current fault has been detected. In Active ORing

applications when one of the input power sources is

shorted, a large reverse current is sourced from the

circuit output through the MOSFET. Depending on

the output impedance of the system, the reverse

current may reach over 60A in some conditions

before the MOSFET is turned off. Such high current

conditions will store energy even in a small parasitic

element. For example, a 1nH parasitic inductance

with 60A reverse current will store 1.8µJ (½Li

When the MOSFET is turned off, the stored energy

will be released and will produce high negative

voltage ringing at the MOSFET source. This event

will create a high voltage difference across the drain

and source of the MOSFET.

The MOSFET current rating and maximum 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 in order to 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

MOSFET power dissipation:

)

(

2

on

Rds

Is

Pd

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)

is 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.

)

(

2

on

Rds

Is

Rth

Pd

Rth

Trise

JA

MOSFET

JA

MOSFET

∗

∗

=

∗

=

,

Where:

JA

Rds(on) and PI2003 sensing:

The PI2003 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

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 PI2003 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.

Under normal conditions the GATE pin output

voltage will rail to the VC voltage minus 0.5V, where

the VC output is regulated to 11V typically to support

any MOSFET with a Vgs rating of ±12V or greater. A

Vgs rating

standard N-Channel MOSFETs.

Picor Corporation • picorpower.com

PI2003

Rev 1.0