August 2004 - Replace -48V ORing Diodes with FETs to Reduce Heat and Save Space

DESIGN FEATURES
Replace –48V ORing Diodes with FETs
to Reduce Heat and Save Space
by James Herr
Introduction
5
DIODE (MBR10100)
4
3
POWER
SAVED
2
1
0
FET (IRFS4710)
0
2
4
6
CURRENT (A)
8
10
Figure 1. FET-based diode circuit saves power
through an external current limiting
resistor (RIN). An internal shunt regulator clamps the voltage at the VCC
pin to 11V above VSS. At power-up,
the initial load current flows through
–48V_RTN
RIN
12k
0.5W
26
R3
33k
3
VCC
LTC4354
DA
DB
1
R1
2k
VA
8
R2
2k
FAULT
4
7
VSS
GB
GA
TO
MODULE
INPUT
6
2, 5
CIN
1µF
D1
LED
M1
IRF3710S
VB
M2
IRF3710S
Figure 2. –36V to –72V at 5A typical design example
–48V_RTN
12k
0.5W
33k
VCC
Regulated MOSFET Drop
Ensures Smooth Switchover
The LTC4354 controls two external
N-channel MOSFETs with the source
pins connected together. This common
source node is then connected to the
VSS pin, which is the negative supply
of the device. The positive supply for
the device is derived from –48V_RTN
the body diode of the MOSFET and
returns to the supply with the lower
terminal voltage. The associated gate
pin immediately starts ramping up and
turns on the MOSFET. The amplifier
regulates the voltage drop between the
source and drain connections to 30mV.
If the load current causes more than
30mV of drop, the gate rises to further
enhance the MOSFET. Eventually the
MOSFET is driven fully on and the
voltage drop is equal to RDS(ON) • ILOAD
(see Figure 2).
When the power supply voltages
are nearly equal, this regulation technique ensures that the load current
is smoothly shared between them
without oscillation. The current level
flowing through each pass transistor
6
POWER DISSIPATION (W)
Critical high availability telecom systems often employ parallel-connected
power supplies or battery feeds to
achieve redundancy and enhance system reliability. Power supply selection
is usually left to ORing diodes, but
there is significant forward voltage
drop in diodes, which reduces efficiency. The voltage drop also reduces the
available supply voltage and dissipates
significant power. A better solution
would retain the diode behavior without the undesirable voltage drop and
the resulting power dissipation.
The LTC4354 is a negative voltage
diode-OR controller that replaces
ORing diodes by driving two external
N-channel MOSFETs as pass transistors. The device maintains a small
30mV voltage drop across the MOSFET
at light load, while at heavy load, the
low RDS(ON) of the external MOSFET
reduces the power dissipation. Lower
power dissipation saves the space and
cost of extra heat sinks.
For example, in a 10A, –48V application, the voltage drop across a
100V Schottky diode (MBR10100) is
around 620mV. Extra PCB space or
additional heat sinking is required to
handle the 6.2W of power dissipation.
A LTC4354 with a 100V N-channel
MOSFET (IRFS4710) as the pass
transistor dissipates only 1.4W of
power—due to the low 14mΩ(max)
RDS(ON) of the MOSFET—that can be
easily dissipated across the existing
PCB. Figure 1 compares the power
dissipation of the Schottky diode and
the MOSFET.
LTC4354
DA
DB
2k
VA = –48V
VB = –48V
GA
LOAD
FAULT
VSS
GB
LED
2k
1µF
IRF540NS
IRF540NS
Figure 3. –48V diode–OR controller monitors and reports open fuses
Linear Technology Magazine • August 2004
DESIGN FEATURES
–48V_RTN
RTN
remaining MOSFET. This raises the
potential at the VSS pin and causes
a large voltage drop across the failed
MOSFET. This can also indicate a
blown fuse in series with the MOSFET
(see Figure 3).
MOSFET short: The MOSFET that is
conducting most or all of the current
has failed short. In normal operation
this does not trigger the fault flag.
But should the power supply with
the lower terminal voltage rise up,
due to excessive load current or it
is replaced by another supply with
higher terminal voltage, a large cross
conduction current will flow between
the supplies. In this case, the voltage
drop across the MOSFET that is not
damaged can easily surpass the fault
threshold.
10k
3
30k
VCC
LTC4354
DA
DB
1
8
2k
GA
FAULT
GB
4
6
2k
VA = –48V
7
VSS
2, 5
CIN1
1µF
LED
–48V OUT
M1
IRF3710
M2
IRF3710
RTN
10k
3
30k
VCC
LTC4354
DA
DB
1
8
2k
VB = –48V
GA
4
FAULT
GB
6
2k
7
VSS
2, 5
CIN2
1µF
Handle Large Currents with
Multiple LTC4354s
LED
M3
IRF3710
M4
IRF3710
Figure 4. Parallel MOSFETS for high current (up to 20A) application
depends on the RDS(ON) of the MOSFET
and the output impedance of the
supplies.
In the case of supply failure, such
as an input supply short to –48V_RTN,
a large reverse current flows from
the –48V_RTN terminal through the
MOSFET that is on. This charges up
the load capacitance, and eventually
flows through the body diode of the
other MOSFET to the second supply. The LTC4354 detects this failure
condition as soon as it appears and
turns off the MOSFET in less than
1µs. This fast turn-off prevents the
reverse current from ramping up to a
damaging level.
that one or more of the following conditions exist.
Current overload: The load condition is too high for the RDS(ON) of the
MOSFET. Extra heat is being generated
due to the large voltage drop across
the pass transistor. A larger MOSFET
with lower RDS(ON) should be used in
the application.
MOSFET open: The MOSFET that
was conducting most or all of the current has failed open. The load current
is being diverted to the other supply
with the higher potential through the
Linear Technology Magazine • August 2004
Low Voltage Operation
Multiple low voltage supplies can
also be diode-ORed together using
LTC4354 to increase reliability. Figure 5 shows the LTC4354 controlling
two logic level N-channel MOSFETs
providing the diode-OR function for
two –5.2V power supplies. The current limiting resistor at the VCC pin
is not needed since the LTC4354 can
continued on page 36
GND
R3
2k
3
Fault Output
Detects Damaged
MOSFETs and Fuses
The LTC4354 monitors each FET and
reports any excessive forward voltage
that indicates a fault. When the pass
transistor is fully on but the voltage
drop across it exceeds the 250mV
fault threshold, the FAULT pin goes
high impedance. This allows an LED
or optocoupler to turn on indicating
Multiple LTC4354s can be connected
in parallel to accommodate large supply currents (see Figure 4). Multiple
MOSFETs can also be connected in
parallel to a single gate drive pin but
at the cost of a longer turn-off time
when the current reverses.
VCC
LTC4354
DA
1
VA = –5.2V
VB = –5.2V
DB
8
GA
4
FAULT
GB
6
LOAD
7
VSS
2, 5
CIN
1µF
D1
LED
M1
Si4466DY
M2
Si4466DY
Figure 5. Low voltage diode-OR saves power and improves reliability
27
DESIGN IDEAS
Operation
The LTC3723 controller’s basic features, its flexibility and support for
secondary synchronous rectifiers
(with adjustable timing) make it
an excellent choice for virtually all
isolated, synchronous topologies.
In this full-bridge application, the
SOT23, LTC4440, 100V, 2.4A high
side driver is used to translate the gate
drive signal to the upper MOSFETs,
Q1 and Q2. The LTC3723 integrated
driver switches the lower MOSFETs
directly. The LTC3723 initial bias voltage is derived via trickle-start resistor
R3. Once switching begins, the IC is
powered from transformer T1.
Output MOSFETs Q12–Q15 are
controlled by the LTC3901secondary
side synchronous MOSFET driver,
PoE Port Current, continued from page 33
0.3V outside the supply rails. Thus,
the maximum current is 600mA when
using a 0.5Ω sense resistor. This is
sufficient to handle an overcurrent
condition before the recommended
500mA fuse blows. The 2kΩ series
resistors limit current in the event of
a fault. The LTC2439-1 is trimmed to
provide greater than 87dB rejection of
both 50Hz and 60Hz, and wideband
LTC4354, continued from page 27
be powered directly from a supply as
low as 4.5V.
Conclusion
The trend in today’s telecom infrastructure is toward higher current and
97
42VIN
96
EFFICIENCY (%)
increased primary to secondary turns
ratio which reduces primary current
and allows the use of lower voltage,
lower loss primary and secondary
MOSFETs.
48VIN
56VIN
95
94
93
6
8
10
12
14
16
LOAD CURRENT (A)
18
20
Figure 4. Efficiency of the circuit in Figure 3
which includes a number of unique
features to ensure safe operation of
the synchronous MOSFETs under
all conditions. The LTC3901 receives
a sequence of alternating input positive and negative input pulses from
the LTC3723 through T2. Zero voltage on the SYNC input (indicating
the freewheeling period) turns both
synchronous MOSFETs on after an
initial negative pulse. Subsequent
positive and negative pulses determine
which synchronous MOSFET should
be off. Incorrect sequences of pulses
cause both synchronous MOSFETs
to turn off. Missing pulses initiate a
user programmable time-out. This
avoids potentially harmful negative
output inductor currents result from
the synchronous MOSFETs being left
on too long (during power down, for
example). Finally, the LTC3901 VDS
comparators monitor the voltage drop
across the synchronous MOSFETs,
offering a second level of protection
against excessive negative inductor
currents.
Conclusion
The LTC3723-1 controller teams up
with the LTC4440 and LTC3901 to
squeeze 240W into 3.3 square inches
of board space. The 12V application
circuit shown takes advantage of the
full bridge transformer utilization
and reduced input range to increase
efficiency beyond 95%.
noise rejection is better than 140dB.
100pF capacitors provide RFI suppression.
Figure 1 shows a typical powered
Ethernet application that supports
both 802.3af-compliant devices and
legacy devices. The LTC4259 controls
the actual switching of power to the
individual ports. The LTC4257 in the
powered device provides classification
information to the LTC4259, which
is then made available to the host
processor via the I2C bus for power allocation purposes. Up to 16 LTC4259s
can be connected to the I2C bus, and
additional LTC2439-1s may be added
to the SPI bus by providing each with
a separate CS line.
smaller module space. The traditional
Schottky diode ORing circuit is increasingly cumbersome. The LTC4354
provides an improved ORing solution
by controlling low RDS(ON) N-channel
MOSFETs. The reduced power dis-
sipation saves board space and cost
associated with extra heat sinks. Furthermore, the LTC4354 monitors and
reports fault conditions, information
not provided by a traditional diode-OR
circuit.
For more information on parts featured in this issue, see
http://www.linear.com/go/ltmag
36
Linear Technology Magazine • August 2004
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