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Texas Instruments External or internal FETs for motor drive in automotive applications (Rev. A) Application notes
Application Report
SLVA968A – January 2019 – Revised February 2019
External or internal FETs for motor drive in
automotive applications
Automotive Systems
ABSTRACT
Modern automobiles use electric motors to drive an increasing number of applications, such as power
windows, locks, seats, mirror adjustments, blowers, and trunk lifts. Transistor-based solid-state drive
circuits are used to switch the current to the motors used in these applications. Designers can choose
motor drive integrated circuits (IC) with either external field-effect transistors (FETs) or internal FETs for
the final drive stage. This document discusses some of the tradeoffs in choosing the external-FET or
internal-FET approach. Additionally, this application report focuses on key comparisons in terms of board
size and thermal performance.
1
2
3
4
5
6
Contents
Introduction ................................................................................................................... 2
Flexibility and Simplicity ..................................................................................................... 4
2.1
External-FET Topology Offers Flexibility ........................................................................ 4
2.2
Internal-FET Topology Offers Simplicity ......................................................................... 4
Board Area Comparisons ................................................................................................... 4
3.1
Board Area Considerations ........................................................................................ 4
3.2
Board Area for External-FET Topology .......................................................................... 5
3.3
Board Area for Internal-FET Topology ........................................................................... 7
3.4
Summary of Board Area Comparison ........................................................................... 8
Thermal Comparisons ..................................................................................................... 10
4.1
Thermal Considerations .......................................................................................... 10
4.2
Thermal Estimates for Gate Driver External-FET Topology ................................................. 12
4.3
Thermal Estimates for Multi-Chip Module Internal-FET Topology .......................................... 13
4.4
Summary of Thermal Comparisons ............................................................................. 13
Summary .................................................................................................................... 15
References .................................................................................................................. 15
Trademarks
All trademarks are the property of their respective owners.
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1
Introduction
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Introduction
In today's automotive industry, many electric motors are added to cars due to the benefits of automation,
enhanced safety, and luxury features. Motors are found in applications such as electric power steering,
brakes, engine and transmission, power seats, windows, doors and trunks. Figure 1 shows an overview of
some of the areas where electric motors are used in a car.
Figure 1. Automotive Electric Motor Application Examples
These electric motors can be brushed DC, brushless DC, or stepper motors. This document focuses on
brushed DC (BDC) motors because they are the most common. But most of the tradeoffs involving
internal or external design also apply to driving brushless DC motors or stepper motors.
The circuit arrangement for a brushed DC motor driver typically consists of a drive stage made up of two
high-side switches and two low-side switches, all of which are controlled by logic. In general the two highside switches and two low-side switches are implemented as n-channel field-effect transistors (FETs). Nchannel FETs are more common than p-channel FETs due to the higher effective current density which, in
turn, is due to the higher mobility of electrons (n-channel charge carriers) compared to holes (p-channel
charge carriers). This arrangement is typically called a full-bridge motor driver, or sometimes an H-bridge
motor driver, due to the shape typically drawn of the electrical schematic.
The full-bridge motor driver is typically implemented using one of the following three topologies:
1. Gate Driver external-FET topology: In this topology, the drive FETs and the FET controller are in
different IC packages. Figure 2 shows an implementation of the motor driver circuit with a controller
chip that includes gate driver, charge pump, control circuits, diagnostics and current sense, along with
four external FETs; that is, the FETs are not in the same IC package as the controller chip. This
arrangement allows the designer to select the controller and the FETs independently, which gives
flexibility to optimize the motor driver design.
2. Multi-chip Module Internal-FET Topology: In this topology, the drive FETs and the FET controller
are in different semiconductor chips, but are encapsulated in the same package in the form of a multichip module (MCM) as Figure 3 shows. In some MCM implementations, some (but not all) of the final
drive-stage FET chips are in the same chip as the controller chip.
3. Monolithic Internal-FET Topology: In this topology, the drive FETs and the FET controller are in the
same IC package. Moreover, all of the drive FETs and the FET controller are in same semiconductor
chip; that is, it is a monolithic chip as Figure 4 shows. This topology is typically used for low motor
current applications.
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Introduction
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The discussion in this document focuses on the Gate Driver External-FET topology or Multi-chip Module
Internal-FET Topology choice.
Topology
Comment
+Supply
PCB
CONTROLLER
FET
FET
Charge Pump
xx
xx
xxx
x
x
Interface
Current Sense
Gate
Drive
x
FET
FET
Drive FET chips and
FET controller chip
are in different IC
packages
Diagnostics
PCB
Package
Chip
Figure 2. Gate Driver External-FET Topology
+Supply
PCB
CONTROLLER
FET
Charge Pump
xx
xx
xxx
x
x
Interface
Current Sense
Gate
Drive
FET
x
Diagnostics
PCB
Package
Drive FET chips and
the FET controller
chip are in the same
IC package. However,
there are multiple
chips.
Die
Figure 3. Multi-Chip Module Internal-FET Topology
+Supply
PCB
FET
CONTROLLER
Charge Pump
xx
xx
xxx
x
x
Interface
Current Sense
Gate
Drive
x
Drive FETs and the
FET controller are in
the same IC package
and the same chip.
Diagnostics
PCB
Package
Die
Figure 4. Monolithic Internal-FET Topology
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Flexibility and Simplicity
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Flexibility and Simplicity
In general, there is a tradeoff between the design flexibility of using external FETs and the design
simplicity of an internal-FET approach. Which choice is better depends on the requirements of the
application as well as the priorities of the circuit designer.
2.1
External-FET Topology Offers Flexibility
External FETs give a designer the flexibility to use one circuit design for a variety of different applications,
simply by allowing designers to choose drive FETs which best suit the requirements of the application.
FETs with larger geometry tend to have lower on-resistance, or RDS(on), for a given gate voltage. Lower
RDS(on) reduces the losses due to the motor current causing voltage drop across the drive FETs. This leads
to power dissipation in each FET, which is given by the product of the square of the FET current and the
FET RDS(on). Note that the maximum current rating of a FET is closely related to RDS(on) of the FET and the
thermal properties of the package.
One possible drawback of choosing larger size FET packages is the overall printed-circuit board (PCB)
size. Perhaps counter-intuitively, Section 3 shows that external-FET implementations are similar in size to
internal-FET devices with comparable performance. Designers should consider both the current rating and
the thermal specification for the FET package; this tradeoff is discussed more in Section 4.
2.2
Internal-FET Topology Offers Simplicity
A motor driver with internal FETs can simplify the design process by providing an all-in-one solution.
Texas Instruments offers several motor drivers with internal FETs, with peak drive current capabilities
ranging from 1 Amp to 10 Amps. These devices minimize the design effort needed for getting started with
a motor drive application, as the internal transistor characteristics are already selected to match the
requirements of the specified motor current.
With the drive FETs in the same IC package as the controller, complete testing of the whole drive circuit
during production assures the device will perform as expected. Having the FETs in the same package as
the drive logic also simplifies monitoring the temperature of the FETs for diagnostic purposes. Additionally,
the internal-FET approach simplifies the bill of materials and layout of the circuit board. Especially for
applications with relatively low motor current, motor drivers with internal FETs can be an attractive
solution.
3
Board Area Comparisons
In many cases the physical size of the PCB is constrained. When board size is limited, internal-FET
drivers may seem to have an inherent advantage due to the reduced number of semiconductor packages.
However, the actual size of the external-FET and internal-FET topologies depends on several variables; in
the following sections we compare board area for the Gate Driver External-FET Topology solution in
Figure 2 and the Multi-chip Module internal-FET Topology solution in Figure 3.
3.1
Board Area Considerations
When comparing the board area for various implementations, there are several factors to consider. These
include:
• Maximum motor current - this is related to RDS(on)
• Maximum motor voltage
• Uni-directional or bidirectional motion - we focus on bidirectional solutions, recognizing that unidirectional motion solutions will be a subset of the full-bridge implementation
• Method of current sensing
• Diagnostic and protection features
• Number of external components needed
• Component spacing rules for manufacturing
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In addition to the FET and the controller chips, several external components are typically required to
implement a complete motor drive solution. These external components may include:
• Decoupling capacitors to stabilize the power supply for the motor driver control circuits
• Bulk capacitors to stabilize the power supply for the internal drive FETs during motor transients
• Resistor to scale the motor current sense signal to a desired voltage range
• Resistor and capacitor to filter the motor current sense signal
• FET, diodes, and resistors to provide reverse battery voltage protection
• Resistors on each signal line to the microcontroller
• Transient voltage suppressor for load dump transients
3.2
Board Area for External-FET Topology
Since the drive FETs can be selected independent of the controller, there are many options to consider
when determining the size of the motor driver solution with external FETs. FET packages are available in
a variety of sizes and configurations. This document discusses the trends in package size versus RDS(on)
for both single-FET implementations and dual-FET implementations of a full-bridge drive circuit.
In addition to the drive FETs forming the full-bridge, the external-FET implementation includes a controller
chip, which typically integrates the necessary gate driver circuits, charge pumps, diagnostic features, and
in many cases current sense amplifiers.
For the Gate Driver external-FET topology in Figure 2, one implementation is to use four single external
FETs for the full-bridge. Figure 5 shows a scatter plot of the data for several FETs, with the board area for
four single FETs plotted on the vertical axis, versus the RDS(on) for one high-side and one low-side FET
plotted on the horizontal axis. The horizontal scale is logarithmic, to better show the wide range of RDS(on)
values available with external FETs. To remove the effect of other factors on these data points, all of the
transistors represented are single n-channel silicon FETs with automotive ratings, and they all have a
maximum VDS rating of 60 V.
Figure 5. Full-Bridge Board Area vs High-Side + Low-Side RDS(on), Single External FETs
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Although a wide variety of options available exist, the general trend is that if only a small board area is
available, the active RDS(on) will be towards the higher values. Conversely, if a high-current application
requires very low RDS(on), the board size will not be as small as lower-current (higher RDS(on)) cases. Single
FETs are also available in packages such as the TO-220, which offer additional flexibility in layout. The
TO-220 can be mounted with the large dimension vertically, so that only the board area used for the three
pins is needed. This also provides another dimension for mounting a heat sink, if needed.
Another option to consider when board space is limited, is to use dual-FET devices, which combine two
FETs in a single package. This can reduce the board size needed for a motor driver without restricting the
available options to only the limited number of motor drivers with internal FETs. Although there are fewer
choices for dual-FET package size, Figure 6 shows the same general trend of increasing board size as
the RDS(on) specification decreases.
Figure 6. Full-Bridge Board Area vs High-Side + Low-Side RDS(on), Dual External FETs
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3.3
Board Area for Internal-FET Topology
Figure 7 illustrates the data for internal-FET topologies. This figure includes both the multi-chip module
and monolithic topologies and shows that the package size for full-bridge devices must increase
significantly to provide RDS(on) reduction. This is primarily due to the larger semiconductor chip size needed
to provide low RDS(on).
Figure 7. Full-Bridge Board Area vs High-Side + Low-Side RDS(on), Internal-FET Devices
Table 1 summarizes the data used to generate Figure 7. It is noted that Table 1 lists some of the
automotive motor drive solutions with internal drive FETs which are available from Texas Instruments and
other semiconductor manufacturers. The selection is limited to devices with voltage ratings of 40 V,
consistent with automotive battery supply systems due to the typical load-dump voltage requirement. The
dimensions are for this motor driver component only, not including any necessary ancillary components.
The listed RDS(on) values used for this comparison are for a path through the driver, including one high-side
FET and one low-side FET.
Table 1. Package Sizes for Examples of Motor Drivers With Internal FETs
Example
Package
Dimensions
(including pins)
Board Area
(mm2)
RDS(on) per H+L pair
Voltage Rating
1
24-pin Thin Small Outline
7.8 mm × 6.4 mm
49.9
150 mΩ (typical)
310 mΩ maximum
40 V absolute maximum
2
16-pin Small Outline
9.9 mm × 6 mm
59.4
100 mΩ (typical)
200 mΩ maximum
38 V absolute maximum, 40 V
with external clamp
3
36-pin Small Outline
Power
10.3 mm x 10.3 mm
106.1
41 mΩ (typical)
80 mΩ maximum
38 V absolute maximum, 40 V
with external clamp
4
36-pin
12.8 mm x 10.3 mm
131.8
150 mΩ (typical)
295 mΩ maximum
45 V absolute maximum
5
28-pin Small Outline
17.9 mm x 10.3 mm
184.4
95 mΩ (typical)
190 mΩ maximum
41 V absolute maximum
6
7-pin Transistor Outline
(two required)
10 mm x 15 mm (two
required)
310
10 mΩ (typical)
28.7 mΩ maximum
40 V absolute maximum
7
30-pin Small Outline
17.2 mm x 19 mm
327
18 mΩ (typical)
38 mΩ maximum
41 V absolute maximum
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Board Area Comparisons
3.4
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Summary of Board Area Comparison
Figure 8 and Figure 9 compare Gate Driver external-FET topology and Multi-chip Module internal-FET
Topology motor driver solutions. The external-FET implementation is documented in TI Design TIDA020008 and uses the DRV8703-Q1 gate driver with two external dual-FET devices. The internal-FET
implementation uses a multi-chip module device available from another manufacturer; it is listed in Table 1
as Example device 3.
Figure 8. High Current Gate Driver External-FET Example
Figure 9. High Current Multi-Chip Module Internal-FET
Example
Figure 8 shows a full-bridge motor driver with a gate driver and external FETs, with the accompanying
passive components. In this external-FET example, the rectangle dimensions are 11.4 mm × 19.3 mm, an
area of 220 mm2. The external FETs can be selected with low RDS(on), thus reducing the power dissipation
and allowing high drive currents. For the dual-FET packages shown, automotive-grade FETs are available
with maximum combined resistance of 23 mΩ and a junction-to-ambient thermal resistance of 95°C/W for
each dual-FET package. Because there are two packages to distribute the power dissipation, the effective
junction-to-ambient thermal resistance is halved. The maximum junction temperature is 175°C, so the
maximum power dissipation is 1.9 W, and a maximum steady-state current of 9.1 A.
Figure 9 shows an internal-FET MCM example with accompanying passive components in the white
rectangle. The rectangle dimensions are 16.8 mm × 14.7 mm, an area of 247 mm2. This particular motor
driver is promoted by the manufacturer as being capable of 35 A of output current. However, when the
thermal properties of the design are considered, the actual steady-state motor current is considerably less
than 10 A. The internal FETs of this example device have a typical combined high-side and low-side
RDS(on) of 41 mΩ, with a maximum combined resistance specification of 80 mΩ. The junction-to-ambient
thermal resistance can be used for a coarse estimate of the thermal properties, and this parameter is in
the range of 45°C/W for this device. Assuming a maximum ambient temperature of 85°C, and a maximum
junction temperature of 150°C, the maximum power dissipation is 1.44 W. For a combined RDS(on) of 80
mΩ, the power equation gives a maximum steady-state current of 4.2 A.
In Table 2 the results are presented in tabular form. The gate driver external-FET topology is slightly more
compact than the multi-chip module internal-FET Topology. The thermal characteristics of these solutions
are compared in Section 4.
Table 2. Board Size Comparison Results
Solution
Board Size (mm ×
mm)
Board Area (mm2)
Figure 8
Gate Driver external-FET topology
11.4 × 19.3
220
Figure 9
Multi-Chip Module internal-FET Topology
14.7 × 16.7
247
Figure
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In summary, the Gate Driver external-FET topology can be equivalent in PCB area as the Multi-Chip
Module internal-FET topology. In addition, the external-FET topology may have significantly higher
steady-state current capability if external FETs with low RDS(on) are selected.
NOTE: Many automotive applications require relatively high peak and continuous motor currents;
examples include windshield wipers, power windows, and power sliding doors. For these
high-current applications, the RDS(on) of the drive FETs is significantly low. When driving
motors with maximum currents greater than 10 A, the size of the FETs becomes a dominant
part of the integrated device size. Many automotive applications have motor currents up to
30 A, or more. Some manufacturers do offer multi-chip module motor drivers with internal
FETs promoted for these high-current applications; the package size of these devices can be
quite large. The key characteristics which cause the package size to increase are the RDS(on)
and voltage ratings of the FETs. The FET size increases as the RDS(on) decreases, and
similarly the size increases as the voltage rating increases.
Designers may assume that motor driver solutions using internal drive FETs provide a
smaller overall size, but in some cases, especially for high-current applications, the layout
area may be very similar. Designers should also carefully compare the specifications of the
components to ensure the two options are truly equivalent.
NOTE: For motor drive applications with relatively low current demands, drive stage transistors with
low RDS(on)values are not needed, and therefore, the FET size can be relatively small. For
example, some motors may have a motor winding resistance in the range of a few Ohms,
with a stall current of 10 A or less, and a normal operating current less than 5 A. This size
motor is used for automotive applications such as power head rests, power sun shades, or
side-mirror folding. A monolithic motor driver with integrated FETs, such as the DRV8873-Q1
device, is a good fit for this type of application. It includes FETs with RDS(on) of 150 mΩ for the
high-side plus low-side transistors, which are active when driving the motor in either
direction. Figure 10 shows the size of this device is about 50 mm2. This device has a very
compact package, and is represented by the data point in the lower right corner of Figure 7.
Compared to the integrated FET solution for low-current applications, it is difficult to
implement an equivalent solution with external FETs in the same compact size. Referring to
Figure 6, even using dual FET devices, most of the available devices with comparable onresistance (less than 200 mΩ) use a 5 mm × 6 mm package as Figure 11 shows, so the
board area for the FETs alone (2 dual FET packages) are over 60 mm2. The complete
solution also requires a gate driver and current sense resistor plus amplifiers, all of which are
integrated into the DRV8873-Q1, along with advanced diagnostics and fault tolerant features.
Thus for low-current applications, devices with integrated FETs can provide a benefit in
terms of reduced board space requirements.
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Figure 10. Package Size of Fully-Integrated Monolithic
Motor Driver for 10-A Peak Current
4
Figure 11. Package Size of Dual-FET Capable of 10-A Peak
Current
Thermal Comparisons
For the relatively low motor current, thermal considerations are typically not as critical as those for higher
power applications. Therefore, this discussion focuses on thermal comparisons for the higher current
applications. Specifically, the thermal performance of Gate Driver external-FET topology and Multi-chip
Module internal-FET Topology is examined.
4.1
Thermal Considerations
For high-power applications, the dissipation of internal heat becomes a significant consideration. When
driving a high-current load, the power dissipation of the controller chip is negligible compared to the power
dissipation of the final drive stage FETs. The power dissipated in the drive FETs depends on the motor
current and the resistance of the FET channel from drain to source (RDS(on)). For a typical full-bridge drive
circuit, the FET power dissipation is calculated as:
Power = I2MOTOR × (RDS(on)High-Side FET + RDS(on)Low-Side FET)
(1)
when a high-side FET and a low-side FET are used to drive the motor current. Further discussion of
power dissipation calculations is given in the Calculating motor driver power dissipation application report.
The path for heat dissipation from the active circuits to the external environment depends on the
implementation of the motor driver as well as the board design. Table 3 shows the thermal paths for
different example implementations. Figure 12, , Figure 13 and Figure 14 show the thermal paths for Gate
Driver external-FET topology, Multi-chip Module internal-FET Topology, and Monolithic internal-FET
Topology respectively.
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Table 3. Heat Flow Paths
Topology
Comment
The separate FET packages will have
thermal paths through their respective
power pads to the PCB.
Figure 12. Heat Flow for Gate Driver External-FET Topology
There can be additional heat flow between
the chips inside the package.
Figure 13. Heat Flow for Multi-Chip Module Internal-FET Topology
For a monolithic device, the primary
thermal flow paths are from the active
device through the power pad to the top
layer copper.
Figure 14. Heat Flow for Monolithic Internal-FET Topology
In all three cases, there is a full-bridge motor driver with either internal FETs or external FETs. During
motor operation, one of the high-side FETs and one of the low-side FETs will be simultaneously turned on
to provide a path for the motor current. The other FETs, making up the complementary path through the
full-bridge, are off. The controller chip typically contributes little to the overall power dissipation. When the
drive FETs are small, sized for lower motor currents, the inclusion of the package area for the controller
chip can provide a significant thermal benefit in terms of a lower thermal resistance. When, the drive FETs
are large compared to the controller chip, the inclusion of the package area for the controller chip is much
less significant.
Thus, the package that includes the controller chip and drive FETs can be more of an advantage for
relatively low motor currents than for high-current applications. TI takes this approach with integrated FET
motor driver devices such as the DRV8873-Q1 for peak drive currents up to 10 A, and with gate driver
devices such as the DRV8703-Q1 for peak currents above 10 A.
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For motor driver designs in automotive applications, the thermal constraints will often be the limiting factor
in designing compact, high-current solutions. Table 4 gives thermal parameters for several of the typical
package products and package types used for automotive motor drivers.
In general, the larger the package, the lower the thermal resistance, thereby allowing better conduction of
heat generated in the device into the circuit board. For multi-chip modules such as are common with
internal-FET drivers, the thermal analysis becomes quite complex with various thermal paths between the
chips as well as from each chip to the board. The specified junction-to-ambient thermal resistance is only
a coarse indicator of the thermal performance that will occur for any specific design, but it is useful for
comparison of the various packages available for motor drivers.
Table 4. Package Thermal Parameters
Example
Package
Dimensions (mm)
Theta JA (°C/W)
Theta JC (°C/W)
1
24-pin thin small outline
7.8 × 6.4
27.8
18.8 (top) 1.0 (bottom)
2
16-pin small outline
9.9 × 6
40.7 (high-side) 55.4 (low-side)
Rthj-pin: 32 (high-side) 45
(low-side)
3
Power SSO-36 TP
10.3 × 10.3
45 ± 25
4 to 4.3
4
36-pin small outline
12.8 × 10.3
46
29
5
28-pin small outline
17.9 × 10.3
35 to 55 (each chip)
20 (high-side) 20 (low-side)
6
7-pin transistor outline
(two required)
10 × 15 (two required)
19 per device
0.55 (high-side) 1.1 (low
side)
7
Power SO-30
17.2 × 19
15 to 22 (1)
1.7 (high-side) 3.2 (lowside)
Dual nchannel FET
LFPAK56D (SOT1205)
5×6
95 (per package)
3.96
(1)
2 half-bridge FETs in parallel, each with Theta JA of 30 to 45°C/W
Another good source of information is found in the TI Training web site; for example, see Power loss and
thermal considerations for gate drivers.
4.2
Thermal Estimates for Gate Driver External-FET Topology
To directly compare the thermal performance of internal and Gate Driver external-FET solutions, the board
size, layer count, and load current can be made the same for both solutions. In Figure 15 and Figure 16
the two solutions are implemented on a two-layer board with the same overall size. In both figures, the
active circuit is driving a 4-Ω resistive load with a supply voltage of 12 V, giving a steady-state load current
of 3 A. The background temperature in both cases is ambient room temperature, about 22°C.
4.2.1
Steady-State 3-A Load Current
With a 3-A load current and typical RDS(on) of 21.25 mΩ for BUK7K25-40E dual FETs, this gives an internal
power dissipation of about 190 mW in each dual-FET package, or about 380 mW total. Using the Theta
JA for the dual-FET package listed in Table 4, the junction-to-ambient temperature delta for each FET
package can be estimated as:
ΔTθJA = 95°C/W × 0.19 W = 18.05°C
(2)
For the ambient temperature of 22°C, this gives an estimated junction temperature of 40°C.
Then, using the Theta JC for the dual-FET package, the junction-to-case temperature delta can be
estimated as:
ΔTJC = 3.96°C/W × 0.19 W = 0.75°C
(3)
which gives an estimated case temperature of about 39°C.
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4.2.2
Steady-State 6-A Load Current
With a 6-A load current and typical RDS(on) of 21.25 mΩ, this gives an internal power dissipation of about
765 mW in each dual-FET package, or about 1.53 W total. Using the Theta JA for the dual-FET package
listed in Table 4, the junction-to-ambient temperature difference for each FET package can be estimated
as:
ΔTJA = 95°C/W × 0.765 W = 73°C
(4)
Then using the Theta JC for the dual-FET package, the junction-to-case temperature delta can be
estimated as:
ΔTJC = 3.96°C/W × 0.765 W = 3°C
(5)
which gives an estimated ambient-to-case temperature rise of about 70°C.
4.3
4.3.1
Thermal Estimates for Multi-Chip Module Internal-FET Topology
Steady-State 3-A Load Current
With a 3-A load current, for a Multi-Chip Module internal-FET device with combined high-side plus lowside typical RDS(on) of 41 mΩ, the internal power dissipation of about 370 mW.
Using the Theta JA for the integrated FET MCM example 3 package, the junction-to-ambient temperature
delta can be estimated as:
ΔTJA = 45°C/W × 0.37 W = 16.65°C
(6)
Then, using the Theta JC for the device, the junction-to-case temperature delta can be estimated as:
ΔT JC= 4.15°C/W × 0.37 W = 1.5°C
(7)
which gives an estimated ambient-to-case temperature rise of about 15°C.
4.3.2
Steady-State 6-A Current
With a 6-A load current , for aMulti-Chip Module internal-FET device with combined high-side plus lowside typical RDS(on) of 41 mΩ, the internal power dissipation is about 1.48 W. Following the same previous
calculations, this gives an estimated ambient-to-case temperature rise of about 60°C. The maximum case
temperature is identified in the image as 77.7°C, about 55°C above the ambient temperature.
4.4
Summary of Thermal Comparisons
Figure 15 and Figure 16 show the thermal comparison for a 3-A load current between the Gate Driver
external-FET topology solution and the Multi-Chip Module internal-FET Topology. Note that the
measurements are in agreement with the theoretical estimates calculated in Section 4.2 and Section 4.3.
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Thermal Comparisons
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Figure 15. Gate Driver External-FET Topology With SteadyState 3-A Load
Figure 16. Multi-Chip Module Internal-FET Topology With
Steady-State 3-A Load
Figure 17 and Figure 18 show the thermal comparison for a 6-A load current between Gate Driver
external-FET topology solution and Multi-Chip Module internal-FET Topology. Note once again that the
measurements are in agreement with the theoretical estimates calculated in Section 4.2 and Section 4.3.
Figure 17. Gate Driver External-FET Topology With SteadyState 6-A Load
14
External or Internal FETs for Motor Drive in Automotive Applications
Figure 18. Multi-Chip Module Internal-FET Topology With
Steady-State 6-A Load
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Summary
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In Table 5 measurement results are presented in tabular form. Specifically, the column with the heading
"Maximum Measured Temperature (°C)" show the maximum measured temperature on the board.
Table 5. Thermal Comparison Results
Test Load
Power (W)
Ambient
Temperature (°C)
Maximum
Measured
Temperature
(°C)
Ambient-toCase
Temperature
Rise (°C)
Figure
Solution
Board Area
(mm2)
Figure 15
Gate driver and
external FET with 42mΩ FETs
220
36
21.5
38.1
16.6
Figure 16
Small 41-mΩ multi-chip
module internal-FET
device
247
36
21.7
36.0
14.3
Figure 17
Gate driver and
external FET with 42mΩ FETs
220
72
22.5
80.2
57.7
Figure 18
Small 41 mΩ multi-chip
module internal-FET
device
247
72
22.5
77.7
55.2
In summary, the external FET solution is slightly more compact than the multi-chip module
internal-FET solution, and has similar thermal characteristics when using FETs with equivalent
RDS(on). The external-FET solution can have significantly better thermal performance due to the
availability of lower RDS(on) of the external FETs.
NOTE: One additional factor when comparing internal-FET solutions to external-FET solutions is the
maximum allowable junction temperature for the devices. The maximum allowable junction
temperature for the internal-FET devices is 150°C, while the maximum allowable junction
temperature for the automotive-qualified external dual-FETs is 175°C, which gives an
additional 25°C of rated temperature margin.
5
Summary
Motor drive solutions using the internal-FET MCM topology simplify the design in terms of component
selection, layout, and bill of materials. For applications with motor currents less than 10 A, integrated FET
motor drivers may also provide size advantages.
Motor drive solutions using the gate driver and external-FET topology offer more flexibility in optimizing the
FET parameters to the particular application. For motor currents greater than 10 A, external FETs may
provide thermal design benefits in terms of spreading the power dissipation into separate packages.
6
References
1.
2.
3.
4.
5.
6.
7.
Texas
Texas
Texas
Texas
Texas
Texas
Texas
Instruments, MOSFET slew rate control training
Instruments, Fundamentals of MOSFET and IGBT gate driver circuits application report
Instruments, Calculating motor driver power dissipation application report
Instruments, Reducing EMI radiated emissions with TI smart gate drive tech note
Instruments, Understanding motor driver current ratings application report
Instruments, Semiconductor and IC package thermal metrics application report
Instruments, Load switch thermal considerations application report
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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 (January 2019) to A Revision .................................................................................................... Page
•
16
Changed 4-mΩ to 42-mΩ in third row of Solution column in the Thermal Comparison Results table. ....................... 15
Revision History
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