The top-side GaN modules are supplied through bootstrap arrangement from the 12-V power supply. This Bootstrap Power Supply dose not have properly return, So it does not work.
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Thank you for the comment.
In motor drive applications using boot-strap supplies in the high-side, it is necessary to turn on the low-side power switches, may be one by one and / or with PWM, to pull the output phases to the negative DC bus to form a return path for each bootstrap capacitor. So is it in this reference design.
Take the U-phase for example, in the up-right corner in page 4 of "TIDA-00915_Isolator_Board_E1.PDF" we can see that the 12V supply can charge the bootstrap capacitor C32 (in fact, there is another one of C17 in parallel with it in "TIDA-00915_Power_Board_E1.PDF") to 12V_T_U via D4 and R26, if the PHASEU is pulled to HVBUS_RTN when the low-side U-phase switch Q2 is turned on. Take a reference to page 4 of "TIDA-00915_Power_Board_E1.PDF" for it.
This is the U-phase's bootstrap capacitor (C32+C17) charge loop. The V-phase and W-phase are similar.