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UCC28064A: Work unstable when switch 2 phase to 1 phase

5321944
Prodigy 30 points
Part Number: UCC28064A

Hi:

When i decrease boost  output power to  the point PHB should turn off ,it  turns off but turns on about 100ms later.It can't stop until power off.

I set PHB=0.9v,it turns on at vcomp=0.9+0.15.Max ton by Rtset is well above 2.7us.

I find when one phase work ton time just change from 2.0us to 2.7us,not doubled.i donnot know why?

Because of ton not doubled,so vout decrease ,and  vcomp rise,it reaches 0.9+0.15V,then PHB work again.

  • 0
    TI__Mastermind 40120 points

    Hello 5321944,

     

    Thank you for your interest in the UCC28064A Interleaved PFC controller.

     

    I believe that you are measuring on-time of the MOSFET by measuring when Vds is low.

    Can you please verify that? My following discussion is based on that premise.

     

    I believe this is a case of excessive Coss in the MOSFETs that you are using. Coss is a combination of drain-to-source capacitance and drain-to-gate capacitance. It is very non-linear with Vds voltage and has very high value when the MOSFET is on (Vds < 1V).

     

    As a consequence, it takes additional time for Coss to charge up until Vds rises rapidly. This makes it appear as if the MOSFET is on longer than the gate-drive.

     

    If we define Ton as the time that Vds is low, then for 2-phase operation Ton2 = Tg2 + Tc2 = 2us, and for 1-phase operation, Ton1 = Tg1 + Tc1 = 2.7us, where Tg is the Vgs time and Tc is the extended Coss charge time. Let us assume that the Coss charge time is the same in both cases.

     

    We expect that Tg1 = 2 x Tg2, and can substitute and solve for Tgx and Tcx in each case.

    We find that for Tg2 = 0.7us, Tg1 = 1.4us (double in 1-phase), and Tc1 = Tc2 = 1.3us will satisfy the timing of both 1-phase and 2-phase operation.

     

    What this shows is that the PWM time of GDA does double when COMP< 0.9V, but the very long charge time of Coss makes the equivalent FET on-time in 1-phase less than 2X the equivalent on-time in 2-phase. As you say, the equivalent on-time does not double and so power drops, COMP rises above the PHB hysteresis, and operation oscillates between 1- and 2-phases.

     

    The high Coss can be due to using a MOSFET that is too large for your power level, or from using an old-generation MOSFET that has higher Coss than compared to Coss of newer-generation MOSFETs of similar Rds(on), or both.

     

    I recommend that you find a new-generation MOSFET with Coss low enough that its Tc time is shorter than the gate-drive time Tg at the PHB threshold. If that is not possible, or if you can only reduce Tc partially, please consider to increase the boost inductance by some amount to increase the Tg on-time. The switching frequencies will be lower and Bmax may be higher, so be sure to account for these effects at full power. If that is not possible, then the circuit shown in Figure 20 (page 19 of the datasheet) can be used to adjust Rtset to extend Tg in 1-phase mode. Since it is at low power, there should be no Bmax issue with the inductor.

     

    Regards,

    Ulrich