ST AN1696 Application note

AN1696

APPLICATION NOTE

L6615, LOAD SHARE CONTROLLER FOR N+1

REDUNDANT, HOT-SWAPPABLE APPLICATION

by Luca Salati

Power supply systems are often designed by paralleling converters in order to improve performance or reliability. To ensure uniform distribution of stresses, the total l oad current should be equally shared among the converters.

This application note describes a redundant system (a demo board is available) composed by three paralleled DC-DC converter modules (synchronous buck t opology, managed by ST L6910) whose output currents are shared through the new ST current sharing controller (L6615).

In this application it is shown the innovative use of a MOSFET as both OR-ing element (replacing ORing diode) and sensing element (R

Introduction

ds(ON

)).

Load sharing is a technique commonly used when powering loads requiring low voltage and high current; for this reason a modular power system is built where two (or m ore) power supplies or DC -DC converters are paralleled and supply the load.

Sharing the output currents is useful to equalize the thermal stress of the different modules providing an advantage in terms of el ec troni c components rel iability (mean time between failure roughly doubles every 10°C decrease in operating temperature).

April 2003

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AN1696 APPLICATION NOTE

In this application, load sharing control is entrusted to ST's L6615 [1] that features automatic master-slave current 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 ) t o a vol t age proporti onal to i t s out put c urrent; the feedback voltage of the others parallel ed power s upplies (slaves) i s t hen tr i mmed by an "adjustment" network 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 nomi nal capabil it y); the f ai l ure of one of the modules can be tol erat ed until the capability of the remai ning power supplies i s enough t o provide the required load current. In thi s 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 sect i on exhibit s a certain amount of di sc harged capacitanc e between t he input terminals: if no inrush current limiting protection is implemented, this will cause a large negative drop on the input bus voltage (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

POWER

SUPPLY #1

CURRENT

SHARING

CONTROL

INPUT

VOLTAGE

CONTROL

POWER

SUPPLY #2

CURRENT

SHARING

CONTROL

POWER

SUPPLY #N

CURRENT

SHARING

CONTROL

BUS

OUTPUT

VOLTAGE

O A

A D

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 use of the R

of this FET as a sensing element as well.

DS(ON)

System Description

The system (fig. 2) is composed of:

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AN1696 APPLICATION NOTE

– three identical sections (daughter boards) able to perform DC-DC conversion starting from +5VDC;

each of them is designed to deliver 3.3V/5A to the load. They must be inserted in the motherboard;

– a motherboard whose input terminals will be connected to a +5V

minals to the load. This board can accommodate up to three DC-DC converters.

On the motherboard there is t he ci rcuitry necessary to perf orm current shari ng (L6615) and to isolat e a f ailed section from the load; it is designed to be adaptable to all power supplies (whose rating are compatible with L6615 absolute maximum ratings) having remote sense pi ns; i n f act onl y changing f ew components it c an be rearranged for new specs.

external source and output ter-

It is so possible to build a system to supply a

10A load at +3.3V in 2+1 redundant conf iguration.

That is,

whenever three sections are running, each of them supplies 3.33A, a value lower than its nominal capability. If one of them is switched off, the sys tem is how ever able to supply the load and each section will car ry 5A. The DC-DC conversion managem ent is entrusted to the L6910 [4]. It is possible to verify that disabling one section (through the relevant switch on the motherboard) does not cause

either overvoltage on the output or overcurrent in other sections. At the same way, enabling one section (with other two already running) does not cause output voltage negative

drop or even short to ground and current sharing is established.

Figure 2. System overview

motherboard

sh bus

10A@+3.3V

GND

+5V

GND

DC-DC

CONVERSION

(daughter board)

DC-DC

CONVERSION

(daughter board)

DC-DC

CONVERSION

(daughter board)

adj

SENSE

CURRENT SHARING (L6615),

ORING FET an d

AUX. CIRCUITRY

SENSE

CURRENT SHARING (L6615)

ORING FET an d

AUX. CIRCUITRY

SENSE

CURRENT SHARING (L6615)

ORING FET an d

AUX. CIRCUITRY

1.0 DAUGHTER BOARD

The L6910 controller drives a s ynchr onous step-down stage at 200KHz; t he internal reference is used for t he regulation. The external power mosfet 's are included in one SO8 package to save space and increase power density.

Fig. 3 shows the schematic of each daughter board and in table 1 the part list is indicated (for the description of this section se e [4 ]).

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AN1696 APPLICATION NOTE

Figure 3. Daughter board schematic

BOOT

VCC

GND

EAREF

BOOT

L6910

COMP

OCSET

R3 C6

VFB

UGATE

PHASE

LGATE

PGND

PGOOD

VREF

C12

C1–C2

C11

C10

C13

R10

R11

VCC

PUMP

OUT

+SOUT

SGND

PGND

PGND SS

Table 1. Part list board L6910

RESISTORS

R1, R9, R10 10 SMD 0805 R7 1K2 SMD 0805

R2 1K5 SMD 0805 R8 10K SMD 0805 R3 2K7 SMD 0805, 1% R9 82 SMD 0805

R4, R5 2.2 SMD 0805 R10 39 SMD 0805

R6 3K75 SMD 0805, 1% R11 680 SMD 0805

CAPACITORS

C1, C2 10µF (TOKIN)

C34Y5U1E106ZTE12

C3, C4,

100nF SMD0805, Ceramic C9, C10 10nF SMD0805, Ceramic

C8, C13

C5 47nF SMD0805, Ceramic C11 330 µF –

C6 N.C. SMD0805, Ceramic

INDUCTOR

L1 10µH T50-52B Core 12T

IC’s

C7, C12 1nF SMD0805, Ceramic

(POSCAP)

6.3V

6TPB330M

U1 L6910 (ST) SO16 NARROW Q1 STS8DNF3L

(ST) SO8

DIODES

D1, D3, D4 1N4148 SOT23 D2 STP130A SMA

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