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  • TI Thinks Resolved

UCC28780: GaN as SR transistor

Prodigy 110 points

Replies: 1

Views: 63

Part Number: UCC28780

Hi,

I'm struggling to achieve as high efficiency as possible using UCC28780 and 3 GaNs as switching components (HS, LS and SR). In most of EVMs I found () you are using 2 GaNs on primary side and one MOSFET as SR. Because I have output voltage 5V I thought it was more optimal to drive GaN directly from output  than using additional winding to drive MOSFET (I'm using UCC24612 as in EVM). Are you see any reason to not use GaN as SR? Currently I have some problems to achieve good efficiency in whole input range that's way maybe it is not recommended. For low input voltage everything seems to be ok (V bulk < 130V) but increasing voltage efficiency more less lineary drops due to switching losses increased. What;s more when pin SET is grounded switching losses increased in HS GaN (switch on in point when Vds is high) but when I tried option with SET pin to Vref switching losses for HS are ok but migrate to SR. I attempted a lot of component combination but without positive result. For most of time I based on your excel calculator. I decided to choose GaNs without integrated driver and driver circuit is done separately so this is main difference comparing to yours EVMs. 

Best Regards

Darek

  • Is 5V the only output from this design? What is your switching frequency, transformer primary inductance, etc? There are other factors to affect the efficiency, too. But to understand your efficiency, please provide the switching waveforms (primary and secondary, as well as the transformer primary winding current) to see if ok.

    We are not using GaN for SR is that we try to reduce cost since in general Si is cheaper on cost while we can still get high efficiency performance. Besides, Si MOSFET body diode forwrad voltage drop is lower than a GaN which body diode voltage drop is higher - so additonal power losses can be resulted.

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