Texas Instruments | High-Side Cutoff Switches for High-Power Automotive Applications (Rev. A) | Application notes | Texas Instruments High-Side Cutoff Switches for High-Power Automotive Applications (Rev. A) Application notes

Texas Instruments High-Side Cutoff Switches for High-Power Automotive Applications (Rev. A) Application notes
High-Side Cutoff Switches for High-Power Automotive
Applications
Mamadou Diallo, Carissa Washam Parish, High Power Drivers
In high power automotive applications relays are
commonly used as cutoff switches, such as battery
load balancing and power distribution. Relays are used
because they can control a high voltage system from a
low power signal. However, they present many design
constraints due to their mechanical nature, cost and
size causing long term reliability issues, slow switching
speeds and board space. Semiconductors, like
MOSFETs and gate drivers, have been widely used to
solve these issues increasing lifetime reliability and
providing fast switching speeds. This tech note will
describe methods to use gate drivers and MOSFETs
as a solid state cutoff switch to replace mechanical
relays in order to extend lifetime and reliability.
Mechanical Relays Design Constraints
A relay is an electromechanical switch which uses the
magnetic field of an energized coil to make or break
electrical contact in a circuit. This coil is typically
controlled by a signal from a low powered circuit such
as a microcontroller, and provides a way to isolate the
two. In automotive applications, the most common
type of relay used is the single-pole double-throw
(SPDT). When the coil is not energized, points Y and
Z are shorted therefore connecting the system to the
battery. When there is a fault condition, the MCU
sends a signal allowing current to pass through the coil
and disconnecting the battery from the rest of the
system as shown in Figure 2.
48V
VDD
X
Y
Coil
Z
MCU
System
Figure 2. SPDT Configuration as Cutoff Switch
During Fault Condition
Figure 1. Block Diagram of a Bidirectional 48V to
12V System in Electric Vehicles
Figure 1 shows a block diagram of a DC-DC converter
seen in Hybrid, Electric and Power Train Systems.
This system helps transfer energy between the 48V
battery and the rest of the low power system (12V
battery). The high voltage system drives large loads
such as traction motor and air conditioning whereas
the low power system supplies low power components
such as safety systems and infotainment. Relays are
often used in these topologies to disconnect the 48V
from the rest of the system in the event of a fault
signal(overcurrent/overvoltage) to protect the rest of
the circuit from the high voltage/current from the
battery.
SNVT010A – November 2018 – Revised November 2018
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Mechanical contacts wear out due to friction and
oxidation. Over time this can lead to slow switching
speeds. Cycling the contact at high voltage or current
can form arcs, liquefying or vaporizing contact metal
and forming pits which reduce the current capability
and contact reliability.
Gate Driver and MOSFETs as Solid State Cutoff
Switch
Designers can avoid these issues by using a gate
driver with a MOSFET device to form a solid state
relay. There are many integrated solutions on the
market for this kind of application, however for higher
power applications, where the MOSFET gate charge
(Qg) is higher, and faster switching rates are desired, a
higher current drive is needed. MOSFETs have higher
switching cycles over lifetime because there are no
physical moving parts therefore no metal fatigue and
High-Side Cutoff Switches for High-Power Automotive Applications Mamadou
Copyright © 2018, Texas Instruments Incorporated
Diallo, Carissa Washam Parish, High Power Drivers
1
www.ti.com
wear. In automotive applications, for example, solid
state drive solutions exhibits infinite effective lifetime
as opposed to mechanical parts. Figure 3 shows the
switching cycle performance of MOSFETs over
mechanical relays. This illustrates a 10x higher lifetime
switching cycle for the semiconductor over the
mechanical counterpart.
1,200,000.00
1,000,000.00
Cycles
800,000.00
Relays
600,000.00
The third portion of the circuit is the low-side driver
configured as a high-side driver where the output of
the level-shifted signal from the MCU drives
UCC27524A-Q1 (dual low-side driver with enable
function capability). Tying the driver's outputs together
allows this topology to meet the fast turn-on/off
requirements of such applications by doubling the
drive current. This topology allows the low-side driver,
referenced to the drain of the lower MOSFET, to
disconnect the battery from the rest of the circuit
during any fault conditions effectively protecting the
system from damage. This discrete solution provides
high power handling capability (>500W) often required
in automotive applications.
MOSFETs
400,000.00
2. Isolated Bias
Supply
200,000.00
0.00
Relays
10V
SN6501Q1
MOSFETs
GND
48V
Figure 3. Switching Cycles Comparison
X
To construct the solid state relay using 2 MOSFETs,
either the drains or the sources are tied together. This
is to prevent current flow through the body diode of the
MOSFETs when there is no conduction. Though the Pchannel FETs drive circuit is easier to implement, highpower and cost-sensitive systems use N-channel FETs
for its lower Rds,on and lower cost. The diagram in
Figure 4 shows the cutoff switch consisting of three
blocks: the level-shifter, the bias supply, and the
driver/power switch.
The external level-shifting portion of the circuit is
required in this example because the low power signal
coming from the MCU needs to be shifted in order to
drive the FET at 48V. This portion would be integrated
into a half-bridge gate driver such as UCC27712-Q1
by using the high-side input and grounding the lowside input. A non-inverting BJT buffer circuit may be
added between the level shifter and the driver's input
to implement faster turn-off at the cost of board space
and component count.
The second portion of this circuit is the external bias
supply where the primary winding of the transformer is
referenced to the controller ground and the secondary
winding referenced to the switch source of the
MOSFET. This bias supply provides the driver with
sufficient drive voltage and eliminates any duty cycle
limitations associated with bootstrap circuits.
2
MCU
VDD
OUTA
OUTB
UCC27524A-Q1
GND
1.Level shifter 3. DRIVER + FET
System
Figure 4. Low-side driver as high-side switch
In summary, this tech note demonstrates a way to
replace relays with MOSFETs and low-side gate
drivers to solve design constraints associated with
mechanical relays. This solution is more viable for its
robustness, reliability and performance over time in
high power automotive applications.
Related Documentation
UCC27524A1-Q1 product folder
UCC27524A1-Q1 datasheet
UCC27528-Q1 product folder
UCC27532-Q1 product folder
SN6501-Q1 product folder
High-Side Cutoff Switches for High-Power Automotive Applications Mamadou
Diallo, Carissa Washam Parish, High Power Drivers
ENA
ENB
INA
INB
GND
SNVT010A – November 2018 – Revised November 2018
Submit Documentation Feedback
Copyright © 2018, Texas Instruments Incorporated
Revision History
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Revision History
NOTE: Page numbers for previous revisions may differ from page numbers in the current version.
Changes from Original (November 2018) to A Revision ................................................................................................ Page
•
•
Changed "Figure 1" ....................................................................................................................... 1
Changed "Section 3 isolated bias supply to section 2 Isolated bias"............................................................... 2
SNVT010A – November 2018 – Revised November 2018
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Copyright © 2018, Texas Instruments Incorporated
Revision History
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