AN1696 APPLICATION NOTE

2/11

In this application, load sharing control is entrusted to ST's L6615 [1] that features automatic master-slave cur-

rent sharing control [2] [3]: the supply that delivers the highest current (sensed by means of an external resistor)

acts as the master and drives a common reference (share bus) to a voltage proportional to its output current;

the feedback voltage of the others paralleled power supplies (slaves) is then trimmed by an "adjustment" net-

work so that they can support their amount of load current. The slave supplies work as current-controlled current

sources.

Moreover a paralleled supply architecture allows achieving redundancy (a system of paralleled power supplies,

each delivering a current lower than its nominal capability); the failure of one of the modules can be tolerated

until the capability of the remaining power supplies is enough to provide the required load current. In this way

an interruptible power supply will be designed, reducing the failure rate of the output bus.

In hot-swappable applications, whenever a section fails, it has to be removed and replaced without turning off

the system and causing significant perturbation to both input and output system buses.

At insertion, each section exhibits a certain amount of discharged capacitance between the input terminals: if

no inrush current limiting protection is implemented, this will cause a large negative drop on the input bus volt-

age (the analysis of this issue is beyond the purpose of this document).

The same problem occurs on the output side whenever the load is already supplied by other running sections:

the discharged output capacitors of the inserted section are a very low impedance that can generate a negative

drop on the load bus. This could trigger the UV/OC protection or cause a false value if a logic circuit reads the

power supply output voltage at its input.

Figure 1. System architecture

This is way an isolating element is introduced on each of the lines connecting the power output of each section

with the load; often an OR-ing diode is used for this purpose but the latest trend is to use an OR-ing FET to save

some points in efficiency.

This, combined with the capability of ST's L6615 load share controller to perform high side sensing, allows the

System Description

The system (fig. 2) is composed of:

L

O

A

D

POWER

SUPPLY #1

&

CURRENT

SHARING

CONTROL

POWER

SUPPLY #2

&

CURRENT

SHARING

CONTROL

POWER

SUPPLY #N

&

CURRENT

SHARING

CONTROL

SHARE

BUS

INPUT

VOLTAGE

OUTPUT

VOLTAGE

L

O

A

D

POWER

SUPPLY #1

&

CURRENT

SHARING

CONTROL

POWER

SUPPLY #2

&

CURRENT

SHARING

CONTROL

POWER

SUPPLY #N

&

CURRENT

SHARING

CONTROL

SHARE

BUS

INPUT

VOLTAGE

OUTPUT

VOLTAGE