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LM5149-Q1: Possible to use with GAN Fets?

Marcel Wilke
Prodigy 40 points
Part Number: LM5149-Q1
Other Parts Discussed in Thread: LM5149

Hi Ti,

i plan to use the LM5149-Q1 for a 48V Vin and 12V 6A out DC/DC Buck Controller. Because of my limited space and the much higher switching loss of traditional Fets im considering using GAN Fets for at least the upper MOS Fets which genereates the higher switching loss, but also the lower one is maybe better a GAN Fet due to no reverse recovery loss. I see the Gates are driven by the VCC which is 5V only so my GAN Fets with 6V Vgs should be OK. Is there any point that I dont see so far which makes it impossible to use GAN Fets instead of Silicium Fets with this controller?

Thanks in advance

Marcel

  • +1
    TI__Mastermind 22299 points

    Hi Marcel,

    Which FETs are you using?

    I have done some evaluation with EPC FETs, LM5149 could drive the FETs and the converter was regulating well initially.

    However there were some instabilities I did not understand, so I would not recommend it.

    Also the GaN third quadrant operation will result in SW node going very negative, even more than a low-side FET body diode.

    This is V_SD source to drain voltage in EPC datasheets.

    The dead time losses will be larger, so even though the QRR losses are reduced it is effectively the same total losses. 

    Hope this helps,

    -Orlando

  • 0 in reply to Orlando Murray
    Prodigy 40 points

    Dear Orlando,

    thanks for your reply. You are right i was focusing on EPC Fets. Sounds not so promising. Then I will try find the best possible Si Fets before investing more time in GAN fets. Since i just need 6A it should be also possible with Si Fets.

    Marcel

  • 0 in reply to Orlando Murray
    Prodigy 40 points

    Dear Orlando, did you also test with only the upper Mosfet using a GAN fet? Then the switch node will not going negative but the switching loss will be much lower because the upper MOS Fet is having the highest switching loss. The QRR loss when using a Si Fet for the lower Fet I can reduce with a parallel power shottky. Maybe this is the way to go.

  • 0 in reply to Marcel Wilke
    TI__Mastermind 22299 points

    Marcel,

    I thought about this mixed topology as well, unfortunately I did not implement it.

    It does sound like a good mix to still benefit from GaN.

    -Orlando