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Texas Instruments Thermal Management to Increase Density in PLC AO Modules Application notes
Thermal Management to Increase Density in PLC AO
Modules
Introduction
IOUT =
Analog output modules for PLCs continue to increase
in density while striving to achieve high accuracy
analog performance. A key challenge in realizing this
goal is managing power dissipation, and in turn, heat.
In practical applications, many of these modules are
connected side-by-side to the same backplane which
can cause excessive heating to each PLC module if
power dissipation is high. Elevated operating
temperatures introduce error as circuit parameters
fluctuate with temperature and can even decrease the
life span of the system due to thermal stresses. This
tech note will demonstrate how the adaptive power
management of DAC8771 and DAC8775 significantly
reduces the power dissipation and temperature of 3wire and 4-wire group isolated, or channel-to-channel
isolated current-output analog output modules.
3-Wire Analog Output Circuit
The basic current mode 3-wire analog output module
consists of a DAC that drives a high-side V-to-I
converter. A reference current flows through the RA,
RSET and NMOS transistor based on the DAC output
voltage. This current is gained by a current mirror and
the loop current is sourced by the PMOS transistor.
Figure 1 shows this common implementation and
Equation 1 shows the transfer function from the DAC
code to the current output.
AVDD
RA
RB
A2
+
DAC
IOUT
+
R A VDAC
RB RSET
(1)
The biasing of the output PMOS, provided by the
negative feedback loop of A2, ultimately controls the
amount of current flowing from AVDD to the load.
Equation 2 shows the relationship between AVDD, IOUT,
RLOAD , and the compliance voltage VC. AVDD must be
greater than the voltage drop of the resistors plus the
compliance voltage to keep the PMOS transistor in
saturation.
A VDD ! IOUT (RLOAD RB ) VC
(2)
The draw-back to this topology is that it can lead to
high on-board power dissipation. The bias point of the
PMOS is related to the variables in Equation 2 as well
as the output current set point. If AVDD is much greater
than the voltage at the point of load, the PMOS will be
required to drop this additional voltage and dissipate
the associated power. The on-board power dissipation
due to the output current is shown in Equation 3. The
power dissipated by the load resistor at the analog
input module is not included.
PDiss
A VDDIOUT
IOUT 2RLOAD
(3)
The highest on-board power dissipation occurs when
the resistance of RLOAD is small and AVDD is large. In
this case most of the power is dissipated in the analog
output module which causes the module to heat and
performance to degrade. One solution to this problem
is to reduce the voltage of AVDD. This will reduce the
power dissipation but the output module will no longer
have the ability to drive large resistive loads. There is
a trade-off between on board power dissipation and
drivable load impedance.
Adaptive Power Management
A1
RLOAD
RSET
GND
Figure 1. 3-Wire Current Transmitter Circuit
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By making a modification to the previous circuit, a
circuit can be developed that has low power
dissipation driving small loads but also the ability to
efficiently drive large loads. This is achieved by
varying the voltage at the source of the PMOS based
on the load impedance and is referred to as adaptive
power management. Figure 2 shows the 3-wire circuit
modified to achieve adaptive power management.
Thermal Management to Increase Density in PLC AO Modules
Copyright © 2018, Texas Instruments Incorporated
1
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AVDD
Buck-Boost
Converter
VREG Vs Vd
RA
RB
A2
DAC8760
+
DAC
Figure 3. Thermal Comparison of DAC8760 and
DAC8771 with Adaptive Power Management
+
A1
IOUT
R LOAD
RSET
GND
Figure 2. 3-Wire Transmitter With Adaptive Power
Management
This circuit adds a buck-boost converter that can
dynamically vary the voltage VREG. The voltages at the
drain and source of the output PMOS are measured to
adjust the output of the buck-boost converter. If a large
load is being driven VREG is increased and if a small
load is connected then the voltage is reduced. This
avoids the situation where the output transistor must
dissipate the full difference between AVDD x IOUT and
IOUT2 x RLOAD. Instead, the buck-boost regulates this
voltage efficiently by varying the duty-cycle and
switching frequency. VREG is set to the voltage required
to drive the load plus the compliance voltage required
to keep the PMOS transistor in saturation. This
topology leads to a significant improvement of onboard power dissipation and reduces heating.
Thermal Comparison
To illustrate the thermal impact of adaptive power
management, the PCB temperature is visualized with
thermal images for two different devices. The first
device is DAC8760, a single channel voltage and
current output DAC designed for 3-wire transmitters.
The second device is DAC8771, which is also a 3-wire
voltage and current output DAC, but the DAC8771
also includes adaptive power management, while the
DAC8760 uses the topology seen in Figure 2.
The first comparison between the two devices is with
AVDD = 24V, RLOAD = 250Ω, and IOUT = 24mA. Figure 3
shows the PCB thermal images for DAC8760 and
DAC8771. The maximum temperature in both cases is
measured at the DAC IC. There is a significant
difference of almost 10⁰C between the device without
adaptive power management and the device that
includes this feature. The thermal impact can be even
greater in the end application where the PCB is
enclosed and multiple channels are required.
2
DAC8771
The second comparison is a more extreme case
where AVDD = 36V, Rload = 10Ω, and Iout = 24mA. In
this case the on board power dissipation is greater due
to the high input voltage and small load
resistor.Figure 4 shows the PCB thermal images for
the DAC8760 and DAC8771. DAC8771 is able to more
efficiently regulate the supply voltage for the current
output while the DAC8760 dissipates additional power
in the output PMOS to regulate the current. This
results in an IC temperature difference of
approximately 19⁰C.
DAC8760
DAC8771
Figure 4. Thermal Comparison of DAC8760 and
DAC8771 With Adaptive Power Management
Conclusion
The adaptive power management included in
DAC8771 offers a significant thermal improvement and
reduces on-chip power dissipation. The buck-boost
converter in DAC8771 can also create all the supply
voltages needed for bipolar operation from a single 12V to 36-V input voltage. This simplifies power supply
design requirements and makes the DAC8771 an ideal
candidate for high density channel-to-channel isolated
designs. The device can be programmed to many
different output ranges and has alarm features that
can improve system safety. For group-isolated
designs, DAC8775 offers a quad-channel device with
the same technology included.
Thermal Management to Increase Density in PLC AO Modules
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