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 load 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 topology, 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 (Rds(ON)).

Introduction

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 more) 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 electronic components reliability (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) to a voltage proportional to its output current; the feedback voltage of the others paralleled power supplies (slaves) is then trimmed 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 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 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
		OUTPUT
	POWER	VOLTAGE
	SUPPLY #2
INPUT	&
	CURRENT
VOLTAGE
	SHARING
	CONTROL	L
		O
		A
	POWER	D
	SUPPLY #N
	&
	CURRENT	SHARE
	SHARING	BUS
	CONTROL

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 RDS(ON) of this FET as a sensing element as well.

System Description

The system (fig. 2) is composed of:

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–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 +5VDC external source and output terminals to the load. This board can accommodate up to three DC-DC converters.

On the motherboard there is the circuitry necessary to perform current sharing (L6615) and to isolate a failed 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 pins; in fact only changing few components it can be rearranged for new specs.

It is so possible to build a system to supply a 10A load at +3.3V in 2+1 redundant configuration. 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 system is however able to supply the load and each section will carry 5A.

The DC-DC conversion management 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
			VSENSE
	DC-DC		RSENSE
	CONVERSION	adj	CURRENT SHARING (L6615),	sh bus
	(daughter board)
			ORING FET and
			AUX. CIRCUITRY
	+5V		VSENSE
	DC-DC		RSENSE
				10A@+3.3V
	CONVERSION	adj	CURRENT SHARING (L6615)
	(daughter board)		ORING FET and
				GND
	GND		AUX. CIRCUITRY
			VSENSE
	DC-DC		RSENSE
	CONVERSION		CURRENT SHARING (L6615)
	(daughter board)	adj	ORING FET and
			AUX. CIRCUITRY

1.0 DAUGHTER BOARD

The L6910 controller drives a synchronous step-down stage at 200KHz; the internal reference is used for the 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 see [4]).

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AN1696 APPLICATION NOTE

Figure 3. Daughter board schematic

									VCC
	D1		R2	C7				D3	D4
					C8				PUMP
		BOOT		OCSET				C12	R11
R1						Q1			C13
			12	3	UGATE			C1–C2
	VCC			11
			15
						R4		L1
				10	PHASE
C4									OUT
	GND
			7
				14	LGATE		D2	C11
									R9
C3			L6910			R5
	SS				PGND
				13
			4						+SOUT
				9	PGOOD
	EAREF	8	5	1	VREF		R7	C10
				6		C9
			COMP
				VFB
			C5	R3			R6	R8	SGND
				C6					R10
									PGND
									SS

Table 1. Part list board L6910

RESISTORS

R1, R9, R10	10	SMD 0805
R2	1K5	SMD 0805
R3	2K7	SMD 0805, 1%
R4, R5	2.2	SMD 0805
R6	3K75	SMD 0805, 1%

R7	1K2	SMD 0805
R8	10K	SMD 0805
R9	82	SMD 0805
R10	39	SMD 0805
R11	680	SMD 0805

CAPACITORS

C1, C2	10μF	(TOKIN)
		C34Y5U1E106ZTE12
C3, C4,	100nF	SMD0805, Ceramic
C8, C13
C5	47nF	SMD0805, Ceramic
C6	N.C.	SMD0805, Ceramic

C7, C12	1nF	SMD0805, Ceramic
C9, C10	10nF	SMD0805, Ceramic
C11	330 μF –	(POSCAP)
	6.3V	6TPB330M

		INDUCTOR
L1	10μH	T50-52B Core 12T
		IC’s
U1	L6910	(ST) SO16 NARROW			Q1	STS8DNF3L	(ST) SO8
						L
		DIODES
D1, D3, D4	1N4148	SOT23			D2	STP130A	SMA

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