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CSD88599Q5DC: Power Loss Curve (Figure 1) Drastically Different from my calculation using TI app note SLVA504 (.246W vs .016W)

Nicholas Layshot4
Prodigy 220 points
Part Number: CSD88599Q5DC

Hi, 

I used figures 1,4,5,6,19 and section 6.7.2 of the CSD88599Q5DC MOSFET datasheet to estimate power loss but this number is far larger then my calculation based on TI application report SLVA504. 

Knowns: VIN = 24V, DC = 50%, FSW = 20KHz, Ron = 1.7 mOhm, I = .614 Amps, Igate_rise = .820A, Igate_fall = .880A, Q_total = 43nC, Rnorm_150_degC = 1.9

Using Figures (1,4,5,6,19 and section 6.7.2): Figure 1 = .255W, Figure 4 = 1, Figure 5 = .964, Figure 6 = 1, Power = Figure 1 x Figure 4 x Figure 5 x Figure 6 = .246 Watts

Using SLVA504 equations: t_rise = Q_total / Igate_rise = 52.439 nS, t_fall = Q_total / Igate_fall = 48.864 nS, Psw_rise = 1/2 x VIN x I x t_rise x fsw = 7.727 mW, Psw_fall = 1/2 x VIN x I x t_fall x fsw = 7.201 mW, Psw = Psw_rise + Psw_fall = 15 mW, Prds = I^2 x RDS x Rnorm = 1.218 mW, P_total = Psw + Prds = 15mW + 1.218mW = 16 mW.

16mW is drastically different from .246W so I'm wondering what I'm doing wrong in my assumptions, calculations or comparisons between the two?

Thanks,

Nick

  • 0
    TI__Prodigy 160 points

    Hi Nick,

    Thanks for posting. We are presently looking into it and will soon get back to you.

    Beth regards,

    Ankan

  • 0 in reply to Ankan De
    TI__Prodigy 160 points
    Hi Nick,

    The power loss mentioned in Figure 1 of the datasheet includes both device loss and core loss of the inductor used while testing. The formula used in slva504 would lead to 0W loss for 0A current. However, as you can see the power loss is about 200mW at near zero current. If you want to fish out the power loss of the device, slva504 would lead to more accurate results.

    The formula used in the datasheet will lead to near accurate result for a high current application. This is owing to the fact that majority of the loss is due to the device instead of the core loss of inductor.

    It should also be noted that the switching loss should be calculated for both HS and LS device while using the formula mentioned in slva504.

    Hope this help. Thanks for posting.

    Ankan