# Texas Instruments | How to Design Isolated Comparators for ±48V, 110V and 240V DC and AC Detection (Rev. A) | Application notes | Texas Instruments How to Design Isolated Comparators for ±48V, 110V and 240V DC and AC Detection (Rev. A) Application notes

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How to Design Isolated Comparators for ±48V, 110V and
240V DC and AC Detection
Suvadip Banerjee, Design Engineer, Isolation Products, Interface Group and
Anant Kamath, Systems Engineer, Isolation Products, Interface Group
Isolated Voltage Monitoring in Industrial
Applications
Many industrial applications need to monitor DC and
AC voltages in the range of 48V to 300V. Examples
include power supply and battery monitors, and
receivers of status signals from sensors, switches and
relays. Isolation is needed to handle ground potential
differences and to protect against high voltage. The
traditional solution for AC detection is to use a bridge
rectifier followed by a Zener diode and an optocoupler
as shown in Figure 1. For DC detection, the same
circuit is used without the bridge rectifier.
5V
VZ
Schmitt Trigger
Buffer
Optocoupler
R1
CIN
R2
Host
Controller
Figure 1. Traditional AC Voltage Detection
Figure 2 shows an implementation of DC voltage
detection with ISO1211. The resistor RSENSE controls
the input current into the ISO1211. The typical voltage
threshold at the SENSE pin is 8.25V. The transition
threshold at the module input is 8.25V plus the voltage
drop across RTHR. The ISO121x devices are rated for
60-V operation. For use with voltages greater than 60V an additional resistor RSHUNT is used. The current
through RSHUNT increases with input voltage, increasing
the voltage drop across RTHR, and preventing the
voltage on the ISO121x pins from exceeding 60V. A
capacitor CIN > 1 nF is used to prevent overshoot on
the SENSE pin beyond 60V during transitions.
The following equations describe the input current
through the ISO1211, the voltage transition threshold
at the module input (VIT), and the maximum allowed
voltage at the module input (VIN_MAX).
IISO121x
ISO121x for 48V to 300V DC Detection
Texas Instruments' ISO1211 and ISO1212 devices
provide a reliable and low-power alternative to
optocouplers for 48-V to 300-V DC and AC detection.
ISO121x devices integrate a 60-V hysteresis
comparator, current limit and galvanic isolation. The
ISO1211 is ideal for channel-to-channel isolation and
the ISO1212 is ideal for multichannel designs.
ISO121x devices offer many performance benefits vs.
optocouplers: precise voltage thresholds, low input
current draw, higher speed, lower Failures In Time
(FIT), higher reliability and operation up to 125°C.
ISO121x devices draw current from the signals they
are monitoring and do not need a separate field side
power supply.
VIT
§ 562 : ·
2.25 mA u ¨
¸
§ 8.25 V
8.25 V RTHR u ¨
¨R
VIN _ MAX
§ 60 V
60 V RTHR u ¨
¨R
ª 562 : º ·
2.25 mA u «
» ¸¸
¬ RSENSE ¼ ¹
ª 562 : º ·
2.25 mA u «
» ¸¸
¬ RSENSE ¼ ¹
Voltage Threshold Calculator
Texas Instruments offers an xls based calculator that
solves the equations described in the previous section:
ISO121x Threshold Calculator for 9V to 300V DC and
AC Voltage Detection. The calculator also provides the
peak current drawn for a given module input voltage.
The calculator has separate sheets for DC and AC
operation.
AC Voltage Detection Using ISO121x
ISO1211
RTHR
SENSE
VIN
+
±
RSHUNT
CIN
VCC1
RSENSE
IN
FGND
OUT
GND1
Figure 2. DC Voltage Detection with ISO1211
SLLA382A – March 2018 – Revised March 2018
Submit Documentation Feedback
Figure 3 shows the application circuit for AC voltage
detection. Low cost diodes such as the BAT54CLT1G
can be used for the bridge rectifier. The equations for
determining the values of the different resistors are
same as for DC detection, except that the equations
will yield peak voltages. The results must be divided by
√2 to obtain the r.m.s. values.
How to Design Isolated Comparators for ±48V, 110V and 240V DC and AC
Detection Suvadip Banerjee, Design Engineer, Isolation Products, Interface Group and Anant Kamath, Systems
Engineer, Isolation Products, Interface Group
1
www.ti.com
Table 1. Suggested Values for DC Applications
RTHR
VDC
System
(V)
ISO1211
VINAC
SENSE
VCC1
Host
Controller
RSENSE
RSHUNT
CIN
IN
OUT
FGND
GND1
Power Reduction and Multi-Level, Multi-Channel
Implementation
Figure 4 shows how ISO1212 can be used for a multichannel application, while reducing average current
draw from the high voltage input pins. The
phototransistor (ex. TLP388, 350V) can be turned on
by the microcontroller for only a short duration when
the measurement is needed, avoiding continous power
dissipation. Also, in this example, IN1 is sampled by
two ISO1212 channels. This enables multi-level
detection with different thresholds.
SENSE1
VCC1
RSENSE
IN1
RSHUNT
OUTA
FGND1
OUT1
SENSE2
OUT2
RTHR1
RSENSE
OUTB
IN2
RSHUNT1
FGND2
IN2
GND1
ISO1212
RTHR
SENSE1
Typ
Max
IIN
(mA)
77
8.5
—
560
24
27
30
2.3
137
22
40
560
54
62
70
3.3
240
250
38
22
560
100
113
126
4.7
RTHR
(kΩ)
RSHUNT
(kΩ)
RSENSE
(Ω)
Transition Threshold (VRMS)
Min
Typ
110
148
33
24
560
58
66
73
240
300
60
15
560
109
124
138
Max
Test Results
Table 1 and Table 2 provide suggested component
values for different values of DC and AC detection.
Figure 5 shows test results from an implementation for
240V DC. A transition threshold of 110V is obtained,
and VSENSE remains below 60V (limit for ISO121x) for
VIN up to 250V. The module input current is less than 5
mA at 250V.
60
55
50
45
40
35
30
25
20
15
10
5
0
-5
VSENSE
VOUT
IIN
50
100
150
Input Voltage, VIN (V)
200
6
5.5
5
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
-0.5
250
D001
IN1
RSHUNT
FGND1
OUT1
SENSE2
OUT2
OUTC
Figure 5. Test Results for 240V DC Detection
OUTD
Table 3. Alternative Device Recommendations
RTHR
RSENSE
RSHUNT
IN2
FGND2
Device
Optimized Parameters
ISO1211
Single-channel isolated
For channel to channel
isolation
ISO1212
Dual-channel isolated
For group isolated inputs.
Lower cost per channel
GND1
ON pulse from
microcontroller
FGND
Phototransistor
TLP388
350V
Figure 4. Reducing Power Consumption in High
Voltage Detection
2
Min
48
RSENSE
IN3
Transition Threshold (V)
110
0
VCC1
RSENSE
(Ω)
VIN_MAX
(VRMS)
Sense and Output Voltage (V)
RTHR
RSHUNT
(kΩ)
VAC
System
(VRMS)
ISO1212
IN1
RTHR
(kΩ)
Table 2. Suggested Values for AC Applications
Figure 3. AC Voltage Detection with ISO1211
For AC inputs, a suitable value for CIN should be
chosen to ensure low ripple on the SENSE pin.
VIN_MAX
(V)
Input Current (mA)
BAT54CLT1G
SLLA370
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Input Module Designs
SLLA379
How To Improve Speed and Reliability of
Isolated Digital Inputs in Motor Drives
How to Design Isolated Comparators for ±48V, 110V and 240V DC and AC
Detection Suvadip Banerjee, Design Engineer, Isolation Products, Interface Group and Anant Kamath, Systems
Engineer, Isolation Products, Interface Group
SLLA382A – March 2018 – Revised March 2018
Submit Documentation Feedback
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