VIV0104THJ

Rev. 1.3

9/2009

Page 13 of 16

V•I CHIP INC. (A VICOR COMPANY) 25 FRONTAGE RD. ANDOVER, MA 01810 800-735-6200

vicorpower.com

+

–

DC

Load

VTM1

VTM2

VTMn

Figure 17 – VTM Array

6.0 FUSE SELECTION

recommended at system level, to provide thermal protection in

case of catastrophic failure.

The fuse shall be selected by closely matching system

requirements with the following characteristics:

• Current rating (usually greater than maximum VTM current)

• Maximum voltage rating (usually greater than the maximum

possible input voltage)

• Ambient temperature

7.0 CURRENT SHARING

The SAC topology bases its performance on efficient transfer

of energy through a transformer, without the need of closed

loop control. For this reason, the transfer characteristic can be

approximated by an ideal transformer with some resistive drop

and positive temperature coefficient.

This type of characteristic is close to the impedance

characteristic of a DC power distribution system, both in

behavior (AC dynamic) and absolute value (DC dynamic).

When connected in an array (with same K factor), the VTM

module will inherently share the load current with parallel

units, according to the equivalent impedance divider that the

system implements from the power source to the point of load.

It is important to notice that, when successfully started, VTMs

are capable of bi-directional operations (reverse power transfer

is enabled if the VTM input falls within its operating range and

the VTM is otherwise enabled). In parallel arrays, because of

the resistive behavior, circulating currents are never

experienced, because of energy conservation law.

General recommendations to achieve matched array

impedances are (see also AN016 for further details):

• to dedicate common copper planes within the PCB to

deliver and return the current to the modules

• to provide the PCB layout as symmetric as possible

• to apply same input/output filters (if present) to each unit