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Original question:

LMG3622: GaN Normally off

LMG3622: Normally-on, Normally-off

R.Fukunaga
Expert 1975 points
Part Number: LMG3622

Tool/software:

I have a question about TI's GaN FET.

(+) LMG3622: GaN Normally off - Interface forum - Interface - TI E2E support forums

(+) Normally ON powerFET - Power management forum - Power management - TI E2E support forums


From the above two threads, I understand as follows.

Please correct any misunderstanding.

1) TI has E-mode and D-mode GaN FETs.

2) TI's D-mode GaN FETs require an external inductor

3) TI's E-mode GaN FETs do not require an external inductor

4) When TI's GaN FETs are used normally-off, the D-mode GaN FETs use direct drive technology, so there is no need to cascode the Si FETs.

Best,

Ryusuke

  • 0
    TI__Expert 3900 points

    Hello Ryusuke,

    Your statements are correct, however, let me add some more information for you to help your understanding.

    Traditional D-mode GaN FETs are normally-on devices, which is unsuitable for switch-mode power supply applications. Switching devices need to block voltage normally to prevent shoot-through phenomenon. Additionally, D-mode GaN FETs require high negative voltage (-14V) to turn off. The easiest way to turn a D-mode GaN FET into a normally-off device is by adding a Silicon FET with GaN FET in a cascode configuration. This has disadvantages: Silicon FET will switch during each cycle and increase switching losses in the system.

    TI has a different approach for D-mode: we do NOT use cascode configuration, instead we use direct-drive. Our GaN FETs are driven directly with a gate driver that generates -14V to turn the FET off, this is why TI GaN D-mode requires an external small inductor and capacitor. This L and C is used for a buck-boost circuit that generates a -14V rail to use for gate drive. A silicon FET is also used in direct-drive GaN FET, but the function is different. This Silicon FET is placed in series with GaN FET, and does not switch, it is only for safety. If for some reason the GaN FET is damaged or does not receive enough power, the Silicon safety FET will turn off to prevent shoot-through.

    Thanks,
    Zach S