AN4402 datasheet(10/39 Pages) STMICROELECTRONICS

10 / 39 page

Demagnetization energy

AN4402

10/39

DocID025557 Rev 1

Demagnetization energy

In this section, we propose to calculate the energy dissipated in the output MOSFETs of the

L99MC6 and L9733 during the switch-off of an inductive load.

We will distinguish the following conditions:

•

the output is configured either as high-side or low-side driver

•

with or without resistor placed in parallel (note RP) to the inductive load

RP is often placed in parallel to the relay coil in automotive applications in order to attenuate

the voltage surge caused by the coil when the circuit is opened (Figure 6).

Figure 6. Relay with spike protection resistor in parallel to the coil

In order to determine EDEMAG, we will take a deeper look at the evolution of the current

flowing through the inductive load (noted iL) and RP (noted iP), between the MOSFET turn-

off and iL reaching zero.

We consider:

•

the time origin as the moment when the MOSFET is switched off

•

the current through the inductor at t = 0 as IL0 = iL(t = 0)

•

the demagnetization time, noted tDEMAG, as the time required for iL to reach zero

•

the energy dissipated in the MOSFET during the switch-off phase is noted EDEMAG

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