AN4402 datasheet(21/39 Pages) STMICROELECTRONICS

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AN4402

Demagnetization energy

3.4

Clamping energy at high battery voltage

3.4.1

Qualitative analysis when the gate-drain clamp is active

The gate-drain clamp is activated

As we will see, the theoretical condition with RL = 0Ω doesn’t provide accurate results,

especially when VBAT is close or equal to VCL_LS.

However, we can draw two qualitative observations from the simplified expression of

EDEMAG (Equation 17).

Equation 17:

EDEMAG is higher than the energy stored in the inductor

Since VBAT < VCL_LS, we see from the simplified expression of EDEMAG(Equation 17), that

the dissipated energy in the output MOSFET is higher than the energy stored in the inductor

(). This is due to the fact that the battery provides additional energy to the output

MOSFET during the turn-off phase.

EDEMAG increases faster than a square function of VBAT

IL0 is proportional to the battery voltage (), and the term

is proportional

to the square of IL0 and therefore to the square of VBAT.

RL > 0 Ω

No parallel resistor to the

inductor: RP = +∞

Equation 26

with

RL = 0 Ω

RP = +∞

Equation 27

with

Table 7. EDEMAG2_HS with VBAT > VCL_LS – VCL_HS (continued)

Conditions

EDEMAG2_HS: VBAT > VCL_LS – VCL_HS

------- V

CL_LS IL0

CL_LS

BAT

–

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

CL_LS

BAT

–

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







–







BAT

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

--- LI

CL_LS

BAT

–

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







BAT

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

DEMAG_LS

∞

→

lim

DEMAG2_LS

∞

→

lim

---LIL0

CL_LS

BAT

–

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







--- LI

BAT

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

--- LI

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