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The design features two sets of MOSFETs (TR2/TR3 in Figure
2) connected in a bi-directional arrangement to handle the
high load current. Without this type of connection, the current
could flow back through the MOSFET body diode to the
battery when the load is deactivated. Each switch requires 10
Vishay SQJQ160E MOSFETs connected in parallel to minimize
resistance and power dissipation. With this arrangement, total
losses can be limited to ~10W at 200A operation. To prevent
greater damage to the load and protect the vehicle wiring in
case of a short circuit, the load current is continuously measured
using the shunt resistor R2 in Figure 2 (4x the Vishay WSLP3921
300µΩ in parallel). When a defined overload current is detected,
the controller can quickly disconnect the battery from the short-
circuited load.
Until the load is deactivated during this short amount of time,
the eFuse can handle a considerable amount of overload current
because of the high pulse drain current capability of the bond
wireless MOSFETs. Also, the continuous current measurement
is used by the vehicle's body control module (via serial interface)
to monitor the state of health (charge level, remaining lifetime,
etc.) of the battery. Another common issue that occurs when first
connecting the battery to the load during vehicle start-up is the
potential in-rush current because of any uncharged capacitor
banks in the load. If this resulting high peak current is allowed to
occur, downstream components could be damaged, and battery
life could be reduced. Therefore, a pre-charge circuit must be
used to limit the in-rush current to an acceptable level.
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D2TO20 Series
SMD Power
Resistors
XMC7K24CA
XClampR™ TVS
Diodes
In this design, a Vishay SQJA84E MOSFET (TR1 in Figure 2),
a Vishay VSS8D5M6 Schottky diode and a Vishay D2TO20
resistor (R1 in Figure 2) are used to limit the in-rush current
to a maximum of 5A at 48V. This thick film resistor provides
superior power handling and heat conduction due to physical
construction and an integrated heatsink. Before energizing the
load, TR1 is switched on for a predefined time (10ms), and the
output voltage X3 in Figure 2 is monitored. If the output voltage
has not reached ~90% of the input voltage after that time, the
process is terminated, assuming a short circuit in the load or
wiring. If the output has reached the appropriate level, then TR1
is switched off, and TR2/TR3 is switched on to energize the load.
Note that the Schottky diode is used to prevent backflow of the
current through the MOSFET body diode when it is off.
In terms of protection features, a Vishay NTCS0805 (NTC1
in Figure 2), featuring polymer terminations for superior
mechanical reliability under thermal stress, is used to monitor
the temperature, and Vishay XMC7K24CA and 5KASMC30A
transient voltage suppressors are connected in series (D1a and
D1b in Figure 2) to protect the components against transients
from the vehicle load. A novel electronic switch and resettable
fuse design has been presented that can be used in any 48V
load application up to 200A and replace mechanical relays that
are prone to excessive arcing and shorter lifetimes. ▼
Figure 2: The eFuse
power stage as shown in
a simplified schematic.
(Source: Vishay)
