Texas Instruments | Current Measurements in Power Supplies (Rev. A) | Application notes | Texas Instruments Current Measurements in Power Supplies (Rev. A) Application notes

Texas Instruments Current Measurements in Power Supplies (Rev. A) Application notes
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Switching Power Supply Current Measurements
Greg Hupp, Current Sensing Products
There are many different switching power supply
topologies available to meet system power
requirements. DC–DC switching converters reduce a
higher voltage DC rail to a lower voltage DC rail.
These converter architectures include buck, boost,
buck-boost, and flyback topologies. DC–AC switching
converters convert a DC input voltage to an AC output
voltage.
As implied by their name, switching converters employ
various switches, transistors/FETs and/or diodes, to
translate the input voltage to the desired output
voltage at high system efficiency levels. The switching
nature of these converters present challenges in trying
to accurately measure the current waveforms. Voltage
node requirements, system control requirements, and
measurement drift are areas to consider when
selecting current sense amplifiers.
Voltage Node Requirements
Each node in the circuit architecture has a different
common-mode voltage and behavior. Measuring
currents at each of these locations has different
characteristics that the measurement circuit must take
into consideration. Figure 1 illustrates the different
nodes of a buck/step-down converter. The circuit
shows a basic circuit consisting of a half H–bridge
output stage with a low-pass filter constructed from an
inductor and capacitor. The control circuitry, output
stage drivers, and load are not shown.
1
H–bridge and are used primarily for
overcurrent/short–circuit detection with a comparator.
Any measurements being made at this node require
high common-mode circuits with the performance to
measure a small differential voltage.
Node 2 is the mid-point of the half H–bridge and
displays the pulse-width modulation (PWM) signal that
switching power supplies are based around. Current
measurements at this location provide the inductor
current for system control and overcurrent/short–circuit
detection. The voltage transitions between the upper
voltage and ground (or negative supply) in the PWM
ratio that is averaged to produce the correct output
voltage. Node 2 voltage will have sharp commonmode transitions, so measurements here need to be
able to handle the transition voltage in magnitude as
well as suppressing the transient in the output
waveform.
Node 3 voltage is the converter output voltage, which
is a DC voltage level with a small voltage ripple when
observed on oscilloscope. Measurements at this
location will have similar requirements to Node 1 and
provide the inductor current for use in system control
and overcurrent/short–circuit detection. Even though
Node 3 voltage is less than Node 1, the desired output
voltage level may still require measurement circuitry to
handle a high common-mode voltage.
Node 4 voltage is tied to ground of the circuit. This
node will see low, close to ground, common-mode
levels so measurements at this location have a
reduced set of requirements compared with the
previously mentioned locations.
3
Other DC–DC switching architectures have similar
behavior as the nodes described above, although they
may be at different locations in the converter circuitry.
L
2
C
LOAD
4
Figure 1. DC–DC Switching Power Supply - Buck
Architecture
Node 1 voltage is tied to the input supply of the
converter. This is the high voltage the converter is
“stepping-down” to the lower output voltage. Current
measurements at this node are measuring the current
flowing through the high-side devices of the half
SBOA176A – December 2016 – Revised July 2018
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Measurement Drift Requirements
Switching power supplies are highly efficient circuits
for voltage level translation, but there are still power
losses in the conversion. These power losses are
system efficiency losses that manifest as thermal
generation or heat. Depending on the power levels of
the converter, this can be a significant thermal source.
The INA240 has a-low thermal drift spec, which means
that the current measurement does not change
significantly due to heat generation. To further reduce
the heat generated, the INA240 comes in different gain
versions, which allow for the decrease in value of the
Switching Power Supply Current Measurements Greg Hupp, Current Sensing Products
Copyright © 2016–2018, Texas Instruments Incorporated
1
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current sense resistor. Traditional amplifiers can have
significant decreases in performance as amplifier gain
increases. By contrast, all gains versions of the
INA240 have excellent electrical specifications
allowing the achievement of high performance levels
across different gain variants. Table 1 provides a
comparison of the power dissipation difference
between gains.
RSENSE
L
C
LOAD
Parameter
INA240
Gain
20V/V
100V/V
200V/V
Input Voltage (mV)
150
30
15
RSENSE (mΩ)
15
3
1.5
Power dissipated (W)
1.5
0.30
0.15
(1) Full-scale output voltage = 3V and current measurement = 10A
System Control and Monitoring Requirements
Most switching power supplies employ closedfeedback systems to provide stable, well regulated
power. In order to provide optimized feedback control,
precision measurements are desired. Amplifier
specifications, like offset and gain errors, can
significantly influence the regulation ability of the
control system. Different feedback methods are used
depending on the system requirements and desired
complexity of the circuitry. Additionally, system power
monitoring is a growing need as designs optimize and
report the power consumption during different
operating modes of the end equipment.
Voltage mode feedback compares a scaled version of
the output voltage to a reference voltage to obtain the
error voltage. This feedback method is relatively
simple, but provides slow feedback as the system
must allow the output voltage to change before
adjustments can be made. Current measurements for
voltage mode feedback generally monitor the load
currents and determine if any short–circuits are
present. The most important current amplifier criteria
for voltage mode feedback converters is the commonmode output voltage of the converter. The output
voltage on these converters ranges from low voltages
used for microprocessors and low voltage digital
circuitry (1.8V to 5V) to high voltages used for 48V or
higher systems. The output waveform, while after the
filter, may still contain noise/transients that can disturb
or cause errors in the measurement.
Current mode feedback adds a feedback loop to the
control system that utilizes the system current. The
current typically used is the inductor current in the
converter (see Figure 2). This provides a much faster
internal loop to run in parallel with the voltage
feedback loop. In general, one of the down sides of
current mode feedback is the susceptibility to
noise/transients on the signal.
2
±
+
Table 1. Power Dissipation Summary(1)
To Feedback/Control Circuit
Figure 2. Current Sensing for Power Supply
Control Feedback
Current mode feedback is generally split into peak
current mode control and average current mode
control. Peak current mode control utilizes the inductor
current directly and therefore any noise or transients
on the signal cause disturbances in the feedback loop.
The INA240 is designed with high CMRR, which helps
to attenuate any potential disturbances or noise due to
the input signal.
Alternative Device Recommendations
Based on the system requirements, additional devices
are available that may provide the needed
performance and functionality. For applications
requiring the lower performance levels than the
INA240, use the LMP8601 family. The LMP8481 is a
bi-directional current sense amplifier used for high
common-mode voltages that do not require the
amplifier to include ground within the input voltage
range.
Table 2. Alternative Device Recommendations
Device
INA253
Optimized
Parameters
Performance Trade-Off
Integrated Low
Inductive Shunt:
+/-15A at TA= 85°C
2mΩ, PWM rejection
LMP8601
Wide CommonMode Input Range,
Small Package
No Enhanced PWM
Rejection, Lower commonmode input range, Reduced
gain options
LMP8481
Wide CommonMode Input Range,
Low power
No Enhanced PWM
Rejection, Reduced gain
options, Common-mode
range does not include
ground
Table 3. Adjacent Tech Notes
SBOA189
Precision Brightness and Color Mixing in LED
Lighting Using Discrete Current Sense Amplifiers
SBOA174
Current Sensing in an H-Bridge
SBOA202
Benefits of a Low Inductive Shunt for Current
Sensing in PWM Applications
Switching Power Supply Current Measurements Greg Hupp, Current Sensing Products
SBOA176A – December 2016 – Revised July 2018
Submit Documentation Feedback
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