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TIDM-1007: Start up code for bootstrap powered GaN daughter card

Jingyuan Zhao1
Prodigy 440 points
Part Number: TIDM-1007
Other Parts Discussed in Thread: LMG3410-HB-EVM,

Dear team,

We are trying to cost down our GaN daughter cards to make our solution more competitive by changing isolated power supply on LMG3410-HB-EVM to bootstrap power supply.

When starting up, each low-side LMG3410  should be driven by a PWM with low duty cycle to charge up bootstrap cap of high-side LMG3410 and avoid inductor current saturation. High-side LMG3410  can't be switched correctly until corresponding fault signal set high.

So any clues on how we should modify the firmware of TIDM-1007 for bootstrap powered GaN daughter cards?

Thanks and Best Regards,

Charles Zhao

  • 0
    TI__Mastermind 43730 points

    Charles,

    Please see the state machine of the PWM

    pwmSwState_normalOperation:

            // wait for positive zero crossing i.e. state 6
            if(acSine < threshold_PZC2  && acSinePrev<0)
            {
                pwm_SwState.enum_pwmSwState=pwmSwState_positiveZeroCrossing1;

                dutyPU=0.004;

                AQ_SW_FORCE_PWMxA_LOW_PWMxB_LOW(LOW_FREQ_PWM_BASE);

                SET_PWM_DBRED(HIGH_FREQ_PWM1_BASE,PFC_PWM_PERIOD);
                SET_PWM_DBFED(HIGH_FREQ_PWM1_BASE,PFC_PWM_PERIOD);

                SET_PWM_DBRED(HIGH_FREQ_PWM2_BASE,PFC_PWM_PERIOD);
                SET_PWM_DBFED(HIGH_FREQ_PWM2_BASE,PFC_PWM_PERIOD);

                SET_PWM_DBRED(HIGH_FREQ_PWM3_BASE,PFC_PWM_PERIOD);
                SET_PWM_DBFED(HIGH_FREQ_PWM3_BASE,PFC_PWM_PERIOD);

                gi.i10=0;
                gi.i6=1;
                gi_out=0;

                clearPWMTripFlags(LOW_FREQ_PWM_BASE);
                clearPWMTripFlags(HIGH_FREQ_PWM1_BASE);
                clearPWMTripFlags(HIGH_FREQ_PWM2_BASE);
                clearPWMTripFlags(HIGH_FREQ_PWM3_BASE);

                ac_cur_ref_inst=0.00;
                ac_cur_ref_inst=0.00;
                closeGiLoop=1;
                closeGvLoop=1;

            }

    Also see

    http://www.ti.com/lit/ug/tidud61b/tidud61b.pdf

    sec 2.4.4

    During startup, the lower FET switch is turned ON before upper switch , and it is soft started. so hopefully you do not need to do anything. 

  • 0 in reply to Manish Bhardwaj
    TI__Prodigy 440 points

    Hi Manish,

    Thanks for your reply!

    I think the only problem left behind is that the voltage of bootstrap cap need to be charged higher than UVLO during soft-start. I'll put up some test on it.

    Thanks and Best Regards,

    Charles Zhao

  • 0 in reply to Manish Bhardwaj
    TI__Prodigy 440 points

    Hi Manish,

    As I look into state machine code of the PWM control, I get a bit confused. According to SLYT650 , the soft-start sequence of zero-crossing should be:

    (1) All switches off => (2) Main FET soft-start => (3) Low freq FET on and sync FET soft-start => (4) Normal operation

    Take positive zero-crossing for an example. Based on my understanding, positiveZeroCrossing1 and positiveZeroCrossing2 seems to be the same for (1) and main FET starts soft-switching during positiveZeroCrossing3, which doesn't fit the soft-start sequence.

    So would you please share some more comments on how these 3 zero-crossing states work? I must get it wrong somewhere.

    Thanks a lot!

    Best Regards,

    Charles Zhao

  • 0 in reply to Manish Bhardwaj
    TI__Mastermind 43730 points

    Hopefully the above diagram explains it a little better, during PZC3 state only the Low side high freq switch is ON. we did modify it slightly from the original proposal in the app note you referred to. 

    -Manish 

  • 0 in reply to Manish Bhardwaj
    TI__Prodigy 440 points

    Hi Manish,

    This diagram really helps a lot. I can fully understand the state machine now.

    Thanks so much for all your time and help!

    Best Regards,

    Charles Zhao