Download DN444 - Ideal Diodes Protect Against Power Supply Wiring Errors

Ideal Diodes Protect Against Power Supply Wiring Errors
Design Note 444
Meilissa Lum
Introduction
High availability systems often employ dual feed power
distribution to achieve redundancy and enhance system
reliability. ORing diodes join the feeds together at the
point of load, most often using Schottky diodes for low
loss. MOSFET-based ideal diodes can be used to replace
Schottky diodes for a significant reduction in power
dissipation, simplifying the thermal layout and improving
system efficiency. Figure 1 shows the LTC4355 and
LTC4354 combining the inputs and returns in a –48V, 5A
dual feed application. This solution reduces the power
dissipation from 6W using Schottky diodes to just 1.1W
with MOSFETs.
With two supply sources and four supply connections
there are plenty of ways to incorrectly connect the wires.
Although the likelihood of a wiring error is small, the cost
is high if downstream cards are not designed to tolerate
such errors. Wiring errors could include reverse polarity
or cross-feed connections. Knowing this, circuit designers are accustomed to using discrete diode solutions to
protect against such mishaps. It is important that active
ideal diodes give similar protection.
Types of Misconnections
Figure 2 shows the correct power supply connections.
RTNA and RTNB are close in potential by virtue of
the common connections to safety ground represented
by RGND.
Figure 3 shows a reversed input connection with RTNA
and NEGA swapped. The associated ideal diodes are
reverse biased, making the wiring error transparent to
the load with BATTERY B providing power.
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FDS3672
RTNA
FDS3672
RTNB
LTC4355
RGND
RTNA
REVERSE
INPUT
PROTECTION
IN1
GATE1 IN2
GATE2 OUT
LTC4355
GND
RTNB
RGND
BATTERY A
LOAD
48V
48V
12k
= BAS21
LOAD
BATTERY B
VCC
1MF
LTC4354
DA
DB
2k
VA
–48V
VB
–48V
GA
GB
NEGB
2k
FDS3672
FDS3672
Figure 1. –48V Ideal Diode-OR
06/08/444
NEGA
VSS
LTC4354
4355 TA06
Figure 2.Correct Power Supply Connections
Figure 4 shows another misconnection with RTNB and
NEGA swapped, so one power supply is connected across
the RTN inputs of the LTC4355 and the other supply across
the –48V inputs of the LTC4354. In this case, the reverse
input protection network of three diodes shown in Figure
1 prevents damage to the LTC4355. The load operates
from BATTERY B, but only after the current has passed
through the ground wiring.
shows the safety ground, RGND, mistakenly connected
to NEGB instead of RTNB. This connects the power
supplies in series and the voltage seen across the load
nears 100V which can cause damage, a situation no different than encountered with a discrete diode solution.
A TransZorb placed across the output protects the load,
until a fuse on the input opens to isolate the high voltage
from the load.
Figure 5 shows BATTERY B installed incorrectly. The
reversed battery has no effect on the load because the
diode connected to NEGB is reverse biased. The voltage
across the LTC4354 can exceed 100V and an external
clamp may be added to protect its DRAIN pin.
Conclusion
In dual feed applications, the supply connections can
be erroneously wired, potentially causing damage to
the load. An ideal diode solution using the LTC4355 and
LTC4354 provides protection similar to Schottkys, but
with much lower power dissipation. The end result is a
compact layout and improved efficiency.
Figures 2-5 have the correct safety ground connections
to RTNA and RTNB. Damage can occur if there is a large
potential difference between RTNA and RTNB. Figure 6
LTC4355
RGND
RTNA
RTNB
BATTERY A
RTNB
BATTERY A
RGND
48V
LTC4355
RGND
RTNA
RGND
48V
LOAD
LOAD
48V
48V
BATTERY B
BATTERY B
NEGA
NEGA
NEGB
NEGB
LTC4354
LTC4354
Figure 3. Reversed RTNA and NEGA Connection
Figure 5. Reversed BATTERY B
LTC4355
RGND
RTNA
RTNB
BATTERY A
RTNB
BATTERY A
RGND
48V
LTC4355
RGND
RTNA
48V
LOAD
48V
BATTERY B
LOAD
48V
BATTERY B
NEGA
NEGA
NEGB
NEGB
RGND
LTC4354
Figure 4. RTNB and NEGA Swapped
LTC4354
Figure 6. GND Misconnected to NEGB
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