AN4402 datasheet(13/39 Pages) STMICROELECTRONICS

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AN4402

Demagnetization energy

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).

CL_LS

BAT

–

BAT

---------------------------------------- RL

DEMAG_LS

-------ln

CL_LS

BAT

–

----------------------------------------

L// Rp

--------------------







DEMAG_LS

CL_LS

t = 0

DEMAG_LS



CL_LS

------------------- e

–

CL_LS

BAT

–

L// Rp

----------------------------------------

–







DEMAG_LS

CL_LS

-------V

CL_LS

L// Rp

-------------------- V

CL_LS

BAT

–

()

–

---------------------

CL_LS

BAT

–

----------------------------------------

⋅

























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