AN4402 datasheet(27/39 Pages) STMICROELECTRONICS

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AN4402

Energy capability and load compatibility

Although the avalanche condition is prevented by the active clamp, the inductive energy

capability of the L99MC6 and L9733 is limited. As the MOSFET operates in linear mode with

a high VDS, a high power is dissipated during the turn-off phase. The sudden increase of the

junction temperature causes a stress to the device. The maximum device capability must

not be exceeded in order to avoid permanent damages.

This aspect must be taken into account during the development phase.

It is possible to identify two main mechanisms that can lead to the device failure:

•

The temperature during the demagnetization rises quickly (depending on the

inductance) and the uneven energy distribution on the power surface can cause the

presence of a hot spot causing the device failure with a single pulse.

•

Like in normal operation, the lifetime of the device is affected by the fast thermal

variation as described by the Coffin-Manson law. A repetitive demagnetization energy

causing a temperature variation above 60K will cause a shorter life time.

These considerations lead to two simple design rules:

•

The energy dissipated in the application conditions during the corresponding

demagnetization time must be lower than the device capability for the same tDEMAG.

•

In case of a repetitive pulse, the average temperature variation of the device should not

exceed 60K at turn-off.

To fulfill these rules, the designer must calculate the energy dissipated in the output mosfet

at turn-off and the corresponding tDEMAG and then compare it with the device capability as

shown in the example Section 4.2.

4.1

Guideline

Step1: Calculation of tDEMAG and EDEMAG

This step consists in calculating the demagnetization time and energy dissipated in the

output under the defined load and application conditions. The relevant formulas are

collected in Table 16 and Table 17.

Step2: Maximum energy capability of the device at the relevant tDEMAG

The designer must determine the device output capability in terms of demagnetization

energy from the datasheet relevant curve with the same demagnetization time.

Note that the curve providing the current capability for inductive loads of the L99MC6 is valid

for the specific conditions: VBAT = 13.5V and RL = 0 Ω (Figure 16).

AN4402 Datasheet(PDF) 27 Page - STMicroelectronics