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LM3414: Unknown spec for calculation power dissipation

Satoshi
Guru 19495 points
Part Number: LM3414

I want to calculate the power dissipation of LM3414, but important spec is not described on datasheet. 

Please let me know about below spec of LM3414;

・Dead time（tD）

・Rise time（trise） and Fall time（tfall）

・Gate capacitance（Qg）

I think that calculation is below, if there wrong point, please let me know.

①Pswf = 1/2 (Vin × Iout × Fsw × Trall)

②Pswr = 1/2 (Vin × Iout × Fsw × Trise)

③Pcond = Iout^2 × RLX

④Pdiode = Vd ×Iout × Fsw × tD

⑤Pdrive = Qg × Vin × fsw

　⇒Ploss = ① + ② + ③ + ④ + ⑤　

Best regards,

Satoshi

  • 0
    TI__Mastermind 25500 points

    Hello Satoshi,

    trise and tfall should really be measured in an application. It will vary with voltages, parasitics, diode, etc...

    I am not sure what you mean by tD, could you clarify? I can try and get the Qg of the internal switch for you.

    In any case I think two of those equations should be:

    Pcond = Iout^2 x RLX x D

    Pdiode = Vd x Iout x (1-D)

    Regards,

    Clint

  • 0 in reply to Clinton Jensen
    Guru 19495 points
    Clinton-san

    Thank you for reply,
    Additional question, I calculated power dissipation on customer condition, result are very large(over 4W).
    Customer condition and equation are below.
    Please let me know about mistake point for equation.

    -----------------------------------------
    [Equation]
    ①Pswf = 1/2 (Vin × Iout × fsw × Tfall)
    ②Pswr = 1/2 (Vin × Iout × fsw × Trise)
    ③Pcond = Iout^2 × RLX × Duty
    ④Pdiode = Vd ×Iout ×（1-Duty）
    ⑤Pdrive = Qg × Vin × fsw
    　⇒Ploss = ① + ② + ③ + ④ + ⑤　

    [Customer condition]
    Vin＝24V
    Iout＝0.35A
    fsw＝500KHz
    Tfall＝1μ
    Trise＝1μ
    RLX＝1.8Ω
    Duty＝1 (Because LED driver application)
    Vd＝1.2V
    Qg＝2.05nC

    [Results]
    ①2.1 = 1/2 (24 × 0.350 × 500000 × 0.000001)
    ②2.1 = 1/2 (24 × 0.350 × 500000 × 0.000001)
    ③0.2205 = (0.35 Ｘ 0.35) × 1.8 × 1
    ④0 = 1.2 ×0.35 ×（1-1）
    ⑤0.0246 = 0.00000000205 × 24 × 500000
    　⇒Ploss_total =4.4451W
    -----------------------------------------

    Best regards,
    Satoshi
  • 0 in reply to Satoshi
    TI__Mastermind 25500 points

    Hello Satoshi-san,

    Two comments:

    1. Trise and Tfall will not be nearly 1us, it should be in the 10s of ns range at most. So the switching losses will be way less than what is calculated here.

    2. The duty cycle (Duty in your equations) will not be 1. It will be approximately Vin/Vled. So if your LED stack voltage is 12V the duty cycle will be 0.5 and if it is 6V the duty cycle will by 0.25 and so on... So your conduction loss in the switch will be lower but the conduction loss in the diode will be higher.

    Regards,

    Clint

  • 0 in reply to Clinton Jensen
    Guru 19495 points
    Clinton-san

    Thank you for advice,

    About Trise and Tfall, are these spec fixed LM3414 spec?
    If yes, please let me know any information of Trise and Tfall.

    Best regards,
    Satoshi
  • 0 in reply to Satoshi
    TI__Mastermind 25500 points

    Hello Satoshi-san,

    The rise and fall times will not really be fixed. They will depend on input and output voltage and will be influenced some by the inductor and diode used. This is a spec that needs to be measured on the bench for a particular application and operating point.

    Regards,

    Clint