PI2007 datasheet(12/19 Pages) VICOR

12 / 19 page



Picor Corporation • picorpower.com

PI2007

Rev 1.3

Page 12 of 19

Where:

PEAK

Peak current in the MOSFET right before it

is turned off.

V :

Input voltage or load voltage at MOSFET

source before input short condition did

occur.

RVS

Reverse fault to MOSFET turn-off time. This

will include PI2007 delay and the MOSFET

turn off time.

PARASITIC

: Circuit parasitic inductance

And the MOSFET avalanche energy during an input

short is calculated as follows:

)

(

)

(

PEAK

PARASITIC

DSS





Where:

Avalanche energy

DSS

)

(

: MOSFET breakdown voltage

MOSFET RDS(on) and maximum steady state power

dissipation are closely related. Generally the lower the

MOSFET RDS(on), the higher the current capability and

the lower the resultant power dissipation. This leads to

reduced thermal management overhead, but will

ultimately be higher cost compared to higher RDS(on)

parts. It is important to understand the primary design

goal objectives

for

the

application

order

effectively trade off the performance of one MOSFET

versus another.

Power dissipation in active ORing circuits is derived

from the total source current and the on-state

resistance of the selected MOSFET

MOSFET power dissipation:

)

(

MOSFET





Where :

Is :

Source Current

RDS(on) : MOSFET on-state resistance

Note:

In the calculation use RDS(on) at maximum MOSFET

temperature

because

RDS(on)

temperature

dependent. Refer to the normalized RDS(on) curves in

the

MOSFET

manufacturers

datasheet.

Some

MOSFET RDS(on) values may increase by 50% at

125°C compared to values at 25°C.

The Junction Temperature rise is a function of power

dissipation and thermal resistance.

)

(

MOSFET

Rth

Trise









Where:

Rth :

Junction-to-Ambient thermal resistance

RDS(on) and PI2007 sensing:

The PI2007 senses the MOSFET source-to-drain

voltage drop via the SP and SN pins to determine the

status of the current through the MOSFET. When the

MOSFET is fully enhanced, its source-to-drain voltage

is equal to the MOSFET on-state resistance multiplied

by the source current, VSD = RDS(on)*Is.

The reverse

current threshold is set for -6mV and when the

differential voltage between the SP & SN pins is more

negative than -6mV, i.e. SP-SN

-6mV, the PI2007

detects a reverse current fault condition and pulls the

MOSFET gate pin low, thus turning off the MOSFET

and preventing further reverse current. The reverse

current fault protection disconnects the power source

fault condition from the redundant bus, and allows the

system to keep running.

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