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AN4402 Datenblatt(PDF) 13 Page - STMicroelectronics |
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AN4402 Datenblatt(HTML) 13 Page - STMicroelectronics |
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13 / 39 page ![]() DocID025557 Rev 1 13/39 AN4402 Demagnetization energy 38 Equation 10 If RP does not satisfy this condition, the energy stored in the inductance would be dissipated only in RL and RP and no additional energy is dissipated in the output MOSFET. In general, RP is high-ohmic enough to satisfy this condition. A low-ohmic value of RP would result in significant power losses during the activation of the relay. Moreover, this would lead to a much longer demagnetization time and possibly decreases the reliability of some loads such as relays. Equation 8 is valid until t = tDEMAG_LS, where tDEMAG_LS is defined by iCL(tDEMAG_LS) = 0: Equation 11 The energy which is dissipated in the low-side driver during the turn-off phase is equal to the integration of the dissipated power VCL_LS iCL(t) from t = 0 to t = tDEMAG. Using the expression of iCL(t) given by Equation 8, we obtain: Equation 12 Finally, the demagnetization energy dissipated in the low-side driver with a parallel resistor to the inductor is: Equation 13 If no parallel resistor to the inductive load is present, it is possible to derive the demagnetization time and energy directly from Figure 8. Another possibility is to determine tDEMAG_LS and EDEMAG_LS in this condition from Equation 11 and Equation 13, when RP tends to + ∞ (and R L // RP tends to RL). R p V CL_LS V BAT – V BAT ---------------------------------------- RL > t DEMAG_LS L R L -------ln V CL_LS V CL_LS V BAT – ---------------------------------------- R L// Rp R L -------------------- = E DEMAG_LS V CL_LS t = 0 tt DEMAG_LS = V CL_LS R L ------------------- e t τ -- – V CL_LS V BAT – R L// Rp ---------------------------------------- – dt = E DEMAG_LS V CL_LS τ R L -------V CL_LS τ R L// Rp -------------------- V CL_LS V BAT – () – 1 R p R L R p + --------------------- V CL_LS V CL_LS V BAT – ---------------------------------------- ⋅ ln + = |
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