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Texas Instruments 0-5 A, Single-Supply, 2 kV Isolated Current Sensing Solution User guides
Tom Hendrick
TI Precision Designs: Reference Design
0-5 A, Single-Supply, 2 kV Isolated Current Sensing
Solution
TI Precision Designs
Circuit Description
TI Precision Designs are analog solutions created by
TI’s analog experts. Reference Designs offer the
theory, component selection, and simulation of useful
circuits. Circuit modifications that help to meet
alternate design goals are also discussed.
This single supply isolated current sensing design can
be used to monitor currents from 0-5 A with up to
2500 Vrms isolation. The isolated current
measurement is accomplished through an isolation
amplifier with a fixed gain of 8 V/V. The 5 V power to
the isolated side of the isolation amplifier (VDD1) is
provided from the user interface power source
(VDD2) using a push-pull driver and small isolation
transformer.
Design Resources
Design Archive
TINA-TI™
AMC1200
SN6501
Ask The Analog Experts
WEBENCH® Design Center
TI Precision Designs Library
All Design files
SPICE Simulator
Product Folder
Product Folder
Isolated (high voltage) Interface
VDD1
User (low voltage) Interface
VDD2
Xfmr Driver
D1
GND
VCC
D2
VDD1
Iload
VDD2
ISO AMP
VDD1
GND
VDD2
Vout
Rsh
IN+
VOUT+
IN-
VOUT-
+
RL
GND1
V
GND2
An IMPORTANT NOTICE at the end of this TI reference design addresses authorized use, intellectual property matters and
other important disclaimers and information.
TINA-TI is a trademark of Texas Instruments
WEBENCH is a registered trademark of Texas Instruments
SLAU521-June 2013-Revised June 2013
0-5A, Single-Supply, 2kV Isolated Current Sensing Solution
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1
Design Summary
The design requirements are as follows:

Generated Isolated Supply Voltage (VDD1): 5.0 Vdc (±3%)

VDD1 Output Current: 8 mA (max)

Isolated Sense Current: 0-5 A

Maximum Shunt Voltage: 250 mV
The design goals and performance are summarized in Table 1. Figure 1 depicts the measured transient
response of the design.
Table 1: Comparison of Design Goals and Simulated/Measured Performance
Goal
Simulated
Offset Voltage
±2 mV
-957.09 µV
Current Measurement
Error (%FSR)
±0.5%
-0.27%
Goal
Measured
±3%
+6-8%
VDD1 Supply Voltage
Figure 1: Simulated Transient Response
2
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2
Theory of Operation
The goals of this design are twofold; the primary goal is to be able to show accurate current
measurements from 0-5 A from an AC current source. The second goal is to power the isolated side of the
current shunt monitor from a 5 V source on the user interface, or output side, of the isolation amplifier.
2.1
Isolated Current Shunt Measurement
The current to be sensed on the isolated side of the circuit (Iload) will be fed through a shunt resistor (Rsh).
Iload generates a voltage across the shunt resistor (Vsh) that is proportional to the value of the shunt
resistor. The differential output voltage of the circuit is the product of the amplifier gain (G) and the voltage
across the shunt Vsh. The AMC1200 output has a fixed common mode voltage based on the VDD2 rail.
When VDD2 is equal to 5 V, the output common-mode voltage is 2.55 V.
Figure 2: Isolation Amplifier Topology
2.2
Isolated Power Supply
Providing a 5 V high side power supply will be realized using a push-pull driver (the SN6501), a pulse
transformer, two rectifiers, and multi-layer ceramic capacitors. Figure 3 depicts the basic transformer
circuit which will provide the necessary power to the input side of the isolation amplifier. VDD1 is the ‘hot’
or isolated, side of the circuit while VDD2 is the user supplied 5 V. The transformer used in this design is
rated for 2500 Vrms isolation.
Figure 3: Isolation Transformer Topology
SLAU521-June 2013-Revised June 2013
0-5A, Single-Supply, 2kV Isolated Current Sensing Solution
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The magnitude of Vsh is proportional to the amount of current flowing through the shunt resistor, Rsh. The
maximum Vsh voltage the amplifier can see before clipping is ±320 mV. To remain in a linear operating
region, the specified maximum differential voltage at the input to the amplifier is limited to ±250 mV. This
is going to be the limiting factor on the size of the shunt resistor. The following equation can be used to
calculate the value of Rsh:
Rsh(max) 
250mV
 0.05
5A
(1)
It is recommended to use the maximum shunt resistance possible to provide the widest dynamic range for
the system. Using a larger shunt also has the negative effect of increasing the power dissipation in the
sense element. As a general rule, use a sense resistor with a power rating of at least 1.5 times the typical
power dissipation expected in the circuit to minimize errors induced by self heating. Power through the
resistor (PRsh) is the product of the current squared and the shunt resistor value as detailed in equation
(2).
PRsh  I 2 * Rsh  5 A2 * 0.05  1.25W
(2)
The gain in this design is fixed by the isolation amplifier at G=8 and can be evaluated using equation (3).
G
4
Vout
Vout
2.0V
V


8
Vsh Rsh * Iload 0.05 * 5 A
V
0-5A, Single-Supply, 2kV Isolated Current Sensing Solution
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(3 )
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3
Component Selection
3.1
Isolation Amplifier
The AMC1200 was chosen for this application because of its high input bandwidth, the low current drawn
on its high-side supply, and its high voltage isolation capability.
3.2
Rectifier Selection
The chosen rectifier diode should possess low-forward voltage to provide as much voltage to the converter
output as possible. When used in high-frequency switching applications the rectifier must also possess a
short recovery time. Schottky diodes meet both of these requirements. The MBR0520L with a typical
forward voltage of approximately 100 mV at 8 mA forward current was used in this low voltage design.
Figure 4: Forward Voltage of the Rectifier
SLAU521-June 2013-Revised June 2013
0-5A, Single-Supply, 2kV Isolated Current Sensing Solution
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3.3
Transformer Selection
To ensure that a magnetic core does not saturate, the magnetics are designed to dissipate the average
stored magnetic flux over each switching cycle of the controller. This is what is known as volt-seconds
balance. The average volt-seconds applied to the magnetics during the switch on time must equal the
average volt-seconds across the magnetics during the off time. To prevent the isolation transformer from
saturating, its volt-seconds (V-t) product must be greater than the maximum volt-seconds product applied
by the SN6501. The maximum voltage delivered by the SN6501 is the nominal converter input plus 10%.
The maximum time this voltage is applied to the primary is half the period of the lowest frequency at the
specified input voltage. The minimum switching frequency of the SN6501 at 5 V operation is 300 kHz.
Therefore the transformer’s minimum V-t product, as determined by equations (1) and (2) in the SN6501
data sheet, is 9.1 µs.
When searching for a suitable transformer, it is necessary to determine the minimum turns ratio required
that will allow the push-pull converter to operate over the specified current and temperature range. This
can be expressed through the ratio of secondary to primary voltage multiplied by a correction factor that
takes the transformer’s typical efficiency into account. Equations (3) through (8) in the SN6501 data sheet
step through the specific requirements for determining the minimum turns ratio for a given application. For
this design, Equation (8) from the SN6501 data sheet (assuming no low drop out regulator is needed) is
used as a starting point to determine the minimum turns ratio requirement. The turns ratio determination is
based on the following equation assuming a forward voltage of 100mV (Vf) across the rectifier with an 8
mA load, the minimum and maximum voltage requirements for the amplifier as noted in design summary,
the transformer correction factor, and drain-source on resistance (RDSon) noted in the SN6501 datasheet,
and maximum current listed in the design summary:
N min  1.031*
3.4
Vf  Vout max
0.1V  5.15V
 1.031*
 1.12
Vout min  RDSon * Iout
4.85V  3 * 8.5mA
(4)
Shunt Resistor
The shunt resistor which provides 250 mV with a 5 A load was calculated to be 0.05Ω. This is a standard
value shunt resistor and is available with a tolerance of 1% and a temperature coefficient as low as
±50ppm/°C.
3.5
Passive Component Selection
The capacitors in the converter circuit are multi-layer ceramic chip (MLCC) capacitors. As with all high
speed CMOS ICs, the SN6501 requires a bypass capacitor in the range of 10 nF to 100 nF. The input
bulk capacitor at the center-tap of the primary is 10 µF, which supports the current into the primary during
the fast switching transients. On the secondary side of the transformer, a bulk capacitor of 22 µF will be
used at the rectifier outputs.
6
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4
Simulation
The TINA-TI™ schematic shown in Figure 5 includes the circuit values obtained in the design process.
Figure 5: TINA-TI™ Schematic of the Isolation Amplifier
The current source is defined as a sinusoidal current of 5 A at a frequency of 5 kHz. The differential output
voltage can be monitored through a standard volt meter or delivered to an analog to digital converter with a
differential input. The output voltage range of the AMC1200 is dependent on the applied VDD2. As
configured here, the output swing is ±2 V (differential) with a common mode level of 2.55 V.
SLAU521-June 2013-Revised June 2013
0-5A, Single-Supply, 2kV Isolated Current Sensing Solution
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4.1
DC Transfer Function
The result of the dc transfer function simulation is shown in Figure 6.
Figure 6: DC Transfer Function
Table 2: Output Voltage versus input Current
Current (A)
4.2
Output Voltage
-5
-1.9960 V
0
-957.09 µV
+5
1.9936 V
Frequency Response
The AC Transfer Characteristics of the circuit from Figure 5 are shown in Figure 7.
Figure 7: AC Analysis
The bandwidth of the isolation amplifier is 92.98kHz. The dc gain according to the simulation is 18.04dB
or 7.97 V/V.
8
0-5A, Single-Supply, 2kV Isolated Current Sensing Solution
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4.3
Error Analysis
A closer analysis of the raw data from figure 6 was used in order to calculate the positive and negative full
scale errors as well as the offset of the differential output voltage. At 0A flowing through the load, the
differential output voltage was -957.09µV. For positive full scale current, the output voltage was 1.9936V.
For negative full scale current, the output voltage was -1.9960. After correcting for the offset voltage, the
following equation was used to calculate %FSR.
% FSR ( pos)  100% *
Iload ( sim)  Iload (ideal )
Iload (max)  Iload (min)
( 5)
 Vout   1 

 *
  Iload (ideal )
G   0.05 

 100% *
5A  0A
 1.9945   1 

*
  5A
8   0.05 

 100% *
 0.27%
5A  0A
The negative full scale error was 0.25%, which also falls within the original design goals.
4.4
VDD1 Error
The output voltage from the transformer, used for the VDD1 supply on the isolation amplifier, was
measured using SN6501 Multi-Transformer EVM. A single AMC1200 draws only 8 mA max, which
hampers the efficiency and regulation capabilities of the power supply. Using the curve found in Figure 8
of the SLLU174 document, the estimated voltage applied to the AMC1200 will be between 5.3 and 5.4 V.
This deviation from 5 V is approximately two times larger than the design goal of 5 V +/-3%. Measuring
directly at the output of the Multi-Transformer EVM, the VDD1 supply was found to be 5.37 V.
Simulating the DC Transfer with 5.4 V applied to VDD1 did show an increase in the offset voltage, but the
overall current measurements remained at the 0.27% level.
4.5
Result Summary
Table 2 summarizes the simulated and estimated performance of the design.
Table 3: Comparison of Design Goals and Simulated/Measured Performance
Goal
Simulated
Offset Voltage
±2mV
-957.09µV
Error (%FSR)
±0.5%
-0.27%
Goal
Measured
5V
5.37 V
VDD1 Supply Voltage
5
Modifications
The components selected for this design were based on the design goals outlined at the beginning of the
design process. Introducing an LDO at the output of the transformer will provide a more stabile VDD1 rail,
but this comes at the additional costs associated with board space, component count, and perhaps a
different transformer depending on the number of current sensing channels needed in the design.
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0-5A, Single-Supply, 2kV Isolated Current Sensing Solution
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Adding an LDO to the secondary side of the transformer would require the circuit designer to account for
the minimum voltage needed at the LDO input to maintain the desired output voltage. Using the
TPS76350 as an example, the regulator needs 75 mV (max) of headroom to maintain 5.0 V out with a 50
mA load. This would provide enough power to drive up to five AMC1200 devices.
A second potential modification to this design would be to replace the 5 V VDD2 source with a 3.3 V
supply. With a slightly larger turns ratio on the transformer, the 5 V necessary for the VDD1 rail on the
AMC1200 could be realized (with or without an LDO). This would provide a ±2 V differential output voltage
from the AMC1200 centered at 1.29 V.
6
Acknowledgements & References
1.
10
SN6501 Multi-Transformer EVM Users Guide (SLLU174)
0-5A, Single-Supply, 2kV Isolated Current Sensing Solution
Copyright © 2013, Texas Instruments Incorporated
SLAU521-June 2013-Revised June 2013
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