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ground.

Besides the power supply feedback loop, the current sharing system introduces another, outer loop. To

avoid interaction between them it is important to design the bandwidth of the sharing loop at least one or-

der of magnitude lower than the bandwidth of the power supply loop.

For the total system, the loop gain is:

where

Typically the compensation network is built by a R-C series.

5

CURRENT SENSE METHODS

Several are the methods to sense the power supply output current; the simplest one is to use a power

resistor (fig. 13a) but increasing load current could require expensive resistor to support the inherent pow-

er dissipation, imposing the use of several paralleled resistor.

Other methods to sense the output current are showed in fig. 13b and 13c:

and to the source of the MOS. Besides providing the sense resistor, the FET is used as "ORing" el-

ement: driving properly its gate, it is possible isolate the power supply output from the load (the body

diode is reversed biased so it doesn't conduct).

This is useful whenever features like hot swap or hot plug are required; compared with the well-known

solution using ORing diode, the ORing FET greatly reduces the power dissipation, in particular:

2. Current transformer: in case of very high load currents, a transformer allows sensing a smaller cur-

rent, obtained through a scaling factor equal to the transformer turn ratio. In this way, the sense re-

sistor power dissipation requirements can be less tight: obviously this is paid with the cost of the

transformer.

In fig. 13c it is showed the simplified output stage of a power supply in forward configuration: through

two current transformers the load current is reproduced in the sensing circuit scaled by a factor N.

that doesn't affect the correct behaviour of the current sharing system because its loop gain is de-

signed with a low bandwidth - at least 2 order of magnitude lower than the switching frequency - that

will cut this high frequency.

G

LOOP s

()

R

SENSE

R

CGA

R

G

--------------- G

()

R

ADJ

R

A

-------------- A

()

1

R

LOA D

------------------

⋅

⋅⋅

⋅⋅

⋅

=

C

C

1

2

π f

CSH

()

⋅⋅

--------------------------------

R

CGA

G

M

⋅

R

G

-----------------------------

R

ADJ

R

A

--------------

R

SENSE

R

LOAD

----------------------

A

PW R f

CSH

()

()

⋅⋅

⋅

⋅

=

R

C

1

2

π f

CSH

⋅⋅

⋅

---------------------------------------------

=

P

DIO DE

()

V

F

I

OUT

R

SE NSE

I

OUT

2

⋅

+

⋅

=

P

MOS

()

R

DS O N

2

⋅

=