Aug 2002 - Low Voltage, High Current DC/DC Power Supply with Load Sharing and Redundancy

DESIGN IDEAS
Low Voltage, High Current DC/DC
Power Supply with Load Sharing and
Redundancy
by Henry J. Zhang and Wei Chen
Introduction
As computer and networking systems
get larger and faster, their supply
currents continue to rise and their
supply voltages continue to drop. Load
currents are high enough to require
that power supply designers use several power supply modules in parallel.
High performance power supplies for
data-processing and communication
equipment must also provide exceptional reliability and fault tolerance.
For example, power systems for mission-critical data processing systems
are must be functional better than
99.999% of the time. To satisfy the
needs of these systems, the power
management solution must provide
load sharing, fault tolerance and redundancy.
This article presents a power management solution that offers all of
these features in a relatively simple
circuit that uses the LTC3729
PolyPhase® controller and LTC4350
hot swappable load-share controller.
About the LTC3729
PolyPhase Controller
The LTC3729 dual current mode
PolyPhase controller provides the performance and reliability required by
low voltage, high current computer
and network systems. The Polyphase
technique interleaves the clock signal
of several paralleled power stages,
thus reducing the input and output
ripple current so less capacitance is
required. Reduced ripple currents significantly improve the reliability and
lifetime of the input and output capacitors. The accurate current sensing
scheme of LTC3729 provides additional reliability. Current sharing
amongst phases is excellent, making
for a uniform thermal distribution,
thus ensuring the reliability of power
semiconductors and output induc34
VIN(BUS)
LTC3729
DC/DC
MODULE
LTC4350
HOT SWAP
&
LOAD
SHARE
•
•
•
•
•
•
LTC3729
DC/DC
MODULE
LTC4350
HOT SWAP
&
LOAD
SHARE
Q2
LOAD
SHARE
BUS
Q2
Q1
VOUT(BUS)
•
•
•
Q1
Figure 1. System block diagram of the DC/DC power supplies with load sharing and redundancy
tors. Other advanced features of
LTC3729 include true remote sensing, integrated high current MOSFET
drivers, overvoltage protection,
foldback current limit, and optional
overcurrent latch-off. All of this adds
up to a reliable and high performance
low voltage, high current supply.
Adding the LTC4350 Hot
Swappable Load Share
Controller
To further improve system reliability,
add the LTC4350 hot swappable load
share controller after the LTC3729.
The LTC4350 allows paralleled power
supplies to share the load with fault
tolerance and redundancy. To share
the load amongst redundant supplies,
the LTC4350 adjusts the output voltage of each supply until the current of
each supply matches the value set by
the share bus. The LTC4350 also
isolates failed supplies by turning off
the series output MOSFETs and identifies failed supplies to the system.
The failed supply can then be removed and replaced with a new unit
without turning off the system power.
The LTC4350 improves system efficiency by allowing the use of low
RDS(ON) output MOSFETs instead of
ORing diodes.
The LTC4350 is a universal load
share controller that works with any
DC/DC controller, such as the
LTC1628, LTC3728, LTC1629 and
LTC1778.
3.3V/40A Output Power
Supply with Load Sharing
and Redundancy
Figures 2a and 2b show a 3.3V/40A
output power supply with load sharing and redundancy. Figure 2a shows
the first part of the circuit: the
LTC3729 controller in a 2-phase, synchronize buck DC/DC converter that
provides 3.3V/40A output from a 5V–
12V bus. The converter only requires
one IC, eight tiny SO-8 size MOSFETs
and two 1µH, low profile, surface
mount inductors. Efficiency is 91%–
93% over the full input voltage range
with a 3.3V/40A output. Figure 3
shows the efficiency of the supply
over a wide 2A to 40A load range.
Figure 2b shows the LTC4350 load
sharing and hot swap circuit. The
load current of each supply is determined by the share bus voltage. For
each channel, the IOUT pin of LTC4350
is connected to the voltage feedback
Linear Technology Magazine • August 2002
Linear Technology Magazine • August 2002
R2
2.7k
C11 4.7nF
R3
10k
470pF
R6
8.66k
C15
470pF
R5 4.7k
C10 100pF
C9 0.01µF
C7
0.1µF
R7
25.5k
SENSE2–
100Ω
C17
1000pF
C1 1000pF
100Ω
SENSE1
–
SENSE2+
100Ω
14
13
12
11
10
9
8
7
6
5
4
3
2
1
SW1
PGOOD
SENSE2 +
SGND PGND
TG2
SW2
BOOST2
BG2
PGND
SENSE2 –
VOS +
VOS
–
VDIFFOUT
SGND
INTVCC
ITH
BG1
EXTVCC
PLLIN
NC
VIN
BOOST1
PLLFLTR
EAIN
SENSE1
TG1
–
CLKOUT
SENSE1 +
U1
LTC3729EG
RUN/SS
100Ω
SENSE1+
CIN:
COUT:
L1, L2:
Q1–Q8:
15
16
17
18
19
20
21
22
23
24
25
26
27
28
C2
1µF
C8
0.47µF
3
2
C16
0.47µF
D1
BAT54A
C7
1µF
(619) 661-6835
(714) 373-7334
(847) 956-0667
(800) 554-5565
C14
4.7µF
1
OS-CON 16SP100M
PANASONIC EEFUE0G18R 180µF 4V
SUMIDA CEP125-1R0
SILICONIX Si7440DP
C12
1µF
R1
10Ω
EXTVCC
Q7
Q5
Q3
Q1
R10
100Ω
+
C22
1µF ×2
Q4
Q2
SENSE1–
R8
0.002Ω
R4
0.002Ω
SENSE1+
C24
10µF
SENSE2–
SENSE2+
L2
1µH
D1
B540C
L1
1µH
D2
B540C
CIN
100µF
16V ×4
COUT
180µF
4V ×6
Q8
Q6
+
VSENSE
(FROM LTC4350)
VOUT (LOCAL)
3.3V/40A
VIN
5V TO 12V
DESIGN IDEAS
Figure 2a. DC/DC converter portion of the redundant, load sharing power supply
35
DESIGN IDEAS
VOUT (LOCAL)
Q10
Q9
Q11
0.002Ω 1% 1W
+
0.002Ω
1%
1W
INTVCC
COUT
180µF
4V ×3
VOUT (BUS)
3.3V
40A
21.5k
1%
100Ω
5%
0.22µF
X5R
0
VSENSE
(TO LTC3729)
100Ω
5%
OPTIONAL
86.6k
1%
200k
1%
30.1K
1%
VCC
GATE
IOUT
RSET
R+
R–
FB
12.4k
1%
0.1µF
X7R
51k
STATUS
SHARE BUS (SB)
STATUS
GAIN
SB
LTC4350
GND
TIMER
0.1µF
X7R
UV
0.1µF
X7R
150k
1%
OV
12.1K
1%
COMP1
COMP2
1000pF
X7R
150Ω
5%
1µF
X7R
Q9, Q10, Q11: SILICONIX Si7866DP
(800) 554-5565
COUT: PANASONIC EEFUE0G18R 180µF 4V (714) 373-7334
Figure 2b. Load sharing and Hot Swap portion of the power supply solution
100
VIN = 5V
95
VIN = 12V
EFFICIENCY (%)
90
85
80
75
70
VOUT = 3.3V
65
0
10
20
30
LOAD CURRENT (A)
40
Figure 3. Measured efficiency of
the LTC3729 circuit
failure to the system through the
STATUS pin. In this design, to simplify the circuit, single-direction
MOSFETs are used in each module
since the LTC3729 also has output
overvoltage and short circuit protection functions.
Figure 4 shows the pulsed load
current waveforms of two paralleled
power supplies with load sharing.
The waveform shows that the two
supplies have good current sharing
from no load to heavy load, 40A.
Figure 5 shows the hot swapping
resistor R10 of the LTC3729. Therefore, the local output voltage VOUT of
LTC3729 can be adjusted until the
current of each supply matches the
value set by the share bus. The
LTC4350 monitors the local output
voltage VOUT of each supply at the UV
(undervoltage) and OV (overvoltage)
pins. Low, high and open circuit faults
are detected in this way by the
LTC4350, which turns off the series
output MOSFET to isolate faulty supplies. The LTC4350 also provides an
open-drain signal to report the local
continued on page 38
Module 1 Output Current
Module 2
Output
Current
VOUT 1 (3.3V)
VOUT 1 (3.3V)
VOUT 2 (3.3V)
VOUT 2 (3.3V)
40A
40A
IOUT 1
~5.5A/DIV
IOUT 1
20A
0A
200µs/DIV
LOAD CURRENT STEP = 0A TO 40A
Figure 4. Pulsed load current of two
paralleled LTC3729 power supplies
with LTC4350
36
IOUT 2
IOUT 2
1ms/DIV
1ms/DIV
Figure 5a. Swapping in module 2
Figure 5b. Swapping out module 2
0A
Figure 5. Hot swapping waveform of two paralleled LTC3729 supplies with LTC4350
Linear Technology Magazine • August 2002
NEW DEVICE CAMEOS
MSOP-8 packaged LT1961 are a high
efficiency 1.5A switch and all the control circuitry required for a complete
current mode buck converter. A patented anti-slope circuit maintains the
1.5A maximum switch current limit
over all duty cycles.
Low switch resistance maintains
high efficiency at a high switching
frequency over the 35V maximum
switch voltage range. A low dropout
internal regulator ensures consistent
performance over the part’s entire
2.7V to 30V input range. The accurate shutdown threshold, which
reduces quiescent current to 6µA,
can be used as a precise undervoltage
lockout. Synchronization allows an
external logic level signal to increase
the internal oscillator frequency from
1.4MHz to 2MHz.
LT6550 and LT6551: 3.3V
Triple and Quad Video
Amplifiers
The LT6550 and LT6551 are triple
and quad video amplifiers designed to
operate from a single 3.3V supply.
These voltage feedback amplifiers
drive double-terminated 50Ω or 75Ω
cables and are configured for a fixed
gain of 2, eliminating either six or
eight external gain setting resistors.
The LT6551 quad is designed for single
supply operation and performance is
fully specified on single 3.3V and 5V
supplies. The LT6550 triple can be
used on either single or split supplies
of ±5V. The LT6550 and LT6551 both
feature 110MHz –3dB bandwidth,
340V/µs slew rate and 3% settling
time in 20ns, making them ideal for
RGB video processing with a maximum screen resolution of 1024 x 768
on a single 3.3V supply.
On a single 3.3V supply, the input
voltage range extends from ground to
1.55V and the output swings to within
400mV of the supply voltage while
driving a 150Ω load. These features,
combined with the ability to accept
RGB video signals without the need
for AC coupling or level shifting of the
incoming signals, make the LT6550
and LT6551 ideal choices for low voltage video applications.
Both the LT6550 and LT6551 are
available in a small 10-pin MSOP
package and utilize a flow-thru pin
out. Each device is available in both
commercial and industrial temperature range versions.
LTC3729/LTC4350, continued from page 36
waveforms of two paralleled modules
with a total of 40A output current.
The settling time for load transients
and hot swap load currents can be
adjusted via the compensation resistors and capacitors on the COMP1
and COMP2 pins of the LTC4350. See
the LTC4350 data sheet for details.
VIN(BUS)
VOUT(BUS A)
LTC4350
LTC3728
DC/DC
SB (A)
VOUT(BUS B)
LTC4350
SB (B)
Redundancy for Multiple
Output Applications
Figure 6 shows a simple and robust
redundant power supply system with
three outputs. In this example, three
LTC3728 (similar to the LTC3729,
but with two outputs) dual output
PolyPhase controllers provide voltage
control, and six LTC4350s provide
hot swappable load sharing. Each
LTC3728 regulates two outputs which
are switched 180-degrees out of phase
to minimize the number of input
capacitors.
Conclusion
The LTC3729 PolyPhase current mode
controller and dual output LTC3728
provide high performance, minimize
component count and increase the
reliability of low voltage, high current
power supplies. These parts, com38
LTC4350
LTC3728
DC/DC
SB (A)
LTC4350
SB (C)
SB (B)
LTC4350
SB (C)
LTC4350
LTC3728
DC/DC
VOUT(BUS C)
Figure 6. Block diagram of a redundant multiple voltage output system
bined with the L TC4350 hot
swappable load share controller, make
for easy-to-design fault tolerant
redundant power supply systems that
are suitable for mission-critical
applications.
Linear Technology Magazine • August 2002
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