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AN4402 Scheda tecnica(PDF) 13 Page - STMicroelectronics

Il numero della parte AN4402
Spiegazioni elettronici  Multichannel drivers driving inductive loads
PDF  39 Pages
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Produttore elettronici  STMICROELECTRONICS [STMicroelectronics]
Homepage  http://www.st.com
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AN4402 Scheda tecnica(HTML) 13 Page - STMicroelectronics

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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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