm)

fo=370kHz, fs=400~500kHz (above)

resonant capacitor: MLCC 6* 8.2nF

separate trans structure : Lk 3.71uH ( separate inductor L = 2.05uH + tran Lk 1.66uH)

                                            Lp =67.5uH ( 16 : 1)

 

Questions :

Q1 : Normally, MLCC vendor don’t provide current ripple specification. Which current specification of MLCC be used for calculation ?

Q2 : As we know, for high frequency, it should reduce inductor value or reduce capacitor value. For PMP20289, What’s the key reason about increase capacitor value significantly and reduce inductor ?  

  • Dose this structure for SR stability due to high current(42A) in secondary side ?

  • Dose it like LLC bobbin structure (increasing primary side Lp value for reduce GaN temperature and decreasing Lk for SR stability) ?

  • Dose this structure for low output voltage application normally ?

 

Q3 : Why fs above fo ?  (Secondary side have FET. If fs above fo, It seems there have higher switching loss. )

 

thanks,

 

  • Hi Peter,

    My response is as follows:

    A1: Please ask the MLCC provider for an official answer. But for me, the idea is to select the ceramic capacitor with low ESR at the operational frequency.The ceramic capacitor I selected does show the ESR value at ~400kHz frequency (product.tdk.com/.../CGJ5L4C0G2H822J160AA.pdf), but it does have low ESR with 3MHz frequeny. What I can do is to use the capacitor in my design and check the temperature rise at full power. As long as the temperature rise of the ceramic capacitor is below 20degC, I think it is good.

    A2: 

    For high power closed loop LLC converter, you tend to have larger resonant capacitor value for higher gain variation to cover ripple voltage and input voltage range. In addition, higher series resonant inductor implies higher power loss on the resonant inductor (both AC and DC losses).
    Therefore, if you want to have high converter efficiency on a high power, high frequency LLC converter, you will need to minimize Lr and maximize Cr. And that is why I do so in this design. It is nothing to do with SR stability.
    Also note that we use the energy stored in Lp to achieve primary MOSFET ZVS. Therefore, maximum Lp value is limited by the ZVS capability. You don’t want to make Lp too large to lost ZVS.
    But we do want to use Lp as large as possible while maintaining ZVS to lower GaN device conduction loss.

    A3: 

    As the Lr is small and the ratio of Lp/Lr is large, the converter frequency variation is large and the covered input voltage range is narrow (380V~400V).
    So if the converter is operated at 380V, fo <fr. Operate at 390V, fo~fr. Operate at 400V, fo>fr.
    The efficiency is slightly lower when operates above fr, but not much.

    Regards,

    Sheng-Yang Yu