LINEAR LTC4350 User Guide

L DESIGN FEATURES

Hot Swap Controller Enables Standard Power Supplies to Share Load

by Vladimir Ostrerov

Introduction

The LTC4350 Hot Swap™ and load +VIN +VOUT

BC847BPN

share controller is a powerful tool for +SENSE

CH1 OUTPUT

developing high availability redun-

POWER SUPPLY

dant and load sharing power supply

Q1 LT1784

systems. It has the unique ability to LOADGND

SINUSOIDAL

work with supplies with any output

GENERATOR

R1 = RPS(SENSE)

stage topology, including output stages –SENSE using synchronous rectification. –VIN –VOUT

Although the LTC4350 does much of the heavy lifting in maintaining a well

Figure 1. Power supply closed-loop Bode plot measurement block diagram

balanced load share system, there are a number of important considerations in designing a stable system. q Redundancy. These systems, the output current of each supply This article deals with some of the often called N + 1 redundant, use must be proportional to the rated more complex design details of the a number of small supplies where supply power. LTC4350. If you are designing a load N units are needed to power the The LTC4350 performs this func- share system and LTC4350 is new load, but a “+1” supply is added tion. It forces multiple, paralleled to you, it may be helpful to first read for redundancy. The theory is, if power supplies to share current. The introductory details in the LTC4350 one supply fails, N units remain concept behind a LTC4350-based data sheet, and the article “Combo/Hot to carry the load. load share controller is simple: a Swap Load Share Controller Allows q Efficiency. If the power system single, overall voltage loop controls the use of Standard Power Modules must support widely ranging the common output while each power in Redundant Power Systems” in the loads, efficiency can be optimized converter is controlled by a local cur- June, 2003 issue of Linear Technology by adjusting the number of rent loop and contributes current in magazine. operating supplies to the load. the common output. Even so, such a

multi-loop feedback system requires

A Little Background careful design. DC/DC converters are paralleled for Active control is achieved by sinking

The LTC4350 Hot Swap and

any of several reasons: additional current from the +SENSE

load share controller is a

q One converter may be pin found on many common converter

powerful tool for developing

insufficient for the required modules, and fooling the converter

power level. For example, an high availability redundant into believing the output voltage is

existing single regulator design and load sharing power something different than what it may be able to handle 100W, would otherwise detect. Increasing

supply systems.

but a new application calls this current causes the output to rise; for up to 200W. Paralleling It has the unique ability decreasing it causes the output to fall. two, production-proven 100W Thus the LTC4350 has a means of

to work with supplies

converters saves time over modulating the output voltage, thereby

with any output stage

developing a new converter controlling the companion converter’s

topology, including output

capable of twice the power. contribution to the system.

q The product may need to be stages using synchronous

scalable. Many rack-based rectification. AC Analysis

systems feature multiple slots, The design of a current sharing power which may be populated at some system involves not only the simple future date, but why install power The key to parallel operation is matter of DC operating conditions, but supply sufficient to operate the balancing the output current of each also AC analysis. This article covers entire rack when only a fraction supply, so that all are equally loaded if the AC design aspects of an LTC4350- of the slots are in use? Paralleling the supplies are identical. If individual based load share system. supplies allows addition of power supplies with different power ratings The design goal is to build a stable on an as-needed basis. are combined for parallel operation, system with maximum bandwidth,

Linear Technology Magazine • June 2008