PI2126

Rev 1.1

the load through the MOSFET. Depending on the output

impedance of the system and the parasitic inductance, the

reverse current in the MOSFET may exceed the source

pulsed current rating (60A) before the PI2126 MOSFET is

turned off.

The peak current during an input short condition is

calculated as follows, assuming that the output has very

low impedance and it is not a limiting factor:

Where:

:

Peak current in PI2126 MOSFET before it is

turned off.

:

Input voltage or load voltage at S pin before

input short condition did occur.

:

Reverse fault to MOSFET turn-off time.

: Circuit parasitic inductance

The high peak current during an input short stores energy

in the circuit parasitic inductance, and as soon as the

MOSFET turns off, the stored energy will be released and

this will produce a high negative voltage and ringing at the

MOSFET source. At the same time the energy stored at the

drain side of the internal MOSFET will be released and

produce a voltage higher than the load voltage. This event

will create a high voltage difference between the drain and

source of the MOSFET. The MOSFET may avalanche, but

this avalanche will not affect the MOSFET performance

because the PI2126 has a fast response time to the input

fault condition and the stored energy will be well below

the MOSFET avalanche capability.

MOSFET avalanche during input short is calculated as

follows:

Where:

:

Avalanche energy

:

MOSFET breakdown voltage (30V)

Power dissipation:

In Active ORing circuits the MOSFET is always on in steady

state operation and the power dissipation is derived from

the total source current and the on-state resistance of the

MOSFET.

The PI2126 internal MOSFET power dissipation can be

calculated with the following equation:

Where:

: MOSFET power dissipation

:

Source Current

:

MOSFET on-state resistance

Note: For the worst case condition, calculate with

proportional to temperature.

Refer to Figure 10 for

125°C junction temperature.

The Junction Temperature rise is a function of power

dissipation and thermal resistance.

Where:

:

Junction-to-Ambient thermal resistance, 46°C/W

This may require iteration to get to the final junction

temperature. Figure 13 and Figure 14 show the PI2126

internal MOSFET final junction temperature curves versus

temperatures and air flow.

EOL