DN534 - Surge Stoppers Ease MIL-STD-1275D Compliance

Surge Stoppers Ease MIL-STD-1275D Compliance
Design Note 534
Dan Eddleman
Introduction
A military vehicle is a tough environment for electronics, where the potential for damaging power supply
fluctuations is high. U.S. Department of Defense MILSTD-1275D sets down the requirements for electronics when powered from a 28V supply, ensuring that
electronics survive in the field.
MIL-STD-1275D compliance can be achieved by brute
force, shunting high energy levels to ground using bulky
passive components. This method does not guarantee
power delivery to downstream electronics and can require replacing damaged protection components when
they do their job. A more compelling solution is to use
high voltage surge stoppers such as the LTC ®4366 and
LT®4363, which use series MOSFETs to limit the output
voltage when faced with input voltage spikes and surges.
A surge stopper reference design for MIL-STD-1275D
is available as Linear Technology demonstration circuit
DC2150A-C. This board limits its output voltage to 44V
when faced with input voltages as high as 250V, while
providing 4A of current to the output in all circumstances
except the ±7V ripple test, when the available current
is reduced to 2.8A. In most circumstances, satisfying
MIL-STD-1275D is as simple as placing this circuit in
front of a 44V tolerant device. A certification report is
available at www.linear.com/demo/DC2150A.
MIL-STD-1275D Requirements
MIL-STD-1275D defines a variety of supply variances,
from steady-state operation to starting disturbances,
spikes, surges, and ripple, and lays down requirements
for each of these conditions in three separate “modes
of operation”:
• Starting mode: starting and cranking conditions
• Normal mode: nominal, fault-free battery supply
• Generator-only: a disconnected battery leaves the
generator directly powering the electronics.
Table 1 compares the MIL-STD-1275D limits for normal
mode and generator-only mode. This article focuses on
generator-only mode since it is the most demanding.
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Table 1. Selected MIL-STD-1275D Specifications in Normal
Operating Mode and Generator-Only Mode
GENERATOR-ONLY
NORMAL
MODE
SPECIFICATION OPERATING MODE
23V < VIN < 33V
Steady State
25V < VIN < 30V
Same as Normal
Spikes
250V, Max.
Energy = 15mJ
Operating Mode
100V Max.,
Surges
40V Max.,
~500ms,
~500ms,
RSOURCE = 20mΩ
RSOURCE = 500mΩ
Ripple
Magnitude ±2V
Magnitude ±7V
Steady State
In generator-only mode, the steady-state supply voltage is between 23V and 33V. In the simplified diagram
in Figure 1, the LT4363 in combination with sense
resistor RSENSE limits the maximum DC current to 4A
minimum/5A typical. This protects the system from
faults that occur at the output and prevents blown
fuses at the input.
Spikes
A spike is generally oscillatory (it rings) and decays
to the steady-state voltage within 1ms. The envelope
of the worst-case MIL-STD-1275D spike is defined by
Figure 2 (for generator-only mode).
In Figure 1, the 250V spike condition is handled by
MOSFET M1, rated to withstand over 300V from drain
to source. During the –250V spike, diode D1 is reversebiased, blocking the spike from M2 and the output. (The
LTC4366 surge stopper withstands reverse voltages
and the –250V spike without additional protection.)
Surges
Surges are transients that last longer than 1ms. Figure 3
shows the limitations for generator-only mode. The
recommended test in MIL-STD-1275D specifies that five
100V pulses of 50ms duration should be applied at the
system input with a 1s repeat time. The envelope of the
surge condition shown in Figure 3 is more difficult to
satisfy, as it does not return to 40V for a full 500ms.
The solution shown satisfies both conditions.
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered
trademarks of Linear Technology Corporation. All other trademarks are the
property of their respective owners.
LIMITED TO
66V
M1
VIN = 28V
–250V
TO
250V
TOLERANT
D1
R2
VDD
R3
12.1k
GATE
LTC4366-2
RSENSE
10mΩ
M2
LIMITED TO
44V
C1
20µF
×12
COUT
VCC
GATE
FB
SNS
OUT
FB
LT4363-2
R4
649k
R5
332k
R6
10k
Figure 1. Simplified MIL-STD-1275D Circuit Diagram
300
110
250V, 70µs
250
90
80
100
VOLTAGE (V)
VOLTAGE (V)
150
STEADY-STATE VOLTAGE (23V–33V)
50
0
−50
MAXIMUM ENERGY
CONTENT OF 15mJ
−100
70
60
50
40V, 500ms
40
30
−150
−200
20
−100V, 1ms
−250
−300
100V, 50ms
100
100V, 1ms
200
10
−250V, 70µs
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
TIME (ms)
1
0
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
TIME (s)
1
Figure 2. Envelope of Spike in Generator-Only Mode
Figure 3. Generator-Only Mode Surge Envelope
During the input surge, M1’s source is regulated to 66V
by the LTC4366, while M2’s source (and the output) is
regulated to 44V by the LT4363. Compared to using a
single MOSFET, this reduces the power that must be
dissipated in the individual MOSFETs and increases
power available at the output.
Starting Mode
Voltage variations caused by the starter motor and
cranking are described by MIL-STD-1275D starting
mode—the supply voltage can drop as low as 6V before recovering to at least 16V within one second and
the steady-state DC voltage within 30 seconds. The
solution presented here typically functions at the 6V
minimum. But it is only guaranteed to work to 8V due
to component tolerances, the most significant being
the loosely specified threshold voltages provided by
MOSFET manufacturers.
Ripple
Ripple refers to 50Hz to 200kHz oscillations of the
supply voltage about its steady-state DC voltage. According to the specification in generator-only mode,
the ripple can be as large as ±7V about the DC steady
state voltage.
Diode D1 in combination with capacitor C1 forms an AC
rectifier that prevents high frequency ripple components
from reaching the output. Note that rising edges of the
input ripple waveform attempt to pull up the output
capacitor, causing the LT4363 to momentarily limit
the current through M2. For this reason, the current
available to the output load during the ripple condition is 2.8A, less than the 4A available during normal
operation. More about the ripple condition and ways
to improve this circuit behavior are described in Linear
Technology Journal of Analog Innovation, Volume 24,
Number 1, “High Voltage Surge Stoppers Ease MILSTD-1275D Compliance by Replacing Bulky Passive
Components.”
Data Sheet Download
www.linear.com/LTC4366
Linear Technology Corporation
Electromagnetic Compatibility Requirements
MIL-STD-1275D refers to another standard, MILSTD-461, regarding electromagnetic compatibility.
Typically, an EMI filter is placed at the input of MILSTD-1275D compliant systems—while surge stoppers
do not eliminate the need for filtering, their linear mode
operation introduces no additional noise.
Conclusion
Linear Technology’s surge stopper products simplify
MIL-STD-1275D compliance by using MOSFETs to block
high voltage input surges and spikes while providing
uninterrupted power to downstream circuitry. Blocking
the voltage with series components avoids the blown
fuses and damage that can occur when circuits attempt
to shunt high energy to ground with bulky passive
components.
For applications help,
call (408) 432-1900
dn534 LT/AP 0115 111K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
 LINEAR TECHNOLOGY CORPORATION 2014
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